Silly MCLR question

[kevj]

Low Voltage.
I suggest you very carefully experiment and verify what your PIC will do with a 1.5 volt supply. The datasheets are silent about such operation. I had a bunch of data loggers where the 3.7 volt battery went completely flat and the processor code was all erased - not scrambled - erased.

Hmm.... that's disturbing. Anyone else have that happen?

I can't imagine they'd design a pic that would zap itself just from a dead battery - I'd imagine all kinds of electronics "dying" all of a sudden just becase a battery went dead, but I guess anything is possible.

Is the idea then to make sure and power the pic *all the way* down - zero volts, power rail tied to ground? That seems a bit extreme to make sure it's totally de-powered, but maybe it's a good idea. I'd expect there are thousands of devices out there where the pic sees some residual voltage from a capacitor or something.

We're going to run a batch of quite a few of these things where the design will be hard to change after the fact, so if they do start to die due to the low voltage, it'll be a major problem to fix them.

[BrianT]

Hello KevJ,
I am not saying Microchip have a problem but they are not specifying anything below 2.0 volts. I will now have to experiment with my loggers on the bench with a resistor across the battery to hasten its decay and see what happens as the voltage falls and the internal impedance of the battery goes up. Just dropping the supply voltage to simulate a decaying battery will not cut it as the real battery internal impedance will vary considerably as the battery decays and I won't see that by just winding down the supply voltage from a bench supply.

It would be nice if any of the PICs had a bandgap reference that the programmer could access. That way you could calculate the battery voltage and put in a software halt before the battery collapsed. I can't see how to measure battery volts via a direct connect PIC ADC referenced off Vdd. There needs to be a voltage reference somewhere in the circuit. The Brownout Detector can flag a 2.0 volt dip so there must be a reference buried inside the PIC somewhere - I just can't see where to find it.

At 1.5 volts you are off the data sheet and will have to build your own maps of what works and what does not.

Cheers
Brian