A000073 Arduino, A000073 Datasheet - Page 4

• Manufacturer Part Number: A000073
• Description: Development Boards & Kits - AVR ARDUINO UNO SMD REV 3
• Manufacturer: Arduino

Specifications of A000073

• Rohs: yes
• Product: Development Kits
• Tool Is For Evaluation Of: ATmega328
• Core: AVR8
• Interface Type: I2C, ICSP, SPI, USB
• Data Bus Width: 8 bit
• For Use With: ATmega

The ATmega328 on the Arduino Uno comes preburned with a
upload new code to it without the use of an external hardware programmer. It
communicates using the original STK500 protocol (reference,
You can also bypass the bootloader and program the microcontroller through the ICSP (In-
Circuit Serial Programming) header; see
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
software
Or you can use the ISP header with an external programmer (overwriting the DFU
bootloader). See
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
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.
this forum thread
(Windows) or the
this user-contributed tutorial
DFU programmer
for details.
these instructions
(Mac OS X and Linux) to load a new firmware.
for more information.
for details.
bootloader
C header
files).
that allows you to
Atmel's FLIP