Dead PICs

[CocaColaKid]

Also note that 78L05 has the input/output pins in reverse order than that you would expect looking at the device facing you and the pins downside, e.g. Out-Gnd-In.

Have you checked that you really have +5 at PIC's pins?

Ioannis

The 78L05 is oriented in the boar correctly and there is only 5V powering the PIC. The thing that's getting me the most is that this is a intermittent problem and I'm running out of things to look at beside the floating inputs. Can these 4 pins just be pulled high or do then have to go to ground?

[BobP]

Hi,

Interesting problem!!

I would suspect a voltage surge on the 5v rail when powering-up. Have you tried increasing the capacitance on the 12v rail to deaden surges hitting the 5v regulator. I have seen some 7805 (not 78L05) regulators 'whistle' on their output giving peaks of 8 volts if the capacitance is too small.

Is there any big inductive loads powered from the supply?

The unused inputs can be tied to ground or 5v with resistors (1k).

Last week 4 PICs went south after passing a functionality test on the bench

Was this a test using the PCB but with nothing connected? Do the PIC's die with just power connected or on load?

If every PIC dies on the bench in different (but identical) PCB's then I would look at the board layout carefully. If you can post a picture of track and component side I will look at it.

[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