Programming a PIC in a 3V system.

Which PIC?

Are you sure? Many support LVP (low voltage programming), especially the ones that operate at low voltages.

Could you post your schematic and be more specific as to what you have done so far?

Probably, but you might also wish to check out

If you cannot use a low voltage programming PIC then you need to isolate the PIC Vcc from the 3.3V system with a diode and use a pull up on Vpp to PIC Vcc.

With this type of isolation the MCLR/VPP, PGC, and PGD are pretty much dictated to the in circuit programming and become very hard to use as I/O pins.

+---------------+ 13.5V -----------+-: MCLR/Vpp RB6 :----------- PGC ! : RB7 :----------- PGD \ : : 22K / : : \ : PIC16F876A : 5V ---------+ / : : Shotkey ! \ : : Diode ! ! : : 3.3V --->!----+--+-: Vcc GND : A C +---------------+

The above ASCII art schematic has worked for me.

And yes the PIC16F876A does support low voltage programming.

My problem was that some of the parts I was getting had the LVP disabled.

10f200

Not this one.

I will try to find a way to post out of Protel.

Thanks, I'll try them.

Donald

This is the circuit I use before.

However, the pins PGC and PGD are connected to 3V chips.

When I programmed the 10f200, my ICD2 put 5V on my 3V logic. The circuit blew-up. ( My circuit did not work any longer and I relpaced the chip and it worked again. )

So, more diodes ???

Donald

Design a custom adapter that sits between the ICD and your circuit. The adapter should disconnect the sensitive logic from those lines. For example you can have a suitably biased transistor in the path to the external logic, and run the base to your custom programming header. WHen you attach your special custom programming adapter, it turns the transistors off and isolates the pgming lines.

You can try to clamp PGC and PGD to 3.0V with resistors and diodes. But this tends to cause more ISP programming problems than it solves.

It's better to design your circuit such that PGC and PGD are connected to 5V tolerant devices. For example, switches or buttons that are normally open when you need to use the ISP and you enable on the internal weak pull up resistors in the PIC for the buttons.

You can usually isolate with series resistors. Will your low-voltage parts actually suffer if they see 5V ?

5V is only needed to clear config bits. (security, WDT, etc)

I simply use a jumper (or SMT SPDT switch like Digikey CAS120GCT.. from Copac Electronics) to switch the break out the PICVCC to 5V from the ICD2

-or- my custom flashing tool).

When done flashing the bootloader, I switch back to 3.3V and power up the rest of my circuitry.

Holding the PIC in reset before switching in 5V as VCC doesn't damage any

3V circuitry attached to the PIC's I/O pins.

If you will be supplying 5V to 3V logic, ensure they have the Ioff parametric as specified by Texas Inst.