Hi Jinx,

I've spent some time today running more tests and finding a more accurate
value of the cap that I'm using to measure the average current usage of the
different circuits. It has been productive. I'm very interested in the
virtual shunt that Peter posted about though.

On Fri, Aug 26, 2005 at 12:54:26PM +1200, Jinx wrote:
> Matthew, think of all this fun you'd have missed out on if
> someone at National hadn't run a red pen through the 3909

:^) I've been learning a great deal because of National's decision.

> With WDT_OFF (PIC never wakes from SLEEP) consumption
> is a few nA. With WDT_ON the consumption is 3.7uA.

My tests show ~5uA current draw while asleep with no WDT. Though my current
measurement setup isn't ideal for such low currents; this is why the virtual
shunt idea is so interesting.

> I think you can save yourself the extra components for ExtRC.

Surprisingly, I believe your right. More below.

> Output is
> a 2us pulse on GPIO,0 every 2.3s. You should be able to extra-
> polate from this what your average is going to be with an LED
> as a load. It would help you to look around for a low current one

I used similar code/setup for two tests. The output pulse to the LED is
about 1ms wide though. It would be very hard to perceive a 2us pulse of
light, atleast it would be very dim.

I ran four tests, and each test was ran using three different 12F629 chips
so that I could compare results. There were no major differences measured
between devices. Tests that used the external RC osc used these values: R=1M
and C=68pF. The resulting instruction cycle frequency was around 4kHz.

All tests used the same circuit built on a breadboard. The only exception is
if a test didn't blink the LED, the LED was removed. The schematic is here:
http://roland.sw.edu/matthew/blinker.png

Test 1. Using the external osc., no WDT, no external loads. After
initialization the pic does a goto $-1. Code is here:
http://roland.sw.edu/matthew/blinker-free.asm
The results are disappointing, a ~280uA current draw across all three
devices. Compare this to the LP osc mode current draw with Fosc=32kHz. Why
does this test show such a high current usage?

Test 2. Similar test to number 1, except that instead of an infinite loop
the device goes to sleep permanently. Code:
http://roland.sw.edu/matthew/blinker-sleep.asm
I can't say much here, but that the current draw is small. :( Around 5uA. I
need a better measurement setup. Virtual shunt here I come!

Test 3. Using the external osc and WDT a LED is flashed for 1ms after each
WDT timeout. Code: http://roland.sw.edu/matthew/blinker-ext.asm
Going to sleep makes the current draw bearable: 58uA.

Test 4. The only difference from test 3, is that the internal osc is
used. The LED flashes for the same (approximate) duration. WDT on.
code: http://roland.sw.edu/matthew/blinker.asm
The average current usage was 53uA.

What do I conclude from this? Contrary to the datasheet, the external RC osc
has an awfully high current usage. It's amazing that test 4 had similar
current draw to test 3. Maybe I'm being fooled by poor equipment? While I
feel I'm measuring everything with a reasonable accuracy, I still need to
build the virtual shunt circuit.

I appreciate everyone's feedback. I learn a lot on this list!

Sincerely,

Matthew.

-- 
Q) Why do C programmers put all the code in the "for" statement?
A) Because they can!
       - Dr. Todd Stevens
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