Autoreset Circuit on Uno

RTS and DTR are a little more bizarre than I thought. I have both an FTDI cable [uses RTS] and
an FTDI Friend [uses DTR]. Don't have the scope handy, but using the DMM, where S-M means Serial
Monitor,

FTDI Cable: RTS=0V [steady] when S-M=off, RTS=5V [steady], when S-M=on.

FTDI Friend: DTR=5V [steady] when S-M=off, DTR=0V [steady], when S-M=on.

So, don't leave out the 100nF cap whatever you do.

Quote from: doov on Today at 05:29:49 PM
I'd like to use that to enable a bluetooth modem which sits on the uart. If it's enabled during programming then it screws with avrdude (presumably because it's driving the tx line at the same time that avrdude is trying to drive the same line.). Thanks!

You should probably wire it so it can be switched on/off via the Arduino and power it on in your setup() function.

Yeah, you cannot really have BT or XBee connected to the Arduino RX line at the same time as the standard
USB port, because the 2 signals conflict, so some XBee shields at least have a switch.