TMR0 stops working

[Darrel Taylor]

Hard to say for sure, but with it working for months at a time, I wouldn't suspect anything in the software.

A couple things I can see that might cause problems ...

You are charging the 3V batteries from a 12v source with only a diode and resistor in series.
That's going to overcharge the batteries severely, and I'm surprised they haven't blown up yet. Then when the voltage gets too high, it'll pass right through to VCC raising the voltage above the F84A's limit.

Then if main power is lost, the F84A will be powered by only 3.6V, which is below the 4v minimum for an F84A.

The F84A doesn't have a Brown-out detector, so it won't reset or anything when the voltage is too low.

Voltages outside the normal operating range can cause just about anything to happen (randomly).<hr>
Lowering the resistor value to MCLR, with a small capacitor to ground may help reduce spurious resets, if that's where it's coming from.

Also, this isn't part of the TMR0 problem, but you shouldn't have a switch on the same pins used for an output. It's a dead short to VCC when the switch is pressed. Maybe with some resistors in series to limit the current, but straight to VCC like that, not a good idea.

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[shahidali55]

Thanks for that Darrel . . .
I've added a .1uf from MCLR to ground, 1k from buttons to VCC. Datasheet states minimum operating voltage is 2 volts. (http://ww1.microchip.com/downloads/e...doc/35007b.pdf)
Sometime back i have also run the clock from a 12 volt lead acid battery (with 7805).
After 3 days, the clock stopped incrementing time, though the display was being multiplexed and the input buttons responded fine.
16F628A also shows the same behavior.
I tried writing a piece of code that resets all TMR0 registers and variables when the time set button is pressed, but no matter what ,TMR0 doesn't seem to come to life until a powerdown reset is done.

[shahidali55]

I think i have found the fault...
Sometimes when i turn on the tubelight in my room, the clock resets or hangs or stops incrementing time,even tough i'm power it from a battery.
Does this mean that i have to provide RF shielding to the PCB ? ? ?

[Darrel Taylor]

I have a similar problem with my breadboards, ... when I turn on the fluorescent light on my desk.

I pretty much ignore it (mostly because I can't fix it).

It doesn't seem to be related to proximity, but seems to be related to it being plugged into the same "Power Strip" as the project.

If I plug the project into the wall, and the light into the power strip, it doesn't happen.

Surely there's an explanation .. (that will surface real soon).
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[shahidali55]

It happens even if i run the clock from a battery.
It sometimes locks up when i plug in the cellphone charger adapter.
Will decoupling capacitors or line filters help?

[Darrel Taylor]

If you don't have decoupling capacitors, it could be a problem.

I usually put a .1uF (100nf) across the power rails for every 2-3 chips.
Some people say 1 cap per chip, but I think that's a little more than necessary.

You shouldn't need RF sheilding.
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[shahidali55]

Decoupling capacitors don't seem to help . . .
Using the 16F628A with MCLR disabled while programming solves the problem (for now)...