Reusing PGC & PGD pins

[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

[Demon]

What would you recommend?

(it's a MeLabs USB programmer)




EDIT: 10K seems to work, but I don't want to shoot myself in the foot down the line. I'd hate to invest a lot of time using pins B6 and B7 as input, only to find out I can't do this later on (I'll be using this exact switch configuration - 10K pulled low followed by 10K/0.01uF).

[richard]

are those switches meant to be active high ?

if so then applying somewhat less than vcc to the pic input pins and all those extra components seems a recipe for non success.
for my money i would use the weak pu and have switches active low [don't press buttons when programming is not to much of a constraint]

[Ioannis]

My practice is what Richard noted. Always have either external pull ups or use the port b with the internal pu enabled and active low switch. No other circuit needed and have the Icsp programmer there just fine.

Ioannis

[Demon]

are those switches meant to be active high ?...

They can be whatever they have to be so that it works perfectly. It's my circuit, and I answer to nobody (except those here that know more than me ).

...applying somewhat less than vcc to the pic input pins and all those extra components seems a recipe for non success....

Oddly, it's working as-is.

...i would use the weak pu and have switches active low....

I just checked, 16F18877 has WPUx register for all ports except pins E0, E1 and E2, so that's a plus.

I'm still doing research on all the processes I need, so the switches can easily become active low.

...don't press buttons when programming is not to much of a constraint...

Definitely agree, I never touch the circuit while it's being programmed.

That being said, is 10K a reasonable isolation resistor in this case?

[Ioannis]

Do not risk reliability and toss the resistor.
Use the internal pull-up with low going switch and you will be just fine.

Ioannis