Greg London wrote: >> ...the designer is using AC coupling. Perhaps that's obvious. When DTR >> goes low, it'll briefly pull down the micro's reset pin. > > The AC coupling I understand.
OK, good. Just to flesh out the rest of the thought to anyone for whom the usage isn't obvious, the designer presumably used AC coupling there because they anticipate DTR staying low for some extended period, yet they want the micro to boot immediately after it is toggled. If the signal was DC coupled (directly wired) the reset pin would be held low causing the micro to be stuck in the reset state. I'm guessing this was done so you can download a program and and exit the downloader without having to care what state DTR remains in. Pulling the USB plug might also cause DTR to remain low perpetually, so in a "program and unplug" scenario, you'd avoid having the micro stuck in reset. > If I use the DTR or RTS to reset the arduino, > that means that DTR should only go low rarely? That's the impression I got. > If its used by the programmer to reset the arduino after > its programmed the chip, I assume DTR doesn't toggle > too often. It was implied that the Sketch downloader tool intentionally toggles DTR upon completion of the download, so I would presume it is under programmer control. > 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. > > I assume...[the FTDI chip]...doesn't toggle [DTR] just because some > data parity was bad and it wants a resend or something. Supposedly the host-side USB driver plus the FTDI chip combine to behave as if the host has a built-in UART and provides the application with a serial port compatible API. I'm sure there are limitations in that emulation, but to the extent that it works, I'd expect DTR to behave just as it would with a real serial port. > 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. > > Reading the wikipedia entry on DTR... > it sounds like its the way to tell the modem to hang up... I don't remember how you'd force a DTR toggle with a typical dumb terminal. It may just happen coincident with opening/closing the port (unless you delve deeper and use ioctls or the like). > I have read some people disconnectiong the USB FTDI DTR/RTS > pin from the atmega reset... Did they say why? Given lots of people are using these FTDI chips with the DTR attached this way, there should be plenty of reports of flaky behavior, if that's what is happening. -Tom _______________________________________________ Hardwarehacking mailing list [email protected] http://lists.blu.org/mailman/listinfo/hardwarehacking
