[HH] arduino help: write protect, USB FTDI, ICSP, all connect to reset

[Tom Metro]

Greg London wrote:
> ...I don't understand how the DTR/RST from the USB FTDI works when
> its connected to reset via a cap.

I'll start with the disclaimer that I haven't done any Arduino hacking.

The generic answer to your question is that the designer is using AC
coupling. Perhaps that's obvious. When DTR goes low, it'll briefly pull
down the micro's reset pin.

Here's a bit of Arduino specific info that might shed some light on why
this arrangement exists:

http://www.sparkfun.com/products/8772

  The major difference with this board is that it brings out the DTR pin
  as opposed to the RTS pin of the FTDI cable. The DTR pin allows an
  Arduino target to auto-reset when a new Sketch is downloaded. This is
  a really nice feature to have and allows a sketch to be downloaded
  without having to hit the reset button. This board will auto reset any
  Arduino board that has the reset pin brought out to a 6-pin connector.

This also explains who is initiating and who is receiving the reset signal.

It seems like there is a typo in one of the documents being referenced
with regards to RTS vs. RST. It seems likely that the schematic you
referenced had the typo, and should have labeled that pin RTS (Request
to Send), as it is sometimes used for the same function[1] as the Data
Terminal Ready (DTR) pin.

This page (about hacking a cheap FTDI to be Arduino compatible):
http://rajeshkovvuri.wordpress.com/2011/11/02/ftdi-type-usb-uart-adapter-for-3-2/

mentions that the FTDI chip also has a RST pin, but says:

  The trace to RST needs to be cut(this RST is related to usb) and
  re-wired to GND

1.
http://en.wikipedia.org/wiki/Data_Terminal_Ready#As_used_on_serial_printers

 -Tom