Douglas Wood wrote:
>
> How do you run code while Vdd is powering up?

Isn't this a typical PIC startup problem?
The MCLR is referenced to Vdd, so if MCLR is
tied to Vdd they will both rise together, and
the the PIC will start executing code possibly
about 2.5v, anyway well before Vdd reaches 5v.

With some simpler circuits I found it useful to
keep the pins in a *known* state, the best for
my apps is as outputs at 0v. Once the time has
elapsed and you are sure the Vdd has stabilised
you can set the pins as inputs where needed
and start all the other processes.

Someone mentioned that during startup the pins
should be held in whichever state will make the
circuit draw the least current, so Vdd reaches 5v
soonest. This is right, but if they are "high"
outputs they will possibly go through undefined
stages as Vdd rises. I prefer with simple circuits
to design the whole thing so "low" outputs are
best during startup mode ("high" outputs drive
things).

Obviously using a MCLR delay cap or a reset chip
would change everything, but again with a simple
circuit the MCLR cap would cause ICSP problems and
a reset chip adds cost and complexity. This is just
one way of going about a difficult problem.
:o)
-Roman

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