Dead PICs

[CocaColaKid]

Looking at the pin function I noticed that porta.5 is shared with MCLR and Vpp. Just wondering if since it's floating that it somehow goes into a programming mode or something.

[Luciano]

Hi,

As a test, can you try to connect 24V DC instead of 24V AC and see if the problem is still there?

Best regards,

Luciano

[Meriachee]

Should the MCLR be tied with a 4k7 to 5v?

[Raflex]

Hello. You need to program RA5/MCLR as I/O port and add diodes between pic and optocouplers. Connect a 10uF cap to the out of 7805 regulator. Try it.

[Squibcakes]

Hi CC,

When your Chip dies, does it smoke, get hot or explode?

What does the 5V, 12V look like when there is no chip plugged in? What do they look like when the chip is in? (ie do either get dragged low from the dead chip?)

Have you checked your wiring, pcb for shorts. Double check that the VSS and VDD are connected to the right pins!

Take out all your electrolytics - it's possible that one of them is failing (shorting) when there is a voltage across them. Is one of them back to front?

I don't think that the floating inputs are going to kill your chip.. Its good practice to tie them to vss, or vdd when not used. I always just link them to ground and in my code set the tris registers to high (ie high impedance).

Oh if your not using the MCLR pin, just set the config fuse to off ie MCLR OFF.

Cheers
Squib

[CocaColaKid]

I think I may have traced the pass to my problem. I think what's been happening is that I have a header that I use to test the boards that is powered all the time. I think I'm applying 24V AC to the board with no ground and then grounding pin PORTB.7 then applying the ground to the chip. I have installed a resistor in the PORTB.7 line and have not seen the problem since then. I will keep checking though but so far so good.

[CocaColaKid]

Hi CC,

When your Chip dies, does it smoke, get hot or explode?

What does the 5V, 12V look like when there is no chip plugged in? What do they look like when the chip is in? (ie do either get dragged low from the dead chip?)

Have you checked your wiring, pcb for shorts. Double check that the VSS and VDD are connected to the right pins!

Take out all your electrolytics - it's possible that one of them is failing (shorting) when there is a voltage across them. Is one of them back to front?

I don't think that the floating inputs are going to kill your chip.. Its good practice to tie them to vss, or vdd when not used. I always just link them to ground and in my code set the tris registers to high (ie high impedance).

Oh if your not using the MCLR pin, just set the config fuse to off ie MCLR OFF.

Cheers
Squib

No explosion or anything exciting like that unfortunately, lol. The PIC does get hot though after it fails as do the regulators.

Typical if it powers up and everything works right the regulators volts I don't think even moves when the PIC is in the circuit or not. When the dead PIC is there the 5V drops to around 4.4V or something as I recall.

What I ended up doing since I noticed I forgot to tie the unused pins somewhere is I made them and output and but them high so they weren't floating.

The MCLR line is set to input mode as I didn't see the sense in putting in another resistor just for that pin.