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

[Greg London]

The AC coupling I understand.

If I use the DTR or RTS to reset the arduino,
that means that DTR should only go low rarely?

I don't know exactly how USB gets faked out into fakey rs232,
so I'm not sure how much  DTR or RTS toggles as part of USB
traffic.

If its used by the programmer to reset the arduino after
its programmed the chip, I assume DTR doesn't toggle
too often.

I've used my PC as a dumb terminal to an Arduino, and
I assume that DTR doesn't toggle much while that connection
is active either.

I wouldn't want my ardiuno to be talking to a terminal
or trying to download a sketch and have DTS pulse low
and throw a reset on me when I didn't want it.

Reading the wikipedia entry on DTR on serial printers
it sounds like its the way to tell the modem to hang up,
so I assume it doesn't toggle too much.

I assume whatever handwavium the USB to rs232 converter
does respects that and allows control of DTR and doesn't
toggle it just because some data parity was bad and it
wants a resend or something.

Also, slogging through the spec that Drew sent for the atmega,
it looks like the reset pin only gets 12v during
parallel programming. and the ICSP is a serial programming
interface, so I'm guessing I don't have to worry about
isolating 12v anyway.

So, as far as voltage isolation is concerned, I'll just put
in a diode on the voltage regulator output, the FTDI 5v pin,
and the ICSP 5v pin, and then nobody gets fried if I hook up
more than one power at a time.

And I'll just keep the cap on the DTR/RTS pin to reset, and assume
it wont toggle much, and assume there isn't any 12v on
reset during serial programming, and not worry about isolating
anything there.

I have read some people disconnectiong the USB FTDI DTR/RTS
pin from the atmega reset, so maybe I'll put a jumper in my design
so I can disconnect it easily if need be.

Greg


> 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
>
>
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