Chip Selection

[AMay]

I do fairly simple stuff using PIC Basic (not “Pro”) mostly using chips to drive switches and as timers. I have had a couple of chips fail (12f629) which has not been a big deal but I wonder about the failures.
Since I expect to be asked, the failures are in a circuit powered by a dc supply that runs parallel to 120v ac lines, so I have a 100uf cap at the input, and a 78L05 regulator, a .1 cap on the 5v side. The out put is a led flasher protected by a 1K resistor. A 16F84A chip uses the same supply a bit south with no problems. All other pins are inputs, internally pulled up except pin 3, pulled up with a 20K resistor. I make my own circuit boards with Press-n-Peel Blue.
I have concluded, with no evidence at all, that the older simpler chip would be more robust than similar more complex chips developed more recently. Say 12f675 or 12f683 or others I don’t know about? (Is there a catalog available to guide chip selection?)
Are any particular chips more rugged than others. Does any one know? Does anyone care. Afterall, all chips that I have used have continued to perform with the one exception so I have no complaint. Since my production runs have never been more than 3, cost is not a big factoe for me.
Comments welcome.

[isaac]

Have you checked with a scope on the dc side if its fluctuating or stable?

[AMay]

No, but I have assumed it is pretty good, as its source is the trickle current to the burglar alarm back up battery. I put my Fluke meter on it, and as I recall the ac ripple south of the vr was OK.

[mackrackit]

What is the source of the inputs? Maybe there is a spike now and then.

[AMay]

The power comes from the trickle charge to a 12 v alarm battery. I have a pretty well filtered supply because of the supply wires running parallel to 120v AC wires.

[mister_e]

I've never, and will never trust any Pull-up thing. As per their definition they are weak. nuff said. In a shielded enclosure with no external inputs, running off battery maybe they're fine... but not something I would use again on a final product since I already got issue with some handheld car instruments I've made years ago. Ununsed i/o are set to input and tied to GND directly, or with low value pull-down.

Some will say set them as output and left them floating... ok .. could be fine as long as you always have supply on your board.

GOOD ground plane will also help.

[mackrackit]

The power comes from the trickle charge to a 12 v alarm battery. I have a pretty well filtered supply because of the supply wires running parallel to 120v AC wires.
The input pins will not handle 12 volts. It is a wonder it works at all.
Or.
You misunderstood the question....

[Charlie]

The input pins will not handle 12 volts. It is a wonder it works at all.
Or.
You misunderstood the question....

Or maybe you missed the original post where he states:

"I have a 100uf cap at the input, and a 78L05 regulator, a .1 cap on the 5v side."

[Demon]

Moved from FAQ.


PIC selection guide:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2551

How robust is a PIC?
http://www.picbasic.co.uk/forum/showthread.php?t=15993

Robert