Downloading programs on a 3.3V powered Picaxe

[fernando_g]

Simple question;
I have a Picaxe project which runs on 3.3V, but requires software updates. I originally programmed the Picaxe in a test socket running at 5V.

I've always used the AXE027 cable, which in my understanding will output 5V TTL levels.

Will the Picaxe's serial input be damaged if I attempt to re-flash it?
And before anyone asks, the Picaxe is soldered to the board.

 

[fernando_g]

Edit: I believe that I will have to "deadbug" the 22k series resistor, which I originally did not include.

 

[AllyCat]

Hi,

Yes, a 22k resistor in series with the programming input (in association with the on-chip "electrostatic protection" diode to the Vdd rail) should keep the PICaxe safe. I hope you haven't hard-wired the serial input pin directly to ground? My advice is always to allow for a resistor from this input to ground - a value of around 100k will guarantee that the PICaxe boots correctly, but you can connect an external programming circuit (i.e. a series 22k resistor and whatever "RS232" adapter you have) without the normal "potential divider" issue (i.e. the 10k being on the "wrong" side of the 22k).

The PICaxe should program fine at 3.3. volts, and potentially at lower Vdds. The only issue I've ever encountered was an incompatibility of the programming interface logic levels (e.g. the serial output from the PICaxe) at very low supply voltages.

Cheers, Alan.

 

[fernando_g]

Thanks Alan, for the advice.
A trick I have learned the hard way is to never hard-wire any unused PICaxe pins.

Nowadays I always leave 0805 placeholders on the PWB, which more than once has allowed me to expand a project.
Fortunately this was the case here!