Hi William,

I'm not in bed yet, but darn it, I'm going to sleep soon!

On Fri, Aug 26, 2005 at 10:23:54PM -0700, William Chops Westfield wrote: 
> On Aug 26, 2005, at 8:54 PM, Matthew Miller wrote:
> 
> >I'm wondering why a 16kHz clocked chip and one clocked at 4MHz,
> >running firmware that does the same thing, have current usages
> >that are so similar. The current use in test 1 still seems high.
> >
> Well, the "obvious" explanation is that the current that you're
> measuring is NOT the dynamic current used by the CPU, but is
> some other static current being consumed somewhere that we haven't
> identified yet.  That would fit the datasheet better; tiny current
> consumed by the CPU at both 16kHz and somewhat higher speeds,
> 200uA going "somewhere else."

That's a good possibility. Tommorow I'm going to breadboard the circuit
again and the only common element will the capacitor I'm using for current
measurements.

> Has anyone else tried measuring such a PIC?  I should have a
> made board somewhere that I can turn into a blinker, but all I've
> got is 12f675s...

I don't know. I've tried to be careful regarding my measurements, but errors
do creap in. Trying to simultaneously disconnect a wire and activate a
stopwatch has a delta t of several hundred milliseconds or worse.

> Just out of curiosity; someone suggested that you might be able
> to run a PIC at 1.5V - have you tried that at all?

I've not tried this suggestion on purpose. A PIC with no external load will
run at 1.5V. I saw this using the _EXTRC_OSC_CLKOUT config option and using
an oscilloscope to measure the output frequency. Without the brown out
circuit enabled a PIC will run at a very low voltage.

It is just that for my application, at around 2.5V the light output of the
LED is very dim. To run at 1.5V I think a voltage multiplier and low Vth
mosfet would be needed; I'm assuming a useful light output here. A 3 volts a
lithium cell has a much better energy capacity and volume than two alkaline
cells. I wish the 12F629 had one open-drain output; the voltage doubler idea
would be easy to use in that case.

I may need to continue this discussion after I've built a virtual shunt
circuit. Everyone, thanks for the input!

Matthew

-- 
"A designer knows he has arrived at perfection not when there is no longer
 anything to add, but when there is no longer anything to take away."
 -- Antoine de Saint-Exupery
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