Arduino Uno SMD
Overview
The Arduino Uno SMD is a version of the Arduino Uno, but uses an surface mount version of the Atmega328P instead of
the through-hole version. This version was made in response to a shortage in supply of the through-hole Atmega328P.
The board is based on the ATmega328 (datasheet). It has 14 digital input/output pins (of which 6 can be used as PWM
outputs), 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset
button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable
or power it with a AC-to-DC adapter or battery to get started.
The Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features
the Atmega8U2 programmed as a USB-to-serial converter.
"Uno" means one in Italian and is named to mark the upcoming release of Arduino 1.0. The Uno and version 1.0 will be
the reference versions of Arduino, moving forward. The Uno is the latest in a series of USB Arduino boards, and the
reference model for the Arduino platform; for a comparison with previous versions, see the index of Arduino boards.
Summary
Microcontroller ATmega328
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
Digital I/O Pins 14 (of which 6 provide PWM output)
Analog Input Pins 6
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 32 KB (ATmega328) of which 0.5 KB used by bootloader
SRAM 2 KB (ATmega328)
EEPROM 1 KB (ATmega328)
Clock Speed 16 MHz
Schematic & Reference Design
EAGLE files: arduino-uno-smd-reference-design.zip
Schematic: arduino-uno-smd-schematic.pdf
Power
The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected
automatically.
External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be
connected by plugging a 2.1mm center-positive plug into the board's power jack. Leads from a battery can be inserted in
the Gnd and Vin pin headers of the POWER connector.
The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may
supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat
and damage the board. The recommended range is 7 to 12 volts.
The power pins are as follows:
VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from
the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying
voltage via the power jack, access it through this pin.
5V. The regulated power supply used to power the microcontroller and other components on the board. This can
come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply.
3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.
GND. Ground pins.
Memory
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The ATmega328 has 32 KB (with 0.5 KB used for the bootloader). It also has 2 KB of SRAM and 1 KB of EEPROM (which
can be read and written with the EEPROM library).
Input and Output
Each of the 14 digital pins on the Uno can be used as an input or output, using pinMode(), digitalWrite(), and
digitalRead() functions. They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal
pull-up resistor (disconnected by default) of 20-50 kOhms. In addition, some pins have specialized functions:
Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are connected to the
corresponding pins of the ATmega8U2 USB-to-TTL Serial chip.
External Interrupts: 2 and 3. These pins can be configured to trigger an interrupt on a low value, a rising or falling
edge, or a change in value. See the attachInterrupt() function for details.
PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output with the analogWrite() function.
SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication using the SPI library.
LED: 13. There is a built-in LED connected to digital pin 13. When the pin is HIGH value, the LED is on, when the pin is
LOW, it's off.
The Uno has 6 analog inputs, labeled A0 through A5, each of which provide 10 bits of resolution (i.e. 1024 different
values). By default they measure from ground to 5 volts, though is it possible to change the upper end of their range
using the AREF pin and the analogReference() function. Additionally, some pins have specialized functionality:
I2C: A4 (SDA) and A5 (SCL). Support I2C (TWI) communication using the Wire library.
There are a couple of other pins on the board:
AREF. Reference voltage (0 to 5V only) for the analog inputs. Used with analogReference().
Reset. Bring this line LOW to reset the microcontroller. Typically used to add a reset button to shields which block
the one on the board.
See also the mapping between Arduino pins and ATmega328 ports.
Communication
The Arduino Uno has a number of facilities for communicating with a computer, another Arduino, or other
microcontrollers. The ATmega328 provides UART TTL (5V) serial communication, which is available on digital pins 0 (RX)
and 1 (TX). An ATmega8U2 on the board channels this serial communication over USB and appears as a virtual com port
to software on the computer. The '8U2 firmware uses the standard USB COM drivers, and no external driver is needed.
However, on Windows, a .inf file is required. The Arduino software includes a serial monitor which allows simple textual
data to be sent to and from the Arduino board. The RX and TX LEDs on the board will flash when data is being
transmitted via the USB-to-serial chip and USB connection to the computer (but not for serial communication on pins 0
and 1).
A SoftwareSerial library allows for serial communication on any of the Uno's digital pins.
The ATmega328 also supports I2C (TWI) and SPI communication. The Arduino software includes a Wire library to simplify
use of the I2C bus; see the documentation for details. For SPI communication, use the SPI library.
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Programming
The Arduino Uno can be programmed with the Arduino software (download). Select "Arduino Uno from the Tools >
Board menu (according to the microcontroller on your board). For details, see the reference and tutorials.
The ATmega328 on the Arduino Uno comes preburned with a bootloader that allows you to upload new code to it
without the use of an external hardware programmer. It communicates using the original STK500 protocol (reference, C
header files).
You can also bypass the bootloader and program the microcontroller through the ICSP (In-Circuit Serial Programming)
header; see these instructions for details.
The ATmega8U2 firmware source code is available . The ATmega8U2 is loaded with a DFU bootloader. The Uno SMD has
a pulldown resistor tying the HWB pin to ground, so all that's needed to enter DFU mode is to briefly short enough to
short pins 5 and 6 of the 8U2 icsp connector. This will connect the 8U2 reset pin to ground. You can then use Atmel's
FLIP software (Windows) or the DFU programmer (Mac OS X and Linux) to load a new firmware. Or you can use the ISP
header with an external programmer (overwriting the DFU bootloader). See this user-contributed tutorial for more
information.
Automatic (Software) Reset
Rather than requiring a physical press of the reset button before an upload, the Arduino Uno is designed in a way that
allows it to be reset by software running on a connected computer. One of the hardware flow control lines (DTR) of the
ATmega8U2 is connected to the reset line of the ATmega328 via a 100 nanofarad capacitor. When this line is asserted
(taken low), the reset line drops long enough to reset the chip. The Arduino software uses this capability to allow you to
upload code by simply pressing the upload button in the Arduino environment. This means that the bootloader can
have a shorter timeout, as the lowering of DTR can be well-coordinated with the start of the upload.
This setup has other implications. When the Uno is connected to either a computer running Mac OS X or Linux, it resets
each time a connection is made to it from software (via USB). For the following half-second or so, the bootloader is
running on the Uno. While it is programmed to ignore malformed data (i.e. anything besides an upload of new code), it
will intercept the first few bytes of data sent to the board after a connection is opened. If a sketch running on the board
receives one-time configuration or other data when it first starts, make sure that the software with which it
communicates waits a second after opening the connection and before sending this data.
The Uno contains a trace that can be cut to disable the auto-reset. The pads on either side of the trace can be soldered
together to re-enable it. It's labeled "RESET-EN". You may also be able to disable the auto-reset by connecting a 110
ohm resistor from 5V to the reset line; see this forum thread for details.
USB Overcurrent Protection
The Arduino Uno has a resettable polyfuse that protects your computer's USB ports from shorts and overcurrent.
Although most computers provide their own internal protection, the fuse provides an extra layer of protection. If more
than 500 mA is applied to the USB port, the fuse will automatically break the connection until the short or overload is
removed.
Physical Characteristics
The maximum length and width of the Uno PCB are 2.7 and 2.1 inches respectively, with the USB connector and power
jack extending beyond the former dimension. Four screw holes allow the board to be attached to a surface or case. Note
that the distance between digital pins 7 and 8 is 160 mil (0.16"), not an even multiple of the 100 mil spacing of the other
pins.
