How long does it take for an 8 bit PIC to stabilize after Power-up?

[Art]

I imagine the power up timer is to wait for the crystal frequency to be stable, and that you could disable it
if you had an external clock source continually running (not in your case).
You could use a reset circuit (a resistor and a capacitor) on the MCLR pin to make a shorter reset time.

[towlerg]

Pretty sure the PWRT is belt and braces. If you don't need to startup in the minimum time and want to be completely stable.

What would be the effect of a few clock cycles long/short slow/fast whatever?

George

George

[vintagecarguy]

I imagine the power up timer is to wait for the crystal frequency to be stable, and that you could disable it
if you had an external clock source continually running (not in your case).
You could use a reset circuit (a resistor and a capacitor) on the MCLR pin to make a shorter reset time.

Thanks for the suggestion. I had considered adding a supercap to the power supply but didn't think to add a capacitor to the MCLR pin to retain the charge and prevent a reset.

Craig

[Art]

That’s actually not what I was suggesting, but instead to disable the powerup timer which usually holds MCLR,
and use the MCLR pin yourself that will lower the power up time to what is acceptable with a stock reset circuit
consisting of a resistor and capacitor. But if you can use a cap on the supply, the thing I’m working on now
continues to multiplex a display for a couple of seconds after power off with two 470uF caps.

[towlerg]

I had a quick look at the datasheet and I see that the Oscillator Start-up Timer is not invoked if an internal clock is selected. I couldn't find a figure for how long the internal clock takes to stabilize but the "Oscillator Wake-up from Sleep start-up time" is 8us (worst case)

I think it's important to note that the power up timer is optional, so MC must forsee a situation where it is not used.

If you believe that some delay after power up is necessary, Art's idea is the way to go.

if you have space, how about a string of NOPs at the reset vector?

George

How long is the power off for?

[Art]

Interesting, I’d have thought it’s the internal RC clock that would be the option wanting to be stabilised,
but really don’t know how the startup of either clock really looks on a scope, etc.

[towlerg]

I’d have thought it’s the internal RC clock that would be the option wanting to be stabilised

Do you have any basis for that or is it just agut feeling? Is it an RC clock? I can't find in the datasheet what kind of osc the internal 4Mhz clock is, perhaps I didn't look hard enough.

Seems to me the internal osc is physically closer, has optimal caps, and a load of other things I can't think of.

Anyway I think your idea of using an RC to hold MCRL for a short while is vcg's only option if he is uncomfortable turning off PWRT (which he has to to achieve 50Ms reliably).

George

[Art]

it’s more than a feeling that it’s an RC clock.
You would rarely need any external clock if it were any better, the clock wouldn’t need a calibration value,
and the clock wouldn’t run faster if the chip is powered from a higher than spec voltage.
It’s only a feeling that an RC clock would take the longest time to stabilise than any other option.