I2C-bus and how to use it - Philips Semiconductors / NXP Semiconductors

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

1995 update

April 1995

1.0 THE I2C-BUS BENEFITS DESIGNERS AND

MANUFACTURERS

In consumer electronics, telecommunications and industrial

electronics, there are often many similarities between seemingly

unrelated designs. For example, nearly every system includes:

•Some intelligent control, usually a single-chip microcontroller

•General-purpose circuits like LCD drivers, remote I/O ports, RAM,

EEPROM, or data converters

•Application-oriented circuits such as digital tuning and signal

processing circuits for radio and video systems, or DTMF

generators for telephones with tone dialling.

To exploit these similarities to the benefit of both systems designers

and equipment manufacturers, as well as to maximize hardware

efficiency and circuit simplicity, Philips developed a simple

bidirectional 2-wire bus for efficient inter-IC control. This bus is

called the Inter IC or I2C-bus. At present, Philips’ IC range includes

more than 150 CMOS and bipolar I2C-bus compatible types for

performing functions in all three of the previously mentioned

categories. All I2C-bus compatible devices incorporate an on-chip

interface which allows them to communicate directly with each

other via the I2C-bus. This design concept solves the many

interfacing problems encountered when designing digital control

circuits.

Here are some of the features of the I2C-bus:

•Only two bus lines are required; a serial data line (SDA) and a

serial clock line (SCL)

•Each device connected to the bus is software addressable by a

unique address and simple master/ slave relationships exist at all

times; masters can operate as master-transmitters or as

master-receivers

•It’s a true multi-master bus including collision detection and

arbitration to prevent data corruption if two or more masters

simultaneously initiate data transfer

•Serial, 8-bit oriented, bidirectional data transfers can be made at

up to 100 kbit/s in the standard mode or up to 400 kbit/s in the

fast mode

•On-chip filtering rejects spikes on the bus data line to preserve

data integrity

•The number of ICs that can be connected to the same bus is

limited only by a maximum bus capacitance of 400 pF.

Figure 1 shows two examples of I2C-bus applications.

1.1 Designer benefits

I2C-bus compatible ICs allow a system design to rapidly progress

directly from a functional block diagram to a prototype. Moreover,

since they ‘clip’ directly onto the I2C-bus without any additional

external interfacing, they allow a prototype system to be modified or

upgraded simply by ‘clipping’ or ‘unclipping’ ICs to or from the bus.

Here are some of the features of I2C-bus compatible ICs which are

particularly attractive to designers:

•Functional blocks on the block diagram correspond with the actual

ICs; designs proceed rapidly from block diagram to final

schematic

•No need to design bus interfaces because the I2C-bus interface is

already integrated on-chip

•Integrated addressing and data-transfer protocol allow systems to

be completely software-defined

•The same IC types can often be used in many different

applications

•Design-time reduces as designers quickly become familiar with

the frequently used functional blocks represented by I2C-bus

compatible ICs

•ICs can be added to or removed from a system without affecting

any other circuits on the bus

•Fault diagnosis and debugging are simple; malfunctions can be

immediately traced

•Software development time can be reduced by assembling a

library of reusable software modules.

In addition to these advantages,the CMOS ICs in the I2C-bus

compatible range offer designers special features which are

particularly attractive for portable equipment and battery-backed

systems.

They all have:

•Extremely low current consumption

•High noise immunity

•Wide supply voltage range

•Wide operating temperature range.

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 2

MICRO-

CONTROLLER

PCB83C528

SDA SCL

MICRO-

CONTROLLER

PCB83C528

MICRO-

CONTROLLER

PCB83C528

MICRO-

CONTROLLER

PCB83C528

MICRO-

CONTROLLER

PCB83C528

NON-VOLATILE

MEMORY

PCF8582E

STEREO/DUAL

SOUND

DECODER

TDA9840

HI-FI

AUDIO

PROCESSOR

TDA9860

SINGLE-CHIP

TEXT

SAA52XX

PLL

SYNTHESIZER

TSA5512

M/S COLOUR

DECODER

TDA9160A

PICTURE

SIGNAL

IMPROVEMENT

TDA4670

VIDEO

PROCESSOR

TDA4685

ON-SCREEN

DISPLAY

PCA8510

MICRO-

CONTROLLER

PCB83C528

SDA SCL

DTMF

GENERATOR

PCD3311

MICRO-

CONTROLLER

PCB83C528

ADPCM

PCD5032

MICRO-

CONTROLLER

P80CLXXX

LINE

INTERFACE

PCA1070

BURST MODE

CONTROLLER

PCD5042

(a) (b)

SU00626

Figure 1. Two examples of I2C-bus applications

(a) a high performance highly-integrated TV set; (b) DECT cordless phone base-station

Philips Semiconductors

The I2C-bus and how to use it

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April 1995 3

1.2 Manufacturer benefits

I2C-bus compatible ICs don’t only assist designers, they also give a

wide range of benefits to equipment manufacturers because:

•The simple 2-wire serial I2C-bus minimizes interconnections so

ICs have fewer pins and there are not so many PCB tracks;

result — smaller and less expensive PCBs

•The completely integrated I2C-bus protocol eliminates the need

for address decoders and other ‘glue logic’

•The multi-master capability of the I2C-bus allows rapid testing and

alignment of end-user equipment via external connections to an

assembly-line computer

•The availability of I2C-bus compatible ICs in SO (small outline),

VSO (very small outline) as well as DIL packages reduces space

requirements even more.

These are just some of the benefits. In addition, I2C-bus compatible

ICs increase system design flexibility by allowing simple

construction of equipment variants and easy upgrading to keep

designs up-to-date. In this way, an entire family of equipment can be

developed around a basic model. Upgrades for new equipment, or

enhanced-feature models (i.e. extended memory , remote control,

etc.) can then be produced simply by clipping the appropriate ICs

onto the bus. If a larger ROM is needed, it’s simply a matter of

selecting a microcontroller with a larger ROM from our

comprehensive range. As new ICs supersede older ones, it’s easy

to add new features to equipment or to increase its performance by

simply unclipping the outdated IC from the bus and clipping on its

successor.

1.3 The ACCESS.bus

Another attractive feature of the I2C-bus for designers and

manufacturers is that its simple 2-wire nature and capability of

software addressing make it an ideal platform for the ACCESS.bus

(Figure 2). This is a lower-cost alternative for an RS-232C interface

for connecting peripherals to a host computer via a simple 4-pin

connector (see Section 19.0).

SU00311A

Figure 2. The ACCESS.bus — a low-cost alternative to an RS-232C interface

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 4

2.0 INTRODUCTION TO THE I2C-BUS

SPECIFICATION

For 8-bit digital control applications, such as those requiring

microcontrollers, certain design criteria can be established:

•A complete system usually consists of at least one microcontroller

and other peripheral devices such as memories and I/O

expanders

•The cost of connecting the various devices within the system must

be minimized

•A system that performs a control function doesn’t require

high-speed data transfer

•Overall efficiency depends on the devices chosen and the nature

of the interconnecting bus structure.

In order to produce a system to satisfy these criteria, a serial bus

structure is needed. Although serial buses don’t have the throughput

capability of parallel buses, they do require less wiring and fewer IC

connecting pins. However, a bus is not merely an interconnecting

wire, it embodies all the formats and procedures for communication

within the system.

Devices communicating with each other on a serial bus must have

some form of protocol which avoids all possibilities of confusion,

data loss and blockage of information. Fast devices must be able to

communicate with slow devices. The system must not be dependent

on the devices connected to it, otherwise modifications or

improvements would be impossible. A procedure has also to be

devised to decide which device will be in control of the bus and

when. And, if different devices with dif ferent clock speeds are

connected to the bus, the bus clock source must be defined. All

these criteria are involved in the specification of the I2C-bus.

3.0 THE I2C-BUS CONCEPT

The I2C-bus supports any IC fabrication process (NMOS, CMOS,

bipolar). Two wires, serial data (SDA) and serial clock (SCL), carry

information between the devices connected to the bus. Each device

is recognised by a unique address — whether it’s a microcontroller,

LCD driver, memory or keyboard interface — and can operate as

either a transmitter or receiver, depending on the function of the

device. Obviously an LCD driver is only a receiver, whereas a

memory can both receive and transmit data. In addition to

transmitters and receivers, devices can also be considered as

masters or slaves when performing data transfers (see Table 1). A

master is the device which initiates a data transfer on the bus and

generates the clock signals to permit that transfer. At that time, any

device addressed is considered a slave.

Table 1. Definition of I2C-bus terminology

TERM DESCRIPTION

Transmitter The device which sends the data to the bus

Receiver The device which receives the data from the bus

Master The device which initiates a transfer, generates clock signals and terminates a transfer

Slave The device addressed by a master

Multi-master More than one master can attempt to control the bus at the same time without corrupting the message

Arbitration Procedure to ensure that, if more than one master simultaneously tries to control the bus, only one is allowed to do

so and the message is not corrupted

Synchronization Procedure to synchronize the clock signals of two or more devices

MICRO-

CONTROLLER

LCD

DRIVER STATIC

RAM OR

EEPROM

GATE

ARRAY ADC

MICRO-

CONTROLLER

SDA

SCL

SU00385

Figure 3. Example of an I2C-bus configuration using two microcontrollers

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 5

The I2C-bus is a multi-master bus. This means that more than one

device capable of controlling the bus can be connected to it. As

masters are usually micro-controllers, let’s consider the case of a

data transfer between two microcontrollers connected to the I2C-bus

(Figure 3). This highlights the master-slave and receiver-transmitter

relationships to be found on the I2C-bus. It should be noted that

these relationships are not permanent, but only depend on the

direction of data transfer at that time. The transfer of data would

proceed as follows:

1. Suppose microcontroller A wants to send information to

microcontroller B:

– microcontroller A (master), addresses microcontroller B (slave)

– microcontroller A (master-transmitter), sends data to

microcontroller B (slave-receiver)

– microcontroller A terminates the transfer.

2. If microcontroller A wants to receive information from

microcontroller B:

– microcontroller A (master) addresses microcontroller B (slave)

– microcontroller A (master-receiver) receives data from

microcontroller B (slave-transmitter)

– microcontroller A terminates the transfer.

Even in this case, the master (microcontroller A) generates the

timing and terminates the transfer.

The possibility of connecting more than one microcontroller to the

I2C-bus means that more than one master could try to initiate a data

transfer at the same time. To avoid the chaos that might ensue from

such an event — an arbitration procedure has been developed. This

procedure relies on the wired-AND connection of all I2C interfaces to

the I2C-bus.

If two or more masters try to put information onto the bus, the first to

produce a ‘one’ when the other produces a ‘zero’ will lose the

arbitration. The clock signals during arbitration are a synchronized

combination of the clocks generated by the masters using the

wired-AND connection to the SCL line (for more detailed information

concerning arbitration see Section 7.0).

Generation of clock signals on the I2C-bus is always the

responsibility of master devices; each master generates its own

clock signals when transferring data on the bus. Bus clock signals

from a master can only be altered when they are stretched by a

slow-slave device holding-down the clock line, or by another master

when arbitration occurs.

4.0 GENERAL CHARACTERISTICS

Both SDA and SCL are bidirectional lines, connected to a positive

supply voltage via a pull-up resistor (see Figure 4). When the bus is

free, both lines are HIGH. The output stages of devices connected

to the bus must have an open-drain or open-collector in order to

perform the wired-AND function. Data on the I2C-bus can be

transferred at a rate up to 100 kbit/s in the standard-mode, or up to

400 kbit/s in the fast-mode. The number of interfaces connected to

the bus is solely dependent on the bus capacitance limit of 400pF.

+VDD

PULL-UP

RESISTORS

DATAN1

OUT

SCLKN1

OUT

SDA (SERIAL DATA LINE)

SCL (SERIAL CLOCK LINE

SCLK

DATA

SCLK

DATAN2

OUT

SCLKN2

OUT

SCLK

DATA

SCLK

SU00386

DEVICE 1 DEVICE 2

Figure 4. Connection of I2C-bus devices to the I2C-bus

Philips Semiconductors

The I2C-bus and how to use it

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April 1995 6

SDA

SCL

DATA LINE

STABLE:

DATA VALID

CHANGE

OF DATA

ALLOWED

SU00361

Figure 5. Bit transfer on the I2C-bus

SDA

SCL SP

SDA

SCL

START

CONDITION STOP

CONDITION

SU00362

Figure 6. START and STOP conditions

5.0 BIT TRANSFER

Due to the variety of different technology devices (CMOS, NMOS,

bipolar) which can be connected to the I2C-bus, the levels of the

logical ‘0’ (LOW) and ‘1’ (HIGH) are not fixed and depend on the

associated level of VDD (see Section 15.0 for Electrical

Specifications). One clock pulse is generated for each data bit

transferred.

5.1 Data validity

The data on the SDA line must be stable during the HIGH period of

the clock. The HIGH or LOW state of the data line can only change

when the clock signal on the SCL line is LOW (see Figure 5).

5.2 START and STOP conditions

Within the procedure of the I2C-bus, unique situations arise which

are defined as START and ST OP conditions (see Figure 6).

A HIGH to LOW transition on the SDA line while SCL is HIGH is one

such unique case. This situation indicates a START condition.

A LOW to HIGH transition on the SDA line while SCL is HIGH

defines a STOP condition.

START and ST OP conditions are always generated by the master.

The bus is considered to be busy after the START condition. The

bus is considered to be free again a certain time after the STOP

condition. This bus free situation is specified in Section 15.0.

Detection of START and ST OP conditions by devices connected to

the bus is easy if they incorporate the necessary interfacing

hardware. However, microcontrollers with no such interface have to

sample the SDA line at least twice per clock period in order to sense

the transition.

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The I2C-bus and how to use it

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6.0 TRANSFERRING DATA

6.1 Byte format

Every byte put on the SDA line must be 8-bits long. The number of

bytes that can be transmitted per transfer is unrestricted. Each byte

has to be followed by an acknowledge bit. Data is transferred with

the most significant bit (MSB) first (Figure 7). If a receiver can’t

receive another complete byte of data until it has performed some

other function, for example servicing an internal interrupt, it can hold

the clock line SCL LOW to force the transmitter into a wait state.

Data transfer then continues when the receiver is ready for another

byte of data and releases clock line SCL.

In some cases, it’s permitted to use a different format from the

I2C-bus format (for CBUS compatible devices for example). A

message which starts with such an address can be terminated by

generation of a STOP condition, even during the transmission of a

byte. In this case, no acknowledge is generated (see Section 9.1.3).

6.2 Acknowledge

Data transfer with acknowledge is obligatory. The

acknowledge-related clock pulse is generated by the master. The

transmitter releases the SDA line (HIGH) during the acknowledge

clock pulse.

The receiver must pull down the SDA line during the acknowledge

clock pulse so that it remains stable LOW during the HIGH period of

this clock pulse (Figure 8). Of course, set-up and hold times

(specified in Section 15.0) must also be taken into account.

Usually, a receiver which has been addressed is obliged to generate

an acknowledge after each byte has been received, except when

the message starts with a CBUS address (see Section 9.1.3).

When a slave-receiver doesn’t acknowledge the slave address (for

example, it’ s unable to receive because it’s performing some

real-time function), the data line must be left HIGH by the slave. The

master can then generate a STOP condition to abort the transfer.

If a slave-receiver does acknowledge the slave address but, some

time later in the transfer cannot receive any more data bytes, the

master must again abort the transfer. This is indicated by the slave

generating the not acknowledge on the first byte to follow. The slave

leaves the data line HIGH and the master generates the STOP

condition.

If a master-receiver is involved in a transfer, it must signal the end of

data to the slave-transmitter by not generating an acknowledge on

the last byte that was clocked out of the slave. The slave-transmitter

must release the data line to allow the master to generate a STOP

or repeated START condition.

START

CONDITION

STOP

CONDITION

SDA

SCL

MSB

12 789 123 – 89

ACK ACK

BYTE COMPLETE,

INTERRUPT WITHIN RECEIVER CLOCK LINE HELD LOW

WHILE INTERRUPTS ARE SERVICED

SU00363

ACKNOWLEDGEMENT

SIGNAL FROM RECEIVER

ACKNOWLEDGEMENT

SIGNAL FROM RECEIVER

Figure 7. Data transfer on the I2C-bus

START

CONDITION

S12 789

DATA OUTPUT BY

TRANSMITTER

DATA OUTPUT

BY RECEIVER

SCL FROM MASTER

CLOCK PULSE FOR ACKNOWLEDGMENT

SU00387

Figure 8. Acknowledge on the I2C-bus

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The I2C-bus and how to use it

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April 1995 8

SU00388

CLK

SCL

WAIT

STATE

START COUNTING

HIGH PERIOD

COUNTER

RESET

Figure 9. Clock synchronization during the arbitration procedure

SU00389

Transmitter 1 Loses Arbitration

DATA 1 ≠ SDA

DATA

SDA

SCL

Figure 10. Arbitration procedure of two masters

7.0 ARBITRATION AND CLOCK GENERATION

7.1 Synchronization

All masters generate their own clock on the SCL line to transfer

messages on the I2C-bus. Data is only valid during the HIGH period

of the clock. A defined clock is therefore needed for the bit-by-bit

arbitration procedure to take place.

Clock synchronization is performed using the wired-AND connection

of I2C interfaces to the SCL line. This means that a HIGH to LOW

transition on the SCL line will cause the devices concerned to start

counting of f their LOW period and, once a device clock has gone

LOW, it will hold the SCL line in that state until the clock HIGH state

is reached (Figure 9). However, the LOW to HIGH transition of this

clock may not change the state of the SCL line if another clock is

still within its LOW period. The SCL line will therefore be held LOW

by the device with the longest LOW period. Devices with shorter

LOW periods enter a HIGH wait-state during this time.

When all devices concerned have counted off their LOW period, the

clock line will be released and go HIGH. There will then be no

difference between the device clocks and the state of the SCL line,

and all the devices will start counting their HIGH periods. The first

device to complete its HIGH period will again pull the SCL line LOW.

In this way, a synchronized SCL clock is generated with its LOW

period determined by the device with the longest clock LOW period,

and its HIGH period determined by the one with the shortest clock

HIGH period.

7.2 Arbitration

A master may start a transfer only if the bus is free. Two or more

masters may generate a START condition within the minimum hold

time (tHD;STA) of the START condition which results in a defined

START condition to the bus.

Arbitration takes place on the SDA line, while the SCL line is at the

HIGH level, in such a way that the master which transmits a HIGH

level, while another master is transmitting a LOW level will switch off

its DATA output stage because the level on the bus doesn’t

correspond to its own level.

Arbitration can continue for many bits. Its first stage is comparison of

the address bits (addressing information is in Sections 9.0 and

13.0). If the masters are each trying to address the same device,

arbitration continues with comparison of the data. Because address

and data information on the I2C-bus is used for arbitration, no

information is lost during this process.

A master which loses the arbitration can generate clock pulses until

the end of the byte in which it loses the arbitration.

If a master also incorporates a slave function and it losesarbitration

during the addressing stage, it’s possible that the winning master is

trying to address it. The losing master must therefore switch over

immediately to its slave-receiver mode.

Figure 10 shows the arbitration procedure for two masters. Of

course, more may be involved (depending on how many masters

are connected to the bus). The moment there is a difference

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 9

between the internal data level of the master generating DATA 1 and

the actual level on the SDA line, its data output is switched off,

which means that a HIGH output level is then connected to the bus.

This will not affect the data transfer initiated by the winning master.

Since control of the I2C-bus is decided solely on the address and

data sent by competing masters, there is no central master, nor any

order of priority on the bus.

Special attention must be paid if, during a serial transfer, the

arbitration procedure is still in progress at the moment when a

repeated START condition or a STOP condition is transmitted to the

I2C-bus. If it’s possible for such a situation to occur, the masters

involved must send this repeated START condition or ST OP

condition at the same position in the format frame. In other words,

arbitration isn’t allowed between:

– A repeated START condition and a data bit

– A STOP condition and a data bit

– A repeated START condition and a ST OP condition.

7.3 Use of the clock synchronizing mechanism

as a handshake

In addition to being used during the arbitration procedure, the clock

synchronization mechanism can be used to enable receivers to

cope with fast data transfers, on either a byte level or a bit level.

On the byte level, a device may be able to receive bytes of data at a

fast rate, but needs more time to store a received byte or prepare

another byte to be transmitted. Slaves can then hold the SCL line

LOW after reception and acknowledgement of a byte to force the

master into a wait state until the slave is ready for the next byte

transfer in a type of handshake procedure.

On the bit level, a device such as a microcontroller without, or with

only a limited hardware I2C interface on-chip can slow down the bus

clock by extending each clock LOW period. The speed of any

master is thereby adapted to the internal operating rate of this

device.

8.0 FORMATS WITH 7-BIT ADDRESSES

Data transfers follow the format shown in Figure 11. After the

START condition (S), a slave address is sent. This address is 7 bits

long followed by an eighth bit which is a data direction bit (R/W) —

a ‘zero’ indicates a transmission (WRITE), a ‘one’ indicates a

request for data (READ). A data transfer is always terminated by a

STOP condition (P) generated by the master. However, if a master

still wishes to communicate on the bus, it can generate a repeated

START condition (Sr) and address another slave without first

generating a STOP condition. Various combinations of read/write

formats are then possible within such a transfer.

Possible data transfer formats are:

–Master-transmitter transmits to slave-receiver. The transfer

direction is not changed (Figure 12)

–Master reads slave immediately after first byte (Figure 13).

At the moment of the first acknowledge, the master-transmitter

becomes a master-receiver and the slave-receiver becomes a

slave-transmitter. This acknowledge is still generated by the slave.

The STOP condition is generated by the master

–Combined format (Figure 14). During a change of direction

within a transfer, the START condition and the slave address are

both repeated, but with the R/W bit reversed. If a master receiver

sends a repeated START condition, it has previously sent a not

acknowledge (A).

START

CONDITION

ADDRESS R/W ACK DATA DATAACK ACK

CONDITION

STOP

SDA

SCL

1–7 8 9 1–7 8 9 1–7 8 9

SU00365

Figure 11. A complete data transfer

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 10

ÎÎÎ

ÎÎ

ÎÎÎÎÎ

ÎÎ

S DATA A/ASLAVE ADDRESS R/W A A

ÎÎÎ

DATA

ÎÎ

‘0’ (WRITE) DATA TRANSFERRED

(n BYTES + ACKNOWLEDGE)

ÎÎ

FROM MASTER TO SLAVE

FROM SLAVE TO MASTER

A = ACKNOWLEDGE (SDA LOW)

A = NOT ACKNOWLEDGE (SDA HIGH)

S = START CONDITION

P = STOP CONDITION

SU00627

Figure 12. A master-transmitter addresses a slave receiver with a 7-bit address.

The transfer direction is not changed

ÎÎ

ÎÎÎÎÎ

ÎÎ

SDATASLAVE ADDRESS R/W A

ÎÎ

A DATA

ÎÎ

(READ) DATA TRANSFERRED

(n BYTES + ACKNOWLEDGE)

ÎÎ

SU00628

Figure 13. A master reads a slave immediately after the first byte

ÎÎ

ÎÎÎÎÎ

ÎÎÎ

SDATASLAVE ADDRESS R/W A

ÎÎ

READ OR WRITE

A/A

ÎÎ

ÎÎÎÎÎ

SLAVE ADDRESS

ÎÎ

R/W

READ OR WRITE

DATAA A/A

(n BYTES

+ ACK.) *

Sr = REPEATED START CONDITION

(n BYTES

+ ACK.) *

DIRECTION OF TRANSFER

MAY CHANGE AT THIS POINT

* TRANSFER DIRECTION OF DATA AND ACKNOWLEDGE BITS DEPENDS ON R/W BITS.

SU00629

Figure 14. Combined format

NOTES:

1. Combined formats can be used, for example, to control a serial memory. During the first data byte, the internal memory location has to be

written. After the START condition and slave address is repeated, data can be transferred.

2. All decisions on auto-increment or decrement of previously accessed memory locations etc. are taken by the designer of the device.

3. Each byte is followed by an acknowledgement bit as indicated by the A or A blocks in the sequence.

4. I2C-bus compatible devices must reset their bus logic on receipt of a START or repeated START condition such that they all anticipate the

sending of a slave address.

Philips Semiconductors

The I2C-bus and how to use it

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April 1995 11

9.0 7-BIT ADDRESSING

(SEE SECTION 13.0 FOR 10-BIT ADDRESSING)

The addressing procedure for the I2C-bus is such that the first byte

after the START condition usually determines which slave will be

selected by the master. The exception is the ‘general call’ address

which can address all devices. When this address is used, all

devices should, in theory, respond with an acknowledge. However,

devices can be made to ignore this address. The second byte of the

general call address then defines the action to be taken. This

procedure is explained in more detail in Section 9.1.1.

9.1 Definition of bits in the first byte

The first seven bits of the first byte make up the slave address

(Figure15). The eighth bit is the LSB (least significant bit). It

determines the direction of the message. A ‘zero’ in the least

significant position of the first byte means that the master will write

information to a selected slave. A ‘one’ in this position means that

the master will read information from the slave.

R/W

SLAVE ADDRESS

LSBMSB

SU00630

Figure 15. The first byte after the START procedure

When an address is sent, each device in a system compares the

first seven bits after the START condition with its address. If they

match, the device considers itself addressed by the master as a

slave-receiver or slave-transmitter, depending on the R/W bit.

A slave address can be made-up of a fixed and a programmable

part. Since it’s likely that there will be several identical devices in a

system, the programmable part of the slave address enables the

maximum possible number of such devices to be connected to the

I2C-bus. The number of programmable address bits of a device

depends on the number of pins available. For example, if a device

has 4 fixed and 3 programmable address bits, a total of 8 identical

devices can be connected to the same bus.

The I2C-bus committee coordinates allocation of I2C addresses.

Further information can be obtained from the Philips representatives

listed on the back cover. T wo groups of eight addresses (0000XXX

and 1111XXX) are reserved for the purposes shown in Table 2. The

bit combination 11110XX of the slave address is reserved for 10-bit

addressing (see Section 13.0).

Table 2. Definition of bits in the first byte

SLAVE

ADDRESS R/ bit DESCRIPTION

0000 000 0 General call address

0000 000 1 START byte

0000 001 X CBUS address

0000 010 X Address reserved for different bus format

0000 011 X Reserved for future purposes

0000 1XX X

1111 1XX X

1111 0XX X 10-bit slave addressing

NOTES:

1. No device is allowed to acknowledge at the reception of the

START byte.

2. The CBUS address has been reserved to enable the inter-mixing

of CBUS compatible and I2C-bus compatible devices in the

same system. I2C-bus compatible devices are not allowed to

respond on reception of this address.

3. The address reserved for a different bus format is included to

enable I2C and other protocols to be mixed. Only I2C-bus

compatible devices that can work with such formats and

protocols are allowed to respond to this address.

9.1.1 General call address

The general call address is for addressing every device connected

to the I2C-bus. However, if a device doesn’t need any of the data

supplied within the general call structure, it can ignore this address

by not issuing an acknowledgement. If a device does require data

from a general call address, it will acknowledge this address and

behave as a slave-receiver. The second and following bytes will be

acknowledged by every slave-receiver capable of handling this data.

A slave which cannot process one of these bytes must ignore it by

not acknowledging. The meaning of the general call address is

always specified in the second byte (Figure 16).

There are two cases to consider:

•When the least significant bit B is a ‘zero’

•When the least significant bit B is a ‘one’.

When bit B is a ‘zero’; the second byte has the following definition:

– 00000110 (H‘06’). Reset and write programmable part of slave

address by hardware. On receiving this 2-byte sequence, all

devices designed to respond to the general call address will reset

and take in the programmable part of their address. Precautions

have to be taken to ensure that a device is not pulling down the

SDA or SCL line after applying the supply voltage, since these low

levels would block the bus

– 00000100 (H‘04’). W rite programmable part of slave address by

hardware. All devices which define the programmable part of their

address by hardware (and which respond to the general call

address) will latch this programmable part at the reception of this

two byte sequence. The device will not reset.

– 00000000 (H‘00’). This code is not allowed to be used as the

second byte.

Sequences of programming procedure are published in the

appropriate device data sheets.

The remaining codes have not been fixed and devices must ignore

them.

Philips Semiconductors

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When bit B is a ‘one’; the 2-byte sequence is a ‘hardware general

call’. This means that the sequence is transmitted by a hardware

master device, such as a keyboard scanner, which cannot be

programmed to transmit a desired slave address. Since a hardware

master doesn’t know in advance to which device the message has

to be transferred, it can only generate this hardware general call and

its own address — identifying itself to the system (Figure 17).

The seven bits remaining in the second byte contain the address of

the hardware master. This address is recognised by an intelligent

device (e.g. a microcontroller) connected to the bus which will then

direct the information from the hardware master. If the hardware

master can also act as a slave, the slave address is identical to the

master address.

In some systems, an alternative could be that the hardware master

transmitter is set in the slave-receiver mode after the system reset.

In this way, a system configuring master can tell the hardware

master-transmitter (which is now in slave-receiver mode) to which

address data must be sent (Figure 18). After this programming

procedure, the hardware master remains in the master-transmitter

mode.

SECOND BYTE

A00000000 X X X X X X B A

LSB

FIRST BYTE

(GENERAL CALL ADDRESS

SU00631

Figure 16. General call address format

ÎÎÎ

ÎÎÎÎ

ÎÎ

SDATA00000000 A

ÎÎÎ

ÎÎÎÎÎÎ

MASTER ADDRESS

ÎÎ

1 A

ÎÎ

DATA

A A

GENERAL

CALL ADDRESS SECOND BYTE

(B)

(n BYTES + ACK.)

SU00632

Figure 17. Data transfer from a hardware master-transmitter

ÎÎÎÎÎÎÎ

ÎÎ

S SLAVE ADDR. H/W MASTER A

ÎÎ

ÎÎÎ

DATA

(n BYTES + ACK.)

ÎÎÎ

R/W

ÎÎÎÎÎÎÎÎ

DUMP ADDR. FOR H/W MASTER

ÎÎ

X A

ÎÎ

WRITE (a)

ÎÎ

ÎÎÎÎÎÎÎÎÎ

DUMP ADDR. FROM H/W MASTER A

ÎÎ

R/W

WRITE

ÎÎÎ

DATAA PA/A

(b)

SU00633

Figure 18. Data transfer by a hardware-transmitter capable of dumping data directly to slave devices

(a) Configuring master sends dump address to hardware master

(b) Hardware master dumps data to selected slave

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9.1.2 START byte

Microcontrollers can be connected to the I2C-bus in two ways. A

microcontroller with an on-chip hardware I2C-bus interface can be

programmed to be only interrupted by requests from the bus. When

the device doesn’t have such an interface, it must constantly monitor

the bus via software. Obviously, the more times the microcontroller

monitors, or polls the bus, the less time it can spend carrying out its

intended function. There is therefore a speed difference between

fast hardware devices and a relatively slow microcontroller which

relies on software polling.

In this case, data transfer can be preceded by a start procedure

which is much longer than normal (Figure 19). The start procedure

consists of:

– A START condition (S)

– A START byte (00000001)

– An acknowledge clock pulse (ACK)

– A repeated START condition (Sr).

After the START condition S has been transmitted by a master

which requires bus access, the START byte (00000001) is

transmitted. Another microcontroller can therefore sample the SDA

line at a low sampling rate until one of the seven zeros in the START

byte is detected. After detection of this LOW level on the SDA line,

the microcontroller can switch to a higher sampling rate to find the

repeated START condition Sr which is then used for

synchronization.

A hardware receiver will reset on receipt of the repeated START

condition Sr and will therefore ignore the START byte.

An acknowledge-related clock pulse is generated after the START

byte. This is present only to conform with the byte handling format

used on the bus. No device is allowed to acknowledge the START

byte.

9.1.3 CBUS compatibility

CBUS receivers can be connected to the I2C-bus. However, a third

bus line called DLEN must then be connected and the acknowledge

bit omitted. Normally, I2C transmissions are sequences of 8-bit

bytes; CBUS compatible devices have different formats.

In a mixed bus structure, I2C-bus devices must not respond to the

CBUS message. For this reason, a special CBUS address

(0000001X) to which no I2C-bus compatible device will respond, has

been reserved. After transmission of the CBUS address, the DLEN

line can be made active and a CBUS-format transmission

(Figure 20) sent. After the STOP condition, all devices are again

ready to accept data.

Master-transmitters can send CBUS formats after sending the

CBUS address. The transmission is ended by a STOP condition,

recognised by all devices.

NOTE: If the CBUS configuration is known, and expansion with

CBUS compatible devices isn’t foreseen, the designer is allowed to

adapt the hold time to the specific requirements of the device(s)

used.

S12 789

SDA

SCL Sr

DUMMY

ACKNOWLEDGE

(HIGH)

ACK

START BYTE 00000001

SU00634

Figure 19. START byte procedure

S P

SDA

SCL

DLEN

START

condition CBUS

address R/W

bit ACK

clock pulse

n – data bits CBUS

address

SU00635

Figure 20. Data format of transmissions with CBUS transmitter/receiver

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The I2C-bus and how to use it

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10.0 ELECTRICAL CHARACTERISTICS FOR

I2C-BUS DEVICES

The electrical specifications for the I/Os of I2C-bus devices and the

characteristics of the bus lines connected to them are given in

Tables 3 and 4 in Section 15.0.

I2C-bus devices with fixed input levels of 1.5 V and 3 V can each

have their own appropriate supply voltage. Pull-up resistors must be

connected to a 5V  10% supply (Figure 21). I2C-bus devices with

input levels related to VDD must have one common supply line to

which the pull-up resistor is also connected (Figure 22).

When devices with fixed input levels are mixed with devices with

input levels related to VDD, the latter devices must be connected to

one common supply line of 5 V  10% and must have pull-up

resistors connected to their SDA and SCL pins as shown in

Figure 23.

Input levels are defined in such a way that:

– The noise margin on the LOW level is 0.1 VDD

– The noise margin on the HIGH level is 0.2 VDD

– As shown in Figure 24, series resistors (RS) of, e.g., 300Ω can be

used for protection against high-voltage spikes on the SDA and

SCL lines (due to flash-over of a TV picture tube, for example).

RPRP

VDD1 = 5V ± 10% VDD2 VDD3 VDD4

BIPOLARCMOSBiCMOSNMOS

SDA

SCL

VDD2 – 4 ARE DEVICE DEPENDENT (e.g., 12V)

SU00636

Figure 21. Fixed input level devices connected to the I2C-bus

RPRP

VDD = e.g., 3V

CMOSCMOSCMOSCMOS

SDA

SCL

SU00637

Figure 22. Devices with wide supply voltage range connected to the I2C-bus

RPRP

VDD1 = 5V ± 10% VDD2 VDD3

BIPOLARNMOSCMOSCMOS

SDA

SCL

VDD2, 3 ARE DEVICE DEPENDENT (e.g., 12V)

SU00638

Figure 23. Devices with input levels related to VDD (supply VDD1) mixed with fixed input level devices (supply V DD2, 3) on the I 2C-bus

RSRS

VDD

I2C

DEVICE

SDA

SCL

RSRS

VDD

I2C

DEVICE RPRP

SU00639

Figure 24. Series resistors (RS) for protection against high-voltage spikes

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10.1 Maximum and minimum values

of resistors Rp and Rs

For standard-mode I2C-bus devices, the values of resistors Rp and

Rs in Figure 24 depend on the following parameters:

– Supply voltage

– Bus capacitance

– Number of connected devices (input current + leakage current).

The supply voltage limits the minimum value of resistor Rp due to

the specified minimum sink current of 3 mA at VOLmax = 0.4 V for

the output stages. VDD as a function of Rpmin

is shown in

Figure 25. The desired noise margin of 0.1VDD for the LOW level,

limits the maximum value of Rs. Rsmax

as a function of Rp is shown

in Figure 26.

The bus capacitance is the total capacitance of wire, connections

and pins. This capacitance limits the maximum value of Rp due to

the specified rise time. Figure 27 shows Rpmax

as a function of bus

capacitance.

The maximum HIGH level input current of each input/output

connection has a specified maximum value of 10 A. Due to the

desired noise margin of 0.2VDD for the HIGH level, this input current

limits the maximum value of Rp. This limit depends on VDD. The

total HIGH level input current is shown as a function of Rp max in

Figure 28.

0481216

DD(V)

minimum

value RP

(kΩ)

max. RS

RS = 0

SU00651

Figure 25. Minimum value of RP as a function of supply

voltage with the value of RS as a parameter

10V

0 400 800 1200 1600

maximum value RS (Ω)

(kΩ)

VDD = 2.5V 5V

15V

SU00652

Figure 26. Maximum value of RS as a function of the value of

RP with supply voltage as a parameter

0 100 200 300 400

bus capacitance (pF)

maximum

value RP

(kΩ)

RS = 0

max. RS

@ VDD = 5V

SU00653

Figure 27. Maximum value of RP as a function of bus

capacitance for a standard-mode I2C-bus

2.5V

0 40 80 120 160

total high level input current (µA)

maximum

value RP

(kΩ)

VDD = 15V

200

10V

SU00654

Figure 28. Total HIGH level input current as a function of the

maximum value of RP with supply voltage as a parameter

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11.0 EXTENSIONS TO THE I2C-BUS

SPECIFICATION

The I2C-bus with a data transfer rate of up to 100 kbit/s and 7-bit

addressing has now been in existence for more than ten years with

an unchanged specification. The concept is accepted world-wide as

a de facto standard and hundreds of different types of I2C-bus

compatible ICs are available from Philips and other suppliers. The

I2C-bus specification is now extended with the following two

features:

•A fast-mode which allows a fourfold increase of the bit rate to 0 to

400 kbit/s

•10-bit addressing which allows the use of up to 1024 additional

addresses.

There are two reasons for these extensions to the I2C-bus

specification:

– New applications will need to transfer a larger amount of serial

data and will therefore demand a higher bit rate than 100 kbit/s.

Improved IC manufacturing technology now allows a fourfold

speed increase without increasing the manufacturing cost of the

interface circuitry

– Most of the 112 addresses available with the 7-bit addressing

scheme have been issued more than once. To prevent problems

with the allocation of slave addresses for new devices, it is

desirable to have more address combinations. About a tenfold

increase of the number of available addresses is obtained with the

new 10-bit addressing.

All new devices with an I2C-bus interface are provided with the

fast-mode. Preferably, they should be able to receive and/or transmit

at 400 kbit/s. The minimum requirement is that they can

synchronize with a 400 kbit/s transfer; they can then prolong the

LOW period of the SCL signal to slow down the transfer. Fast-mode

devices must be downward-compatible which means that they must

still be able to communicate with 0 to 100 kbit/s devices in a 0 to

100 kbit/s I2C-bus system.

Obviously, devices with a 0 to 100 kbit/s I2C-bus interface cannot

be incorporated in a fast-mode I2C-bus system because, since they

cannot follow the higher transfer rate, unpredictable states of these

devices would occur.

Slave devices with a fast-mode I2C-bus interface can have a 7-bit or

a 10-bit slave address. However, a 7-bit address is preferred

because it is the cheapest solution in hardware and it results in the

shortest message length. Devices with 7-bit and 10-bit addresses

can be mixed in the same I2C-bus system regardless of whether it is

a 0 to 100 kbit/s standard-mode system or a 0 to 400 kbit/s

fast-mode system. Both existing and future masters can generate

either 7-bit or 10-bit addresses.

12.0 FAST-MODE

In the fast-mode of the I2C-bus, the protocol, format, logic levels and

maximum capacitive load for the SDA and SCL lines quoted in the

previous I2C-bus specification are unchanged. Changes to the

previous I2C-bus specification are:

– The maximum bit rate is increased to 400 kbit/s

– T iming of the serial data (SDA) and serial clock (SCL) signals has

been adapted. There is no need for compatibility with other bus

systems such as CBUS because they cannot operate at the

increased bit rate

– The inputs of fast-mode devices must incorporate spike

suppression and a Schmitt trigger at the SDA and SCL inputs

– The output buf fers of fast-mode devices must incorporate slope

control of the falling edges of the SDA and SCL signals

– If the power supply to a fast-mode device is switched off, the SDA

and SCL I/O pins must be floating so that they don’t obstruct the

bus lines

– The external pull-up devices connected to the bus lines must be

adapted to accommodate the shorter maximum permissible rise

time for the fast-mode I2C-bus. For bus loads up to 200pF, the

pull-up device for each bus line can be a resistor; for bus loads

between 200pF and 400pF, the pull-up device can be a current

source (3mA max.) or a switched resistor circuit as shown in

Figure 37.

13.0 10-BIT ADDRESSING

The 10-bit addressing does not change the format in the I2C-bus

specification. Using 10 bits for addressing exploits the reserved

combination 1111XXX for the first seven bits of the first byte

following a START (S) or repeated START (Sr) condition as

explained in Section 9.1. The 10-bit addressing does not affect the

existing 7-bit addressing. Devices with 7-bit and 10-bit addresses

can be connected to the same I2C-bus, and both 7-bit and 10-bit

addressing can be used in a standard-mode system (up to

100 kbit/s) or a fast-mode system (up to 400 kbit/s).

Although there are eight possible combinations of the reserved

address bits 1111XXX, only the four combinations 11110XX are used

for 10-bit addressing. The remaining four combinations 1111 1XX are

reserved for future I2C-bus enhancements.

13.1 Definition of bits in the first two bytes

The 10-bit slave address is formed from the first two bytes following

a START condition (S) or a repeated START condition (Sr).

The first seven bits of the first byte are the combination 11110XX of

which the last two bits (XX) are the two most-significant bits (MSBs)

of the 10-bit address; the eighth bit of the first byte is the R/W bit

that determines the direction of the message. A ‘zero’ in the least

significant position of the first byte means that the master will write

information to a selected slave. A ‘one’ in this position means that

the master will read information from the slave.

If the R/W bit is ‘zero’, then the second byte contains the remaining

8 bits (XXXXXXXX) of the 10-bit address. If the R/W bit is ‘one’, then

the next byte contains data transmitted from a slave to a master.

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13.2 Formats with 10-bit addresses

Various combinations of read/write formats are possible within a

transfer that includes 10-bit addressing. Possible data transfer

formats are:

–Master-transmitter transmits to slave-receiver with a 10-bit

slave address. The transfer direction is not changed

(Figure 29). When a 10-bit address follows a START condition,

each slave compares the first seven bits of the first byte of the

slave address (11110XX) with its own address and tests if the

eighth bit (R/W direction bit) is 0. It is possible that more than one

device will find a match and generate an acknowledge (A1). All

slaves that found a match will compare the eight bits of the

second byte of the slave address (XXXXXXXX) with their own

addresses, but only one slave will find a match and generate an

acknowledge (A2). The matching slave will remain addressed by

the master until it receives a STOP condition (P) or a repeated

START condition (Sr) followed by a dif ferent slave address

–Master-receiver reads slave- transmitter with a 10-bit slave

address. The transfer direction is changed after the second

R/W bit (Figure 30). Up to and including acknowledge bit A2, the

procedure is the same as that described for a master-transmitter

addressing a slave-receiver. After the repeated START condition

(Sr), a matching slave remembers that it was addressed before.

This slave then checks if the first seven bits of the first byte of the

slave address following Sr are the same as they were after the

START condition (S), and tests if the eighth (R/W ) bit is 1. If there

is a match, the slave considers that it has been addressed as a

transmitter and generates acknowledge A3.

The slave-transmitter remains addressed until it receives a STOP

condition (P) or until it receives another repeated START condition

(Sr) followed by a different slave address. After a repeated START

condition (Sr), all the other slave devices will also compare the

first seven bits of the first byte of the slave address (11110XX)

with their own addresses and test the eighth (R/W) bit. However,

none of them will be addressed because R/W = 1 (for 10-bit

devices), or the 11110XX slave address (for 7-bit devices) does

not match)

–Combined format. A master transmits data to a slave and

then reads data from the same slave (Figure 31). The same

master occupies the bus all the time. The transfer direction is

changed after the second R/W bit

–Combined format. A master transmits data to one slave and

then transmits data to another slave (Figure 32). The same

master occupies the bus all the time

–Combined format. 10-bit and 7-bit addressing combined in

one serial transfer (Figure 33). After each START condition (S),

or each repeated START condition (Sr), a 10-bit or 7-bit slave

address can be transmitted. Figure 33 shows how a

master-transmits data to a slave with a 7-bit address and then

transmits data to a second slave with a 10-bit address. The same

master occupies the bus all the time.

ÎÎÎÎÎ

ÎÎÎ

SSLAVE ADDRESS

1st 7 BITS A1

ÎÎ

ÎÎÎ

DATA A

ÎÎÎ

DATA PA/A

11110XX 0

ÎÎÎ

R/W

(WRITE)

ÎÎÎÎÎ

SLAVE ADDRESS

2nd BYTE A2

SU00640

Figure 29. A master-transmitter addresses a slave-receiver with a 10-bit address

ÎÎÎÎÎÎ

ÎÎ

SSLAVE ADDRESS

1st 7 BITS A1

ÎÎ

DATA

ÎÎ

A P

11110XX 0

ÎÎÎ

R/W

(WRITE)

ÎÎÎÎÎ

SLAVE ADDRESS

2nd BYTE A2

ÎÎ

ÎÎÎÎÎÎ

SLAVE ADDRESS

1st 7 BITS

11110XX 1

ÎÎ

R/W

(READ)

A3 DATA

ÎÎ

SU00641

Figure 30. A master-receiver addresses a lave-transmitter with a 10-bit address

ÎÎÎÎÎÎ

ÎÎ

SSLAVE ADDRESS

1st 7 BITS A

ÎÎ

DATA

ÎÎ

A P

11110XX 0

ÎÎÎ

R/W

(WRITE)

ÎÎÎÎÎ

SLAVE ADDRESS

2nd BYTE A

ÎÎ

ÎÎÎÎÎÎ

SLAVE ADDRESS

1st 7 BITS

11110XX 1

ÎÎ

R/W

(READ)

A DATA

ÎÎ

ÎÎÎ

DATA A

ÎÎÎ

DATA A/A

SU00642

Figure 31. Combined format. A master addresses a lave with a 10-bit address,

then transmits data to this slave and reads data from this slave

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ÎÎÎÎÎÎ

ÎÎ

SSLAVE ADDRESS

1st 7 BITS A

ÎÎ

DATA A P

11110XX 0

ÎÎÎ

R/W

(WRITE)

ÎÎÎÎÎ

SLAVE ADDRESS

2nd BYTE A

ÎÎ

ÎÎÎÎÎ

SLAVE ADDRESS

1st 7 BITS

11110XX 0

ÎÎÎ

R/W

(WRITE)

ÎÎÎ

DATA

ÎÎÎ

DATA A

ÎÎÎ

DATA A/A

ÎÎÎÎÎ

SLAVE ADDRESS

2nd BYTE A A/A

SU00643

Figure 32. Combined format. A master transmits data to two slaves, both with 10-bit addresses

ÎÎÎÎÎÎ

ÎÎ

S7-BIT

SLAVE ADDRESS A

ÎÎ

DATA A P

ÎÎÎ

R/W

(WRITE)

ÎÎ

ÎÎÎÎÎ

1st 7 BITS OF 10-BIT

SLAVE ADDRESS

11110XX 0

ÎÎÎ

R/W

(WRITE)

ÎÎÎ

DATA

ÎÎ

DATA A

ÎÎ

DATA A/A

ÎÎÎÎÎ

2nd BYTE OF 10-BIT

SLAVE ADDRESS A A/A

SU00644

Figure 33. Combined format. A master transmits data to two slaves, one with a 7-bit address, and one with a 10-bit address

NOTES:

1. Combined formats can be used, for example, to control a serial memory. During the first data byte, the internal memory location has to be

written. After the START condition and slave address is repeated, data can be transferred.

2. All decisions on auto-increment or decrement of previously accessed memory locations etc. are taken by the designer of the device.

3. Each byte is followed by an acknowledgement bit as indicated by the A or A blocks in the sequence.

4. I2C-bus compatible devices must reset their bus logic on receipt of a START or repeated START condition such that they all anticipate the

sending of a slave address.

14.0 GENERAL CALL ADDRESS AND START

BYTE

The 10-bit addressing procedure for the I2C-bus is such that the first

two bytes after the START condition (S) usually determine which

slave will be selected by the master. The exception is the ‘general

call’ address 00000000 (H‘00’). Slave devices with 10-bit addressing

will react to a ‘general call’ in the same way as slave devices with

7-bit addressing (see Section 9.1.1).

Hardware masters can transmit their 10-bit address after a ‘general

call’. In this case, the ‘general call’ address byte is followed by two

successive bytes containing the 10-bit address of the

master-transmitter. The format is as shown in Figure 17 where the

first DATA byte contains the eight least-significant bits of the master

address.

The START byte 00000001 (H‘01’) can precede the 10-bit

addressing in the same way as for 7-bit addressing (see

Section 9.1.2).

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15.0 ELECTRICAL SPECIFICATIONS AND TIMING

FOR I/O STAGES AND BUS LINES

The I/O levels, I/O current, spike suppression, output slope control

and pin capacitance for I2C-bus devices are given in Table 3. The

I2C-bus timing is given in Table 4. Figure 34 shows the timing

definitions for the I2C-bus.

The noise margin for HIGH and LOW levels on the bus lines for

fast-mode devices are the same as those specified in Section 10.0

for standard-mode I2C-bus devices.

The minimum HIGH and LOW periods of the SCL clock specified in

Table 4 determine the maximum bit transfer rates of 100 kbit/s for

standard-mode devices and 400 kbit/s for fast mode devices.

Standard-mode and fast-mode I2C-bus devices must be able to

follow transfers at their own maximum bit rates, either by being able

to transmit or receive at that speed or by applying the clock

synchronization procedure described in Section 7.0 which will force

the master into a wait state and stretch the LOW period of the SCL

signal. Of course, in the latter case the bit transfer rate is reduced.

Table 3. Characteristics of the SDA and SCL I/O stages for I2C-bus devices

PARAMETER SYMBOL STANDARD-MODE

DEVICES FAST-MODE

DEVICES UNIT

Min. Max. Min. Max.

LOW level input voltage: VIL V

fixed input levels –0.5 1.5 –0.5 1.5

VDD-related input levels –0.5 0.3VDD –0.5 0.3VDD

HIGH level input voltage: VIH V

fixed input levels 3.0 *1) 3.0 *1)

VDD-related input levels 0.7VDD *1) 0.7VDD *1)

Hysteresis of Schmitt trigger inputs: Vhys V

fixed input levels n/a n/a 0.2 –

VDD-related input levels n/a n/a 0.05VDD –

Pulse width of spikes which must be suppressed

by the input filter tSP n/a n/a 0 50 ns

LOW level output voltage

(open drain or open collector): V

at 3 mA sink current VOL1 00.4 0 0.4

at 6 mA sink current VOL2 n/a n/a 0 0.6

Output fall time from VIHmin to VILmax with a bus

capacitance from 10 pF to 400 pF: tof ns

with up to 3 mA sink current at VOL1 –2503) 20 + 0.1Cb2) 250

with up to 6 mA sink current at VOL2 n/a n/a 20 + 0.1Cb2) 2503)

Input current each I/O pin with an input

voltage between 0.4 V and 0.9VDDmax Ii–10 10 –104) 104) A

Capacitance for each I/O pin Ci– 10 – 10 pF

n/a = not applicable

1. Maximum VIH = VDDmax + 0.5 V

2. Cb = capacitance of one bus line in pF.

3. The maximum tf for the SDA and SCL bus lines quoted in Table 4 (300 ns) is longer than the specified maximum tof for the output stages

(250 ns). This allows series protection resistors (Rs)to be connected between the SDA/SCL pins and the SDA/SCL bus lines as shown in

Figure 37 without exceeding the maximum specified tf.

4. I/O pins of fast-mode devices must not obstruct the SDA and SCL lines if VDD is switched off.

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 20

Table 4. Characteristics of the SDA and SCL bus lines for I2C-bus devices

PARAMETER SYMBOL STANDARD-MODE

I2C-BUS FAST-MODE

I2C-BUS UNIT

Min. Max. Min. Max.

SCL clock frequency fSCL 0100 0 400 kHz

Bus free time between a STOP and START

condition tBUF 4.7 – 1.3 – s

Hold time (repeated) START condition.

After this period, the first clock pulse is generated tHD;STA 4.0 – 0.6 – s

LOW period of the SCL clock tLOW 4.7 – 1.3 – s

HIGH period of the SCL clock tHIGH 4.0 – 0.6 – s

Set–up time for a repeated START condition tSU;STA 4.7 – 0.6 – s

Data hold time: tHD;DAT

for CBUS compatible masters

(see NOTE, Section 9.1.3) 5.0 – – – s

for I2C–bus devices 01) – 01) 0.92) s

Data set–up time tSU;DAT 250 – 1003) –ns

Rise time of both SDA and SCL signals tr– 1000 20 + 0.1Cb4) 300 ns

Fall time of both SDA and SCL signals tf– 300 20 + 0.1Cb4) 300 ns

Set–up time for STOP condition tSU;STO 4.0 – 0.6 – s

Capacitive load for each bus line Cb– 400 – 400 pF

All values referred to VIHmin and VILmax levels (see Table 3).

1. A device must internally provide a hold time of at least 300 ns for the SDA signal (referred to the VIHmin of the SCL signal) in order to bridge

the undefined region of the falling edge of SCL.

2. The maximum tHD;DAT has only to be met if the device does not stretch the LOW period (tLOW) of the SCL signal.

3. A fast-mode I2C-bus device can be used in a standard-mode I2C-bus system, but the requirement tSU;DAT w250 ns must then be met. This

will automatically be the case if the device does not stretch the LOW period of the SCL signal. If such a device does stretch the LOW period

of the SCL signal, it must output the next data bit to the SDA line t r max + tSU;DAT = 1000 + 250 = 1250 ns (according to the standard-mode

I2C-bus specification) before the SCL line is released.

4. Cb = total capacitance of one bus line in pF.

tSP

tBUF

tHD;STA

PP S

tLOW tR

tHD;DAT

tHIGH tSU;DAT

tSU;STA

tHD;STA

tSU;STO

SDA

SCL

SU00645

Figure 34. Definition of timing on the I2C-bus

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 21

16.0 APPLICATION INFORMATION

16.1 Slope-controlled output stages of fast-mode

I2C-bus devices

The electrical specifications for the I/Os of I2C-bus devices and the

characteristics of the bus lines connected to them are given in

Tables 3 and 4 in Section 15.0.

Figures 35 and 36 show examples of output stages with slope

control in CMOS and bipolar technology. The slope of the falling

edge is defined by a Miller capacitor (C1) and a resistor (R1). The

typical values for C1 and R1 are indicated on the diagrams. The

wide tolerance for output fall time tof given in Table 3 means that the

design is not critical. The fall time is only slightly influenced by the

external bus load (Cb) and external pull-up resistor (Rp). However,

the rise time (tr) specified in Table 4 is mainly determined by the bus

load capacitance and the value of the pull-up resistor.

16.2 Switched pull-up circuit for fast-mode

I2C-bus devices

The supply voltage (VDD) and the maximum output LOW level

determine the minimum value of pull-up resistor Rp (see

Section 10.1). For example, with a supply voltage of

VDD = 5V ± 10% and VOLmax = 0.4V at 3mA,

Rp min (5.5 – 0.4)/0.003 = 1.7 kW. As shown in Figure 38, this value

of Rp limits the maximum bus capacitance to about 200pF to meet

the maximum tr requirement of 300 ns. If the bus has a higher

capacitance than this, a switched pull-up circuit as shown in

Figure 37 can be used.

The switched pull-up circuit in Figure 37 is for a supply voltage of

VDD = 5V ± 10% and a maximum capacitive load of 400pF. Since it

is controlled by the bus levels, it needs no additional switching

control signals. During the rising/falling edges, the bilateral switch in

the HCT4066 switches pull-up resistor Rp2 on/off at bus levels

between 0.8 V and 2.0 V. Combined resistors Rp1 and Rp2 can

pull-up the bus line within the maximum specified rise time (tr) of

300 ns. The maximum sink current for the driving I2C-bus device

will not exceed 6 mA at VOL2 = 0.6 V, or 3 mA at VOL1 = 0.4 V.

Series resistors Rs are optional. They protect the I/O stages of the

I2C-bus devices from high-voltage spikes on the bus lines, and

minimize crosstalk and undershoot of the bus line signals. The

maximum value of Rs is determined by the maximum permitted

voltage drop across this resistor when the bus line is switched to the

LOW level in order to switch off Rp2.

50kΩRP

I/O

VDD

VSS

2pF N2

INPUT

CIRCUIT

VDD

VSS

SDA OR SCL

BUS LINE

SU00646

Figure 35. Slope-controlled output stage in CMOS technology

20kΩRP

I/O

GND

5pF T2

INPUT

CIRCUIT

VDD

VSS

SDA OR SCL

BUS LINE

SU00647

Figure 36. Slope-controlled output stage in bipolar technology

1/4 HCT4066

1.3kΩ

≤100Ω

I/O

≤100Ω

I/O

400pF max.

RP1

1.7kΩ

VCC

GND

VDD

5V ± 10%

SDA OR SCL

BUS LINE

VSS

SU00648

Figure 37. Switched pull-up circuit

bus capacitance (pF)

maximum

value Rp

(kΩ)

7.5

6.0

4.5

3.0

1.5

00 100 200 300 400

RS = 0

max. RS

@ VDD = 5V

SU00649

Figure 38. Maximum value of RP as a function

of bus capacitance for meeting the tR MAX requirement

for a fast-mode I2C-bus

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 22

16.3 Wiring pattern of the bus lines

In general, the wiring must be so chosen that crosstalk and

interference to/from the bus lines is minimized. The bus lines are

most susceptible to crosstalk and interference at the HIGH level

because of the relatively high impedance of the pull-up devices.

If the length of the bus lines on a PCB or ribbon cable exceeds

10cm and includes the VDD and VSS lines, the wiring pattern must

be:

SDA

VDD

VSS

SCL

If only the VSS line is included, the wiring pattern must be:

SDA

VSS

SCL

These wiring patterns also result in identical capacitive loads for the

SDA and SCL lines. The VSS and VDD lines can be omitted if a PCB

with a VSS and/or VDD layer is used.

If the bus lines are twisted-pairs, each bus line must be twisted with

a VSS return. Alternatively, the SCL line can be twisted with a VSS

return, and the SDA line twisted with a VDD return. In the latter case,

capacitors must be used to decouple the VDD line to the VSS line at

both ends of the twisted pairs.

If the bus lines are shielded (shield connected to VSS), interference

will be minimized. However, the shielded cable must have low

capacitive coupling between the SDA and SCL lines to minimize

crosstalk.

16.4 Maximum and minimum values of resistors

Rp and Rs for fast-mode I2C-bus devices

The maximum and minimum values for resistors Rp and Rs

connected to a fast-mode I2C-bus can be determined from

Figure 25, 26 and 28 in Section 10.1. Because a fast-mode I2C-bus

has faster rise times (tr) the maximum value of Rp as a function of

bus capacitance is less than that shown in Figure 27. The

replacement graph for Figure 27 showing the maximum value of Rp

as a function of bus capacitance (Cb) for a fast mode I2C-bus is

given in Figure 38.

17.0 DEVELOPMENT TOOLS

17.1 Development tools for 8048 and 8051-based systems

PRODUCT DESCRIPTION

OM1016 I2C-bus demonstration board with microcontroller, LCD, LED, Par. I/O, SRAM, EEPROM, Clock, DTMF generator,

AD/DA conversion, infrared link.

OM1018 manual for OM1016

OM1020 LCD and driver demonstration board

OM4151 I2C-bus evaluation board (similar to OM1016 above but without infrared link).

OM5027 I2C-bus evaluation board for low-voltage, low-power ICs & software

17.2 Development tools for 68000-based systems

PRODUCT DESCRIPTION

OM4160 Microcore-1 demonstration/evaluation board:

SCC68070, 128K EPROM, 512K DRAM, I2C, RS-232C, VSC SCC66470, resident monitor

OM4160/3 Microcore-3 demonstration/evaluation board:

128K EPROM, 64K SRAM, I2C, RS-232C, 40 I/O (inc. 8051-compatible bus), resident monitor

OM4160/3QFP Microcore-3 demonstration/evaluation board for 9XC101 (QFP80 package)

17.3 17.3 Development tools for all systems

PRODUCT DESCRIPTION

OM1022 I2C-bus analyzer.

Hardware and software (runs on IBM or compatible PC) to experiment with and analyze the behaviour of the I2C-bus

(includes documentation)

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 23

18.0 SUPPORT LITERATURE

DATA HANDBOOKS

Semiconductors for radio and audio systems

IC01a 1995

IC01b 1995

Semiconductors for television and video systems

IC02a 1995

IC02b 1995

IC02c 1995

Semiconductors for telecom systems

IC03 1995

I2C Peripherals

IC12

8048-based 8-bit microcontrollers

IC14 1994

Wireless communications

IC17 1995

Semiconductors for in-car electronics

IC18

80C51-based 8-bit microcontrollers

IC20 1995

68000-based 16-bit microcontrollers

IC21

Desktop video

IC22 1995

Brochures/leaflets/lab. reports

I2C-bus compatible ICs and support overview

I2C-bus control programs for consumer applications

Microcontrollers, microprocessors and support overview

Application notes for 80C51-based 8-bit microcontrollers

OM5027 I2C-bus evaluation board for low-voltage, low-power ICs

& software

P90CL301 I2C driver routines

User manual of Microsoft Pascal I2C-bus driver (MICDRV4.OBJ)

User’s guide to I2C-bus control programs

Philips Semiconductors

The I2C-bus and how to use it

(including specifications)

April 1995 24

19.0 APPLICATION OF THE I2C-BUS IN THE

ACCESS.bus SYSTEM

The ACCESS.bus (bus for connecting ACCESSory devices to a

host system) is an I2C-bus based open-standard serial interconnect

system jointly developed and defined by Philips and Digital

Equipment Corporation. It is a lower-cost alternative to an RS-232C

interface for connecting up to 14 inputs/outputs from peripheral

equipment to a desk-top computer or workstation over a distance of

up to eight metres. The peripheral equipment can be relatively low

speed items such as keyboards, hand-held image scanners, cursor

positioners, bar-code readers, digitizing tablets, card readers or

modems.

All that’s required to implement an ACCESS.bus is an 8051-family

microcontroller with an I2C-bus interface, and a 4-wire cable

carrying a serial data (SDA) line, a serial clock (SCL) line, a ground

wire and a 12V supply line (500mA max.) for powering the

peripherals.

Important features of the ACCESS.bus are that the bit rate is only

about 20% less than the maximum bit rate of the I2C-bus, and the

peripherals don’t need separate device drivers. Also, the protocol

allows the peripherals to be changed by ‘hot-plugging’ without

re-booting.

As shown in Figure 39, the ACCESS.bus protocol comprises three

levels: the I2C-bus protocol, the base protocol, and the application

protocol.

The base protocol is common to all ACCESS.bus devices and

defines the format of the ACCESS.bus message. Unlike the I2C-bus

protocol, it restricts masters to sending and slaves to receiving data.

One item of appended information is a checksum for reliability

control. The base protocol also specifies seven types of control and

status messages which are used in the system configuration which

assigns unique addresses to the peripherals without the need for

setting jumpers or switches on the devices.

The application protocol defines the message semantics that are

specific to the three categories of peripheral device (keyboards,

cursor locators, and text devices which generate character streams,

e.g., card readers) which are at present envisaged.

Philips offers computer peripheral equipment manufacturers

technical support, a wide range of I2C-bus devices and development

kits for the ACCESS.bus. Hardware, software and marketing

support is also offered by DEC.

KEYBOARD

PROTOCOL LOCATOR

PROTOCOL TEXT

PROTOCOL REAL-TIME

CONTROL

PROTOCOL

BASE

PROTOCOL

I2C

PROTOCOL

SOFTWARE

PROTOCOLS

HARDWARE

PROTOCOLS

SU00650

Figure 39. ACCESS.bus protocol hierarchy