Programming; Automatic (Software) Reset - Arduino Mega ADK User Manual

Programming

The ATmega2560 on the Arduino MEGA ADK comes pre-burned with a bootloader (the same
on Mega 2560) that allows you to upload new code to it without the use of an external
hardware programmer. It communicates using the original STK500v2 protocol (reference, C
header files).
You can also bypass the boot loader and program the microcontroller through the ICSP (In-
Circuit Serial Programming) header using Arduino ISP or similar; see these instructions for details.
The ATmega8U2 firmware source code is available in the Arduino repository.
The ATmega8U2 is loaded with a DFU boot loader, which can be activated by:
On Rev1 boards: connecting the solder jumper on the back of the board (near the map of
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Italy) and then resetting the 8U2.
On Rev2 or later boards: there is a resistor that pulling the 8U2/16U2 HWB line to ground,
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making it easier to put into DFU mode. 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 boot loader).

Automatic (Software) Reset

Rather then requiring a physical press of the reset button before an upload, the Arduino
MEGA ADK 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 ATmega2560 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 MEGA ADK 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 MEGA ADK.
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 MEGA ADK 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.
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