Low Power PICAXE 08M - BOD

I've been interested in using the PICAXE in solar powered circuits. I received a new batch of PICAXE 08M's that were firmware Rev 9.2 with the BOD (Brown Out Detector) command, as was discussed previously.

I ran a series of tests comparing the current usage PAUSE and NAP commands with the BOD enabled and disable. NAP puts the PICAXE in the low power mode.
<code><pre><font size=2 face='Courier'>
BOD Enabled BOD Disabled
Clock Pause Nap Pause Nap
4 MHz 0.74ma 0.09ma 0.66ma &lt;0.01ma
</font></pre></code>
Rechecking the BOD enabled/disabled current with a more acccurate meter gave 0.093 vs 0.0018 milliamps.

All the tests were run with a 5 volt regulated power supply and the current measured during the PAUSE or NAP command.

Also, all pins were set to outputs, to avoid floating inputs and the programming connector disconnected.

At the same time, I also underclocked the PICAXE to see the effect on power usage, by poking to the $8F location. Hoping to further reduce the power usage.

All values are in milliamps
<code><pre><font size=2 face='Courier'>
BOD Enabled BOD Disabled
Clock Pause Nap Pause Nap
8 MHz 1.28 0.09 1.16 &lt;0.01
4 MHz 0.74 0.09 0.66 &lt;0.01
2 MHz 0.52 0.09 0.45 &lt;0.01
1 MHz 0.40 0.09 0.32 &lt;0.01
500 kHz 0.34 0.09 0.26 &lt;0.01
250 kHz 0.32 0.09 0.23 &lt;0.01
125 kHz 0.30 0.09 0.22 &lt;0.01
31 kHz 0.12 0.09 0.03 &lt;0.01
</font></pre></code>
Looking at the data, it would seem that underclocking could reduce the power requirements. Except there is the trade off in speed. At 4 MHZ, the PICAXE runs at approximately 2000 steps per second, while at 31 kHz it will run at approximately 16 steps per second, 128 times slower but only at 1/25th the current While the current requirements are less, the longer step time seems to offset any overall power savings. This would have to be confirmed in a real world situtation where actual calculations are performed.

TANSTAAFL!

Myc



Edited by - Admin on 16/07/2006 00:54:38

Thanks a lot for all this data affecting low power apps - I haven't got the new chips but am looking forward to it!

Sounds very useful for solar-power stuff... an underclocked 08 might be just the thing to give some extra brains to BEAM robots.

PICAXE and BEAM make for a great combination, almost as good as peanut butter and jelly.

Most BEAM circuits: solar engines, photovores, photopoppers, pummers, and heads are easily done with a PICAXE 08M.

The brown out function and underclocking bring the current requirements down below that of the 74AC240, which is the core chip in BEAM.

The PICAXE/BEAM combination has already been nicknamed as SMARTBEAM or PBJ (PICAXE BEAM joined).

Thanks Myc for the detailed analysis.

This is very useful, not just for BEAM, but for any application where a PICAXE can go into a lower power mode, such as data logging, and even when waiting for an interrupt or a key press.

The trade-off with speed of execution is probably not that bad in practice. Having gone into minimal-power mode, the PICAXE can switch to a normal execution speed after it has woken-up and detects there is something to do. Even at 31kHz, the code to detect that should execute in just a few mS which will give acceptable response times for many programs, and then a switch to 4MHz/8MHz gives normal full-speed execution until the PICAXE sleeps again.