Reset Circuit on the PropPlug - Why do we need it???

Cluso99

Firstly, let me explain why the reset circuit is used...

The DTR pin is used to reset the prop. DTR is a negative active at TTL level (i.e. 0V = DTR ON, 3V3 = DTR OFF). When the DTR pin is steady state (either on or off), the transistor circuit is turned off, and no reset takes place. When the DTR pin is turned off, the DTR (TTL) pin goes high and this charges the series capacitor. The transistor turns on until the cap is discharged by the ground 10K resistor, creating a ground pulse on the Reset pin of the prop.

It seems to me that an inverter from the DTR output pin of the FT232R chip would serve just as well, although some timing change may be necessary in the software, since currently the reset pulse is being timed from the start of DTR going OFF, rather than timed from DTR going on. Anyone done any timing here???

Now, how could we avoid this circuit totally???

If we changed the software, such as the PropTool (as an option), or bst (as an option), or Propellant or indeed any other download software, we could indeed avoid this additional hardware (transistor circuit or an inverter). How???

Well, if we the software download code had an option to invert the use of the DTR pin, this should work (maybe a timing change required, but it should work from what I can see). So, the software would pulse/set DTR ON, then a small delay, and set DTR OFF. The prop would be reset when DTR was ON. Once DTR goes OFF the propeller would startup. So, the download software would be required to keep DTR OFF. This means that the software would be required to remain active and DTR held OFF.

However, this method would allow lots of other standard plugs to work. It would happily program the prop's eeprom simply. Then the user could remove a reset (DTR) link and continue working.

This is certainly not an elegant solution, but it gives a programming option without having to build the reset circuit.

Perhaps an alternative would be to just have a simple download program that would achieve this from an already compiled program.

Any thoughts or comments???

Ariba

You can program some settings of the FT232R with the FT_Prog tool. One of these settings is the polarity of the DTR output.
So no inverter needed. Just a 10nF capacitor between /DTR out and RESn input (BOE must be Low).
I use such a circuit since years to program my Propellers.

Andy

mpark

What does the capacitor do?

Mike Green

The capacitor differentiates the /DTR signal so you get a short negative going and positive going pulse with fairly sharp edges when /DTR goes low, then when it goes high again. The idea is that /DTR goes low when the comm port is opened, then goes high when the comm port is closed. That's a long time and you don't want the Prop held in reset all that time. This way, the Prop is quickly reset, the Prop Tool downloads the program, and the fact that the Prop may be reset again after downloading when the Prop Tool closes the port doesn't really matter.

Ariba

For downloading code with the PropTool I think you don't need the capacitor. But for communicating with a Terminal or other PC Application which control the DTR line on Comm open and close the C helps. With a C you "only" get a Reset on comm close. As Mike said the C differentiate the signal, but only the negative edge of DTR causes a reset.

Andy

Cluso99

Mike, Is there a reason the prop cannot or should not be held in reset???

My objective is to remove the requirement for the extra components.

Another option could be to use one of the CBUSn pins? (postedit: RI is an input to FT chip)

Andy: Perhaps I should look at the FT-Prog tool to see what could be done.

Postedit:
OK looked at the FT-Prog tool and the lines can be inverted, so we could use this and give a short pulse to the DTR line. I didn't find anything about configuring the CBUSn pins.