Hardware or Software

Hello,

I seem to have hit a issue that I cannot pinpoint. I was not there when it happened and have not looked at it yet but I have been told that a board that mounts on top of a series string of batteries. The pic is not starting up at this time however the regulator is putting out the requisite 3.3 volts. Could it have something to do with having voltage dividers feeding the a/d channels on the pic before the pic is up and running? Any thoughts as to what may be happening/causing problems here?

The board worked prior to it being attached to the battery pack and then once attached it worked and operated as designed. The pack and board were taken apart the other day and when reassembled the pic did not come back.

Thanks again in advanced.

Quote:

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Originally Posted by bmagistro

Hello,

Could it have something to do with having voltage dividers feeding the a/d channels on the pic before the pic is up and running? Any thoughts as to what may be happening/causing problems here?

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I think you are on the right track in this assumption, either it blew or erased the PIC.

I was afraid of it blowing the pic. I wrote one pic off to ESD because I knew I was being careless but didn't think it was ESD, just didn't want to think it got blown when working on it at one point. What are your thoughts on using a MOSFET/BJT as an electronic switch that enables the A/D lines to the pic once the pic up and running. I'd like to limit the number of mechanical switches if possible since it would not be practical to flip several mechanical switches after a "power failure" or similar issue.

Some time ago I designed a battery charger for my tools and had the same situation, although I never had a problem with a blown pic when the battery was connected to the pic inputs. To be safe, I would suggest putting a P-ch Mosfet with a low resistance such as a BS 250 in series with the input divider and have the pic turn them on when ready.
Regards,
misterg

You can apply voltage to an input before the PIC has power as long as you keep the current below the 'maximum input current' for your device. I believe that is +/-20mA for most devices. This number can be found in the ABSOLUTE MAXIMUM RATINGS listing in the datasheet.
All PICs have (essentially) a diode between the input pins and the supply rails. This also means that if you apply a voltage to an input pin, you can power the PIC from the input pin alone. I once had trouble with a PIC not resetting on power up. It turns out that the current into the input pin was preventing the supply voltage from dropping low enough (even when the PICs power supply was shut off) to allow the PICs internal RESET circuit to operate.