Resetting Prop via FT232R

cmd

I'm not quite understanding the reset circuit in the Propeller Proto Board schematic:

http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/32212-32812-PropellerProtoBoard-v1.3.pdf

Is DTR normally high and the upload software pulses it low to reset the Propeller?
Or is it the other way around?

I have a spare open-drain inverting buffer on my PCB and I'd like to use that to reset the Propeller instead of adding the transistor and associated components to the board layout, but I'm not clear about the polarity of DTR. Could someone clarify?

Dr_Acula

Normally high or normally low depends on where you measure it as the signal gets inverted (twice in some circuits).

On the propeller chip reset is active low. If reset is high or open circuit then the propeller runs (I think there is a weak pullup resistor inside the propeller chip). See the schematic - there is a little pushbutton that pulls the reset low. You can put a transistor in parallel with that or one of your buffers and do the same thing.

I tend to use max3232 chips on a real serial port rather than FT232 chips on a USB port. On a real RS232 port, resting is minus 12V and active is plus 12V. So to send a reset you make the DTR line go -12V to +12V to -12V. The max3232 inverts this so the signal then goes 3V to 0V to 3V. This goes through a high pass filter made up of a capacitor and a resistor, and the transition from 0V to 3V briefly turns on the base of a transistor and pulls the propeller reset line low. With a propplug all of that is internal so it goes from a USB signal to a reset pulse. I like to have a D9 plug on the side of my propeller boards because you can then use that as a comms port to other things once it has been programmed (eg GPS or radio modules or other propeller boards - anything that uses RS232). But I think most people tend to use the propplug.

So if you can briefly pull the reset line of the propeller low that should work.

Rayman

I believe DTR is active low. But, you can reprogram the FT232 to make it active high.
I think it would work either way though because the pulse signal goes through a capacitor on it's way to the transistor base.
So, the low to high transition is what reset's the Prop.