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Note: Some revisions of this device may incorporate deviations from published specifications known as errata. Multiple revisions of any device
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FEATURES
Integrated NV SRAM, Real-Time Clock
(RTC), Crystal, Power-Fail Control Circuit,
and Lithium Energy Source
Clock Registers are Accessed Identically to
the Static RAM; These Registers Reside in
the 16 Top RAM Locations
Century Byte Register (i.e., Y2K Compliant)
Totally Nonvolatile with Over 10 Years of
Operation in the Absence of Power
Precision Power-On Reset
Programmable Watchdog Timer and RTC
Alarm
BCD-Coded Year, Month, Date, Day, Hours,
Minutes, and seconds with Automatic Leap-
Year Compensation Valid Up to the Year
2100
Battery Voltage-Level Indicator Flag
Power-Fail Write Protection Allows for 10%
VCC Power-Supply Tolerance
Lithium Energy Source is Electrically
Disconnected to Retain Freshness Until
Power is Applied for the First Time
Also Available in Industrial Temperature
Range: -40°C to +85°C
PIN CONFIGURATIONS
DS1556
1M, Nonvolatile, Y2K-Compliant
Timekeeping RAM
www.maxim-ic.com
R
ST VCC
A15
I
R
Q
/
FT
W
E
A13
A8
A9
A11
O
E
A10
C
E
DQ7
DQ5
DQ6
DQ4
DQ3
13
1
2
3
4
5
6
7
8
9
10
11
12
14
31
A14
A7
A5
A4
A3
A2
A1
A0
DQ1
DQ0
32
30
29
28
27
26
25
24
23
22
21
19
20
A16
A12
A6
DQ2
GND 15
16 18
17
Encapsulated DIP
Maxim
DS1556
TOP VIEW
IRQ
/FT
1
2
3
A15
A16
R
ST
VCC
O
E
C
E
DQ7
DQ6
DQ5
DQ4
DQ3
DQ2
DQ1
DQ0
GND
4
5
6
7
8
9
10
11
12
13
14
15
16
17
N.C.
A14
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
A13
A12
A11
A10
A9
A8
A7
A6
A5
A4
A3
A2
A1
A0
34 N.C.
X1 GND
VBAT X2
PowerCap Module Board
(Uses DS9034PCX PowerCap)
W
E
Maxim
DS1556
19-5500; Rev 9/10
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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PIN DESCRIPTION
A0–A16 - Address Input
DQ0–DQ7 - Data Input/Outputs
IRQ/FT - Interrupt, Frequency Test Output (Open Drain)
RST - Power-On Reset Output (Open Drain)
CE - Chip Enable
OE - Output Enable
WE - Write Enable
VCC - Power Supply Input
GND - Ground
N.C. - No Connection
X1, X2 - Crystal Connection
VBAT - Battery Connection
ORDERING INFORMATION
PART TEMP RANGE
VOLTAGE
(V) PIN-PACKAGE TOP MARK**
DS1556-70+ 0°C to +70°C 5.0 32 EDIP (0.740a) DS1556+070
DS1556-70IND+ -40°C to +85°C 5.0 32 EDIP (0.740a) DS1556+070 IND
DS1556P-70+ 0°C to +70°C 5.0 34 PowerCap* DS1556P+70
DS1556P-70IND+ -40°C to +85°C 5.0 34 PowerCap* DS1556P+70 IND
DS1556W-120+ 0°C to +70°C 3.3 32 EDIP (0.740a) DS1556W+120
DS1556W-120IND+ -40°C to +85°C 3.3 32 EDIP (0.740a) DS1556W+120 IND
DS1556WP-120+ 0°C to +70°C 3.3 34 PowerCap* DS1556WP+120
DS1556WP-120IND+ -40°C to +85°C 3.3 34 PowerCap* DS1556WP+120 IND
+Denotes a lead(Pb)-free/RoHS-compliant package.
*DS9034-PCX+ or DS9034I-PCX+ required (must be ordered separately).
**A “+” in top mark denotes a lead(Pb)-free device. An “IND” anywhere on the top mark indicates an industrial temperature grade device.
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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DESCRIPTION
The DS1556 is a full-function, year-2000-compliant (Y2KC), real-time clock/calendar (RTC) with an
RTC alarm, watchdog timer, power-on reset, battery monitor, and 128k x 8 nonvolatile static RAM. User
access to all registers within the DS1556 is accomplished with a byte-wide interface as shown in Figure 1.
The RTC registers contain century, year, month, date, day, hours, minutes, and seconds data in 24-hour
BCD format. Corrections for day of month and leap year are made automatically.
The RTC registers are double-buffered into an internal and external set. The user has direct access to the
external set. Clock/calendar updates to the external set of registers can be disabled and enabled to allow
the user to access static data. Assuming the internal oscillator is turned on, the internal set of registers is
continuously updated, which occurs regardless of external registers settings to guarantee that accurate
RTC information is always maintained.
The DS1556 has interrupt (IRQ/FT) and reset (RST) outputs which can be used to control CPU activity.
The IRQ/FT interrupt output can be used to generate an external interrupt when the RTC register values
match user programmed alarm values. The interrupt is always available while the device is powered from
the system supply and can be programmed to occur when in the battery-backed state to serve as a system
wake-up. Either the IRQ/FT or RST outputs can also be used as a CPU watchdog timer, CPU activity is
monitored and an interrupt or reset output will be activated if the correct activity is not detected within
programmed limits. The DS1556 power-on reset can be used to detect a system power down or failure
and hold the CPU in a safe reset state until normal power returns and stabilizes; the RST output is used
for this function.
The DS1556 also contains its own power-fail circuitry, which automatically deselects the device when the
VCC supply enters an out of tolerance condition. This feature provides a high degree of data security
during unpredictable system operation brought on by low VCC levels.
PACKAGES
The DS1556 is available in two packages (32-pin DIP and 34-pin PowerCap module). The 32-pin DIP
style module integrates the crystal, lithium energy source, and silicon all in one package. The 34-pin
PowerCap module board is designed with contacts for connection to a separate PowerCap (DS9034PCX)
that contains the crystal and battery. This design allows the PowerCap to be mounted on top of the
DS1556P after the completion of the surface mount process. Mounting the PowerCap after the surface
mount process prevents damage to the crystal and battery due to the high temperatures required for solder
reflow. The PowerCap is keyed to prevent reverse insertion. The PowerCap Module board and PowerCap
are ordered separately and shipped in separate containers. The part number for the PowerCap is
DS9034PCX.
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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Figure 1. Block Diagram
Table 1. Operating Modes
VCC CE OE WE DQ0–DQ7 MODE POWER
VIH X X High-Z Deselect Standby
VIL X VIL D
IN Write Active
VIL V
IL V
IH D
OUT Read Active
VCC > VPF
VIL V
IH V
IH High-Z Read Active
VSO < VCC <VPF X X X High-Z Deselect CMOS Standby
VCC <VSO < VPF X X X High-Z Data Retention Battery Current
DATA READ MODE
The DS1556 is in the read mode whenever CE (chip enable) is low and WE (write enable) is high. The
device architecture allows ripple-through access to any valid address location. Valid data will be available
at the DQ pins within tAA after the last address input is stable, providing that CE and OE access times are
satisfied. If CE or OE access times are not met, valid data will be available at the latter of chip enable
access (tCEA) or at output enable access time (tOEA). The state of the data input/output pins (DQ) is
controlled by CE and OE. If the outputs are activated before tAA, the data lines are driven to an
intermediate state until tAA. If the address inputs are changed while CE and OE remain valid, output data
will remain valid for output data hold time (tOH) but will then go indeterminate until the next address
access.
DATA WRITE MODE
The DS1556 is in the write mode whenever WE and CE are in their active state. The start of a write is
referenced to the latter occurring transition of WE or CE. The addresses must be held valid throughout the
cycle. CE and WE must return inactive for a minimum of tWR prior to the initiation of a subsequent read
or write cycle. Data in must be valid tDS prior to the end of the write and remain valid for tDH afterward. In
Maxim
DS1556
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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a typical application, the OE signal will be high during a write cycle. However, OE can be active
provided that care is taken with the data bus to avoid bus contention. If OE is low prior to WE
transitioning low, the data bus can become active with read data defined by the address inputs. A low
transition on WE will then disable the outputs tWEZ after WE goes active.
DATA-RETENTION MODE
The 5V device is fully accessible and data can be written and read only when VCC is greater than VPF.
However, when VCC is below the power-fail point VPF (point at which write protection occurs) the
internal clock registers and SRAM are blocked from any access. When VCC falls below the battery switch
point VSO (battery supply level), device power is switched from the VCC pin to the internal backup lithium
battery. RTC operation and SRAM data are maintained from the battery until VCC is returned to nominal
levels.
The 3.3V device is fully accessible and data can be written and read only when VCC is greater than VPF.
hen VCC falls below VPF, access to the device is inhibited. If VPF is less than VSO, the device power is
switched from VCC to the internal backup lithium battery when VCC drops below VPF. If VPF is greater
than VSO, the device power is switched from VCC to the internal backup lithium battery when VCC drops
below VSO. RTC operation and SRAM data are maintained from the battery until VCC is returned to
nominal levels.
All control, data, and address signals must be powered down when VCC is powered down.
BATTERY LONGEVITY
The DS1556 has a lithium power source that is designed to provide energy for the clock activity, and
clock and RAM data retention when the VCC supply is not present. The capability of this internal power
supply is sufficient to power the DS1556 continuously for the life of the equipment in which it is
installed. For specification purposes, the life expectancy is 10 years at 25C with the internal clock
oscillator running in the absence of VCC. Each DS1556 is shipped from Maxim with its lithium energy
source disconnected, guaranteeing full energy capacity. When VCC is first applied at a level greater than
VPF, the lithium energy source is enabled for battery backup operation. Actual life expectancy of the
DS1556 will be much longer than 10 years since no internal battery energy is consumed when VCC is
present.
INTERNAL BATTERY MONITOR
The DS1556 constantly monitors the battery voltage of the internal battery. The Battery Low Flag (BLF)
bit of the Flags Register (B4 of 1FFF0h) is not writable and should always be a 0 when read. If a 1 is ever
present, an exhausted lithium energy source is indicated and both the contents of the RTC and RAM are
questionable.
POWER-ON RESET
A temperature compensated comparator circuit monitors the level of VCC. When VCC falls to the power
fail trip point, the RST signal (open drain) is pulled low. When VCC returns to nominal levels, the RST
signal continues to be pulled low for a period of 40 ms to 200 ms. The power-on reset function is
independent of the RTC oscillator and thus is operational whether or not the oscillator is enabled.
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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CLOCK OPERATIONS
Table 2 and the following paragraphs describe the operation of RTC, alarm, and watchdog functions.
Table 2. Register Map
DATA
ADDRESS B7 B
6 B
5 B
4 B
3 B
2 B
1 B
0 FUNCTION RANGE
1FFFFh 10 Year Year Year 00-99
1FFFEh X X X 10
Month Month Month 01-12
1FFFDh X X 10 Date Date Date 01-31
1FFFCh X Ft X X X Day Day 01-07
1FFFBh X X 10 Hour Hour Hour 00-23
1FFFAh X 10 Minutes Minutes Minutes 00-59
1FFF9h OSC 10 Seconds Seconds Seconds 00-59
1FFF8h W R 10 Century Century Control 00-39
1FFF7h WDS BMB4 BMB3 BMB2 BMB1 BMB0 RB1 RB0 Watchdog
1FFF6h AE Y ABE Y Y Y Y Y Interrupts
1FFF5h AM4 Y 10 Date Date Alarm Date 01-31
1FFF4h AM3 Y 10 Hours Hours Alarm Hours 00-23
1FFF3h AM2 10 Minutes Minutes Alarm Minutes 00-59
1FFF2h AM1 10 Seconds Seconds Alarm Seconds 00-59
1FFF1h Y Y Y Y Y Y Y Y Unused
1FFF0h WF AF 0 BLF 0 0 0 0 Flags
X = Unused, Read/Writable Under Write and Read Bit Control AE = Alarm Flag Enable
Y = Unused, Read/Writable Without Write and Read Bit Control ABE = Alarm in Battery-Backup Mode Enable
FT = Frequency Test Bit AM1 to AM4 = Alarm Mask Bits
OSC = Oscillator Start/Stop Bit WF = Watchdog Flag
W = Write Bit AF = Alarm Flag
R = Read Bit 0 = 0 (Read Only)
WDS = Watchdog Steering Bit BLF = Battery Low Flag
BMB0 to BMB4 = Watchdog Multiplier Bits RB0 to RB1 = Watchdog Resolution Bits
CLOCK OSCILLATOR CONTROL
The clock oscillator can be stopped at any time. To increase the shelf life of the backup lithium battery
source, the oscillator can be turned off to minimize current drain from the battery. The OSC bit is the
MSB of the Seconds Register (B7 of 1FFF9h). Setting it to a 1 stops the oscillator, setting to a 0 starts the
oscillator. The DS1556 is shipped from Maxim with the clock oscillator turned off, OSC bit set to a 1.
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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READING THE CLOCK
When reading the RTC data, it is recommended to halt updates to the external set of double-buffered RTC
Registers. This puts the external registers into a static state allowing data to be read without register
values changing during the read process. Normal updates to the internal registers continue while in this
state. External updates are halted when a 1 is written into the read bit, B6 of the Control Register
(1FFF8h). As long as a 1 remains in the Control Register read bit, updating is halted. After a halt is
issued, the registers reflect the RTC count (day, date, and time) that was current at the moment the halt
command was issued. Normal updates to the external set of registers will resume within 1 second after the
read bit is set to a 0 for a minimum of 500s. The read bit must be a zero for a minimum of 500s to
ensure the external registers will be updated.
SETTING THE CLOCK
The MSB bit, B7, of the Control Register is the write bit. Setting the write bit to a 1, like the read bit,
halts updates to the DS1556 (1FFF8h to 1FFFFh) registers. After setting the write bit to a 1, RTC
Registers can be loaded with the desired RTC count (day, date, and time) in 24-hour BCD format. Setting
the write bit to a 0 then transfers the values written to the internal RTC Registers and allows normal
operation to resume.
CLOCK ACCURACY (DIP MODULE)
The DS1556 is guaranteed to keep time accuracy to within 1 minute per month at 25C. The RTC is
calibrated at the factory by Maxim using nonvolatile tuning elements, and does not require additional
calibration. For this reason, methods of field clock calibration are not available and not necessary. The
electrical environment also affects clock accuracy, and caution should be taken to place the RTC in the
lowest-level EMI section of the PC board layout. For additional information, refer to Application Note
58.
CLOCK ACCURACY (PowerCap MODULE)
The DS1556 and DS9034PCX are each individually tested for accuracy. Once mounted together, the
module will typically keep time accuracy to within 1.53 minutes per month (35 ppm) at 25°C. The
electrical environment also affects clock accuracy, and caution should be taken to place the RTC in the
lowest-level EMI section of the PC board layout. For additional information, refer to
Application Note 58.
FREQUENCY TEST MODE
The DS1556 frequency test mode uses the open drain IRQ/FT output. With the oscillator running, the
IRQ/FT output will toggle at 512 Hz when the FT bit is a 1, the Alarm Flag Enable bit (AE) is a 0, and
the Watchdog Steering bit (WDS) is a 1 or the Watchdog Register is reset (Register 1FFF7h = 00h). The
IRQ/FT output and the frequency test mode can be used as a measure of the actual frequency of the
32.768 kHz RTC oscillator. The IRQ/FT pin is an open-drain output that requires a pullup resistor for
proper operation. The FT bit is cleared to a 0 on power-up.
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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USING THE CLOCK ALARM
The alarm settings and control for the DS1556 reside within Registers 1FFF2h to 1FFF5h. Register
1FFF6h contains two alarm enable bits: Alarm Enable (AE) and Alarm in Backup Enable (ABE). The AE
and ABE bits must be set as described below for the IRQ/FT output to be activated for a matched alarm
condition.
The alarm can be programmed to activate on a specific day of the month or repeat every day, hour,
minute, or second. It can also be programmed to go off while the DS1556 is in the battery-backed state of
operation to serve as a system wake-up. Alarm mask bits AM1 to AM4 control the alarm mode. Table 3
shows the possible settings. Configurations not listed in the table default to the once per second mode to
notify the user of an incorrect alarm setting.
Table 3. Alarm Mask Bits
AM4 AM3 AM2 AM1 ALARM RATE
1 1 1 1 Once per second
1 1 1 0 When seconds match
1 1 0 0 When minutes and seconds match
1 0 0 0 When hours, minutes, and seconds match
0 0 0 0 When date, hours, minutes, and seconds match
When the RTC Register values match Alarm Register settings, the Alarm Flag bit (AF) is set to a 1. If
Alarm Flag Enable (AE) is also set to a 1, the alarm condition activates the IRQ/FT pin. The IRQ/FT
signal is cleared by a read or write to the Flags Register (Address 1FFF0h) as shown in Figure 2 and 3.
When CE is active, the IRQ/FT signal may be cleared by having the address stable for as short as 15 ns
and either OE or WE active, but is not guaranteed to be cleared unless tRC is fulfilled. The alarm flag is
also cleared by a read or write to the Flags Register but the flag will not change states until the end of the
read/write cycle and the IRQ/FT signal has been cleared.
Figure 2. Clearing IRQ Waveforms
C
E, 0V
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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Figure 3. Clearing IRQ Waveforms
The IRQ/FT pin can also be activated in the battery-backed mode. The IRQ/FT will go low if an alarm
occurs and both ABE and AE are set. The ABE and AE bits are cleared during the power-up transition,
however an alarm generated during power-up will set AF. Therefore, the AF bit can be read after system
power-up to determine if an alarm was generated during the power-up sequence. Figure 4 illustrates alarm
timing during the battery-backup mode and power-up states.
Figure 4. Backup Mode Alarm Waveforms
C
E=0
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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USING THE WATCHDOG TIMER
The watchdog timer can be used to detect an out-of-control processor. The user programs the watchdog
timer by setting the desired amount of time-out into the 8-bit Watchdog Register (Address 1FFF7h). The
five Watchdog Register bits BMB4 to BMB0 store a binary multiplier and the two lower order bits RB1
to RB0 select the resolution, where 00=1/16 second, 01=1/4 second, 10=1 second, and 11=4 seconds. The
watchdog timeout value is then determined by the multiplication of the 5-bit multiplier value with the
2-bit resolution value. (For example: writing 00001110 in the Watchdog Register = 3 x 1 second or
3 seconds.) If the processor does not reset the timer within the specified period, the Watchdog Flag (WF)
is set and a processor interrupt is generated and stays active until either the Watchdog Flag (WF) is read
or the Watchdog Register (1FFF7h) is read or written.
The most significant bit of the Watchdog Register is the Wa tchdog Steering Bit (WDS). When set to a 0,
the watchdog will activate the IRQ/FT output when the watchdog times out.
When WDS is set to a 1, the watchdog will output a negative pulse on the RST output for a duration of
40ms to 200ms. The Watchdog Register (1FFF7h) and the FT bit will reset to a 0 at the end of a
watchdog timeout when the WDS bit is set to a 1.
The watchdog timer resets when the processor performs a read or write of the Watchdog Register. The
time-out period then starts over. The watchdog timer is disabled by writing a value of 00h to the
Watchdog Register. The watchdog function is automatically disabled upon power-up and the Watchdog
Register is cleared. If the watchdog function is set to output to the IRQ/FT output and the frequency test
function is activated, the watchdog function prevails and the frequency test function is denied.
POWER-ON DEFAULT STATES
Upon application of power to the device, the following register bits are set to a 0:
WDS = 0, BMB0 to BMB4 = 0, RB0 to RB1 = 0, AE = 0, ABE = 0.
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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ABSOLUTE MAXIMUM RATINGS
Voltage Range on Any Pin Relative to Ground……………………..……………………………………………..-0.3V to +6.0V
Storage Temperature Range
EDIP........................………………………………………………………………...................................-40°C to +85°C
PowerCap..............………………………………………………….…………………………………..-55°C to +125°C
Lead Temperature (soldering, 10s).………..........................................................................................................................+260°C
Note: EDIP is han d or wave-soldered only.
Soldering Temperature (reflow, PowerCap)………………………………….....................................................................+260°C
This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation
sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect
reliability.
OPERATING RANGE
RANGE TEMP RANGE VCC
Commercial 0°C to +70°C 3.3V 10% or 5V 10%
Industrial -40°C to +85°C 3.3V 10% or 5V 10%
RECOMMENDED DC OPERATING CONDITIONS
(Over the Operating Range)
PARAMETER SYMBOL
CONDITIONS MIN TYP MAX UNITS
VIH VCC = 5V 10% 2.2 VCC +
0.3V V
Logic 1 Voltage All Inputs
(Note 1) VIH VCC = 3.3V 10% 2.0 VCC +
0.3V V
VIL VCC = 5V 10% -0.3 +0.8
Logic 0 Voltage All Inputs
(Note 1) VIL VCC = 3.3V 10% -0.3 +0.6
DC ELECTRICAL CHARACTERISTICS
(VCC = 5.0V 10%, Over the Operating Range.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Active Supply Current ICC (Notes 2, 3, 11) 30 85 mA
TTL Standby Current
(CE = VIH) ICC1 (Notes 2, 3) 4 6 mA
CMOS Standby Current
(CE VCC - 0.2V) ICC2 (Notes 2, 3) 2 6 mA
Input Leakage Current
(Any Input) IIL -1 +1 A
Output Leakage Current
(Any Output) IOL -1 +1 A
Output Logic 1 Voltage
(IOUT = -1.0 mA) VOH (Note 1) 2.4 V
VOL1 IOUT = 2.1 mA, DQ0–7
Outputs (Note 1) 0.4 V
Output Logic 0 Voltage VOL2 IOUT = 7.0 mA, IRQ/FT,
and RST Outputs
(Notes 1, 5) 0.4 V
Write Protection Voltage VPF (Note 1) 4.20 4.50 V
Battery Switchover Voltage VSO (Notes 1,4) VBAT V
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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DC ELECTRICAL CHARACTERISTICS
(VCC = 3.3V 10%, Over the Operating Range.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Active Supply Current ICC (Notes 2, 3, 11) 20 30 mA
TTL Standby Current
(CE = VIH) ICC1 (Notes 2, 3) 2 6 mA
CMOS Standby Current
(CE VCC - 0.2V) ICC2 (Notes 2, 3) 1 4 mA
Input Leakage Current
(Any Input) IIL -1 +1 A
Output Leakage Current
(Any Output) IOL -1 +1 A
Output Logic 1 Voltage
(IOUT = -1.0 mA) VOH (Note 1) 2.4 V
VOL1 IOUT = 2.1 mA, DQ0–7
Outputs (Note 1) 0.4 V
Output Logic 0 Voltage VOL2 IOUT = 7.0 mA, IRQ/FT
and RST Outputs
(Notes 1, 5) 0.4 V
Write Protection Voltage VPF (Note 1) 2.75 2.97 V
Battery Switchover Voltage VSO (Notes 1,4) VBAT
or VPF V
Figure 5. Read Cycle Timing Diagram
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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AC CHARACTERISTICS—READ CYCLE
(Over the Operating Range)
VCC = 5.0V 10% VCC = 3.3V 10%
PARAMETER SYMBOL MIN MAX MIN MAX
UNITS
Read Cycle Time tRC 70 120 ns
Address Access Time tAA 70 120 ns
CE to DQ Low-Z tCEL 5 5 ns
CE Access Time tCEA 70 120 ns
CE Data Off Time tCEZ 25 40 ns
OE to DQ Low-Z tOEL 5 5 ns
OE Access Time tOEA 35 100 ns
OE Data Off Time tOEZ 25 35 ns
Output Hold from Address tOH 5 5 ns
AC CHARACTERISTICS—WRITE CYCLE
(Over the Operating Range)
VCC = 5.0V 10% VCC = 3.3V 10%
PARAMETER SYMBOL
MIN MAX MIN MAX
UNITS
Write Cycle Time tWC 70 120 ns
Address Access Time tAS 0 0 ns
WE Pulse Width tWEW 50 100 ns
CE Pulse Width tCEW 60 110 ns
Data Setup Time tDS 30 80 ns
Data Hold Time (Note 9) tDH1 5 5 ns
Data Hold Time (Note 10) tDH2 5 5 ns
Address Hold Time (Note 9) tAH1 5 0 ns
Address Hold Time
(Note 10) tAH2 5 5 ns
WE Data Off Time tWEZ 25 40 ns
Write Recovery Time tWR 5 10 ns
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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Figure 6. Write Cycle Timing, Write-Enable Controlled
Figure 7. Write Cycle Timing, Chip-Enable Controlled
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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POWER-UP/DOWN CHARACTERISTICS—5V
(VCC = 5.0V 10%, Over the Operating Range.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
CE or WE at VIH, Before
Power-Down tPD 0 s
VCC Fall Time: VPF(MAX) to VPF(MIN) tF 300 s
VCC Fall Time: VPF(MIN) to VSO t
FB 10 s
VCC Rise Time: VPF(MIN) to VPF(MAX) t
R 0 s
VPF to RST High tREC 40 200 ms
Expected Data-Retention Time
(Oscillator On) tDR (Notes 6, 7) 10 years
Figure 8. Power-Up/Down Waveform Timing (5V Device)
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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POWER-UP/DOWN CHARACTERISTICS—3.3V
(VCC = 3.3V 10%, Over the Operating Range.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
CE or WE at VIH, Before
Power-Down tPD 0 s
VCC Fall Time: VPF(MAX) to VPF(MIN) tF 300 s
VCC Rise Time: VPF(MIN) to VPF(MAX) t
R 0 s
VPF to RST High tREC 40 200 ms
Expected Data-Retention Time
(Oscillator On) tDR (Notes 6, 7) 10 years
Figure 9. Power-Up/Down Waveform Timing (3.3V Device)
CAPACITANCE
(TA = +25°C)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Capacitance on All Input Pins CIN (Note 1) 14 pF
Capacitance on IRQ/FT, RST,
and DQ Pins CIO (Note 1) 10 pF
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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AC TEST CONDITIONS
Output Load: 50 pF + 1TTL Gate
Input Pulse Levels: 0.0 to 3.0V
Timing Measurement Reference Levels:
Input: 1.5V
Output: 1.5V
Input Pulse Rise and Fall Times: 5 ns
NOTES:
1. Voltage referenced to ground.
2. Typical values are at +25C and nominal supplies.
3. Outputs are open.
4. Battery switchover occurs at the lower of either the battery voltage or VPF.
5. The IRQ/FT and RST outputs are open drain.
6. Data-retention time is at +25C.
7. Each DS1556 has a built-in switch that disconnects the lithium source until VCC is first applied by the
user. The expected tDR is defined for DIP modules and PowerCap modules as a cumulative time in the
absence of VCC starting from the time power is first applied by the user.
8. RTC modules (DIP) can be successfully processed through conventional wave-soldering techniques
as long as temperature exposure to the lithium energy source contained within does not exceed
+85C. Post-solder cleaning with water-washing techniques is acceptable, provided that ultrasonic
vibration is not used.
In addition, for the PowerCap:
a. Maxim recommends that PowerCap Module bases experience one pass through solder reflow
oriented with the label side up (“live-bug”).
b. Hand soldering and touch-up: Do not touch or apply the soldering iron to leads for more than
3 seconds. To solder, apply flux to the pad, heat the lead frame pad and apply solder. To remove
the part, apply flux, heat the lead frame pad until the solder reflow and use a solder wick to
remove solder.
9. tAH1, tDH1 are measured from WE going high.
10. tAH2, tDH2 are measured from CE going high.
11. tWC = 200ns.
PACKAGE INFORMATION
For the latest package ou tline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-”
in the package code indicates RoHS status only. Pack age drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO.
32 EDIP MDF32+1 21-0245 —
34 PWRCP PC2+6 21-0246 —
DS1556 1M, Nonvolatile, Y2K-Compliant Timeke eping RAM
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Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses
are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
2010 Maxim Integrated Products Maxim and the Dallas logo are registered trademarks of Maxim Integrated Products.
REVISION HISTORY
REVISION
DATE DESCRIPTION PAGES
CHANGED
9/10
Updated the Ordering Information table to include only lead-free parts; updated the Absolute
Maximum Ratings section to include the storage temperature range and lead and soldering
temperatures for EDIP and PowerCap packages; added Note 11 to the ICC parameter in the DC
Electrical Characteristics tables (for 5.0V and 3.3V) and the Notes section; replaced the
package outline drawings with the Package Information table
2, 11, 12, 17
