Reusing PGC & PGD pins

[Demon]

i like to connect some critical or dangerous hardware to one of those pins, like the enable pin for the high power laser or the step input for the Z axis.
that way you really know when reprograming is happening

Pics or it didn't happen.

[richard]

its very easy to do.
on a pic curiosity board the pins are just there begging you to use them. you can kinda forget the programmer is on board
when there is no need to plug and unplug it at all.

[tumbleweed]

To be clear, I will NOT have the circuit powered when I'm programming. The PCB must be removed from the main board in order to be programmed (I use the same connector for operation and for programming).

Specifically, now I'd like to connect the output of a 74HC14 on PGC or PGD

I wouldn't do that. Your safest bet would be to have a jumper you can remove to isolate the output of the HC14 from the PGC/PGD pin while you're programming.

[pedja089]

I conect ICSP directly to pins. And from konektor, other part of circuit connect via 2K resistor. If you use pin as inputs, then other hardver driving state of pin. So not to have colision with programmer, put some resistors.
For MCLR use 10K resistor. As it can reach 13V. I generaly avoid using MCLR.
But this makes it hard to use debuger....

[tumbleweed]

And from konektor, other part of circuit connect via 2K resistor.

You have to be careful when using the pin as an input.

While a series R can help isolate the pin from the ICSP programmer, if the external circuit is driving the pin high then this can change the idle state of the pin that the PIC sees during programming from low to high depending on the R value. The ICSP typically has 4.7K pulldowns on the PGC/PGD pins, so the series R creates a voltage divider which you may need to take into account too.

[Demon]

Does this look "safe" to use an opto-coupler to be able to reuse ICSP Clock & Data pins?




I'm using my old MeLabs USB programmer with stock header; circuit is powered by wall adapter/7805.


EDIT:

1. Should I be better to use an opto with a CurrentTranferRatio of 0%?

2. Is anything stopping me from doing the same thing to MCLR with quad opto?

[Ioannis]

If you think that through the opto you can transfer the High and also the Low state of the 74HC14 then be sure it won't happen. When opto is ON, it will transfer the High state only. The Low state of the HC14 output cannot pass because collector of opto is reverse biased. You need then a pull down on the emmitter of the opto, which will have loading effect on the ICSP programmer.

Best and simpler is to use a simple and cheap resistor to isolate the 74HC14.

Ioannis