Quote Originally Posted by badrad
As for my problem, since then, I had to rework my pcb and included the following:
1. left the unused pins alone, but defined them as outputs. (now this might sound contrary and I did initially ground them and assign as inputs, but the tech people at Microchip recommended this to me).
2. put in sufficient ground plane.
3. installed MOV at my A/D input as well as at the power input.
4. increased my A/D input resistor from 1K to 20K.
5. keep my fingers crossed...
I agree with the ground plane idea, i should said it before.

For MOV at the A/D input... yes/no/toaster it depend of where the signal come from but some 1n5817 could do the job... it's always case by case anyways.

Increasing the A/D input resistor... i can't see how it will help and it's totally out of the Microchip spec... well in the datasheet.

But for the left pin open and set them to output... OOOOOOH no i totally disagree. I did it in my first projects as this is what Microchip sugested to me too. I had a loooooooooad of unit that comes back to me after few months... PIC burn.... Solution, set to input, tie them to ground. Most of those unit were dedicated to do some test in car engine... YEAH you want noise... that's one of the best place to test it. After the modification, i place some units
  • 2 X close to the alternator
  • 2 X close to the sparks plug
  • 2 X close to distributor
in my cars, my friends car and run with them for 1 months. Those unit had no metal shielded box... only sit on a plastic Ziploc bag... with a touch of silicone. After the test... no more problem with those modified, most of those unmodified still had the same problem after the test.

I still produce those units. I never, never, never see the same problem. now exactly 5 347 are on the market. If i look to my warranty/exchange list and i remove those from the first edition, i have only 8 which come back for a 7 segment problem... Big deal !!!

I never use internal pull-up for final product... always place an external pull-up 1-4.7K... yeah more power hungry but save my butt in many case.

I'll be curious to see your actual schematic and board layout to see what i can suggest to you.