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

[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.

[towlerg]

Looked in Mid-range family MCU ref manual and you're right, it is an RC clock.

Having read the blurb I was surprised to find that although devices have a calibration value written to them, it is the responsibility of the application to fetch that value and write to the appropriate register.

George

[Art]

Last edited by towlerg; Today at 00:38. Reason: old and stupid

That’s a bit rough!

It will loose time with temperature, and any supply voltage fluctuation (batteries).
It’s ultra cheap, you can use serial, but not keep or measure time with it.

[Art]

That’s an interesting find.
You could probably cycle the calibration value at run time then.

for value = mincalvalue to maxcalvalue
calvalue = value
turn led on
pause
turn led off
pause
next value

You could probably calibrate it yourself with a good freq counter,
and double check their work or what they thought was operating temperature.

[Demon]

Does anyone have an answer to the original question?

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

I'm trying to clean up code and wondering just how much we should pause at program start. I have code with pauses of all sorts of lengths.

How about:
- without LCD
- with standard 4x20 LCD

16F18877, also an 8-bit PIC.

[richard]

How about:
- without LCD
- with standard 4x20 LCD

a pic is ready as soon as its osc is stable; as discoverable from the oscstat reg, a handful of microseconds at worst
a LCD can take 100's of milliseconds, 500 not uncommon. its not the same across version/model/maker either
its usually pretty obvious when you have tried to talk to a LCD before its listening

i feel you have the cart before the horse

[Ioannis]

Up to this moment I did not had any special needs for a fast power up, so the 1 sec pause worked just fine for me.

But I am pretty sure that each osc will have its own time to stabilise. So, for 18877, page 112 of the datasheet, says that there is a timer named OST (Oscillator Startup Timer) that counts 1024 cycles of from OSC1, if the Oscillator is set to LP, XT or HS (all these are external oscillators).

For any case of oscillator selection (internal or external), there is OSCSTAT register that displays the status of that module in the appropriate bits (page 122).

Page 603 shows the wake up time that any osc selection needs.

Other PIC have same data also, so you may look accordingly.

Ioannis