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Another question along with this one.

The guide I've been following says that after you reset the 8u2 by connecting the two pins shown below, the board should be undetectable by the Arduino IDE. I have successfully done so, but the board is still using COM22 as an Arduino Mega in the device manager. This other guide I found shows the board becoming a LibUSB-Win32 device. Anyone know what's going on?

Pins to connect

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Did you solder the 10k resistor in? (mentioned in the previous step in the first link) – Oli Glaser Dec 2 '11 at 1:07
The Mega has it built in. On Rev2 or later boards: there is a resistor that pulling the 8U2/16U2 HWB line to ground, making it easier to put into DFU mode. – Jeffrey Dec 2 '11 at 1:14
Ah, I saw it mention a certain revision didn't need it but I wasn't sure which one you had. – Oli Glaser Dec 2 '11 at 2:17
So you have your Mega plugged in, you short circuit those two pins for a second or so and let it go. At that stage, if everything is alright your Windows should start searching/installing drivers and new device under your Computer should appear (where Camera and other stuff appears - at least on Win 7) as ATMEL DFU or something similarly titled. So are you doing all of the mentioned in that order? Btw, doublecheck if you have that resistor soldered in if you didn't purchase the board directly from arduino.cc – Mihailo Dec 2 '11 at 10:36
@Jeffrey - The above image is a Rev2 Uno, not a Mega. The image of the Mega is at arduino.cc/en/uploads/Main/ArduinoMega2560_R3_Front_450px.jpg. – Kevin Vermeer Dec 2 '11 at 13:53
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2 Answers

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I have confirmed that, on my UNO R2, if I bridge the two connections shown briefly, the 8U2 resets and enters DFU mode. Instantly, the logs on my Linux system that it is connected to show that the ACM device (what would be COM22 for you) has been removed, and replaced with a generic USB device. The dfu-programmer command can then communicate with it fine.

The bridging of the connection should be done:

  1. while the Arduino is connected to the computer and powered up,
  2. briefly, with a bit of wire, resistor leg, screwdriver, tweezers, etc - whatever you have to hand.

Removing power from the Arduino and re-connecting it returns it back to normal operation.

  • Incidentally, the UNO R2 has that block of 6 pins populated with a header - makes it easier...
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Apparently, my mega did not have the 10k resistor built in for whatever reason. Adding the resistor on let it enter DFU mode without a hitch.

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