dealing with 20b/30b/wp34s power drain


fed up with burning button cells?

cells die during flashing?

here is the answer!

is fine on desk too.



Hi Hugh, you created the backpacker's calculator d:-)


more seriously, i do think HP should consider a model with a slightly thicker part to accommodate maybe AAA batteries.

This would give a much extended battery life, could afford to run the processor faster on average, plus it could provide the current to drive a micro SD card slot.

my hack shown in the pictures is the result of some goose chase regarding weak button batteries. basically, i had a situation where the unit did flash, but drained the battery whilst doing so to the point that it wouldnt operate the unit. i thought i'd bricked it until i tried driving from a psu.

incidentally, im thinking the WP34s rom requires more current to run, since a weak battery wouldnt activate it, but the same battery worked in another unit. something to watch out for.

-- hugh.


incidentally, im thinking the WP34s rom requires more current to run

I'm working on it...


To be more precise:

I'm now able to power the processor down when there is no program running and no key down. This lowers the current draw to 0.15 mA. I have the feeling that this is just the PIO being active for wake up.

During a programmed pause, the CPU stays on and the current draw is between 0.25 and 0.3 mA. With full power at 32 MHz, the demands are significantly higher. I measured 15 mA while a program was running.

In my very first attempts, in CPU off mode the draw was higher than with the PIO off but the CPU on. That was until I figured out that I had the pull-up resistors active on the outputs which were tied low. I must have measured the current flowing through these resistors (summing up to 0.4 mA).

Tim, what does the 20b firmware demand on the batteries in idle mode? Can I get below my 0.15 mA?

BTW, the new image is on SF in the repository.


Paul has reported problems with the stability of the last build (699). As a measure, I've relaxed the power saving strategies slightly. Turning of the flash rom while idling doesn't seem to save enough power to be worth the hazzle. So it's gone for now. Going to processor off mode is now postponed to half a second after the last keystroke. You can see it by watching the tiny equals sign in the display.

The new release is in the repository. If it is reported to be stable I'll put it in the standard download.


Once again! I've posted another update of the flash image to the SVN repository. The auto power down feature which is supposed to turn the calculator off after about three minutes of inactivity seems still to be a bit flaky. Please try out the new build!


All that effort to save replacing some twenty five cent button cells :-)

- Pauli


All that effort to save replacing some twenty five cent button cells

a more accurate response would have been :

All that effort to save THE INCONVENIENCE OF replacing some twenty five cent button cells

it's not so much the cost of the batteries i suspect, but the hassle of having to change them so often. the cost of a pair of 2032 cells is miniscule compared to the cost of the petrol consumed driving down to the battery shop, and the time taken for the trip.


Buy them in bulk lots from eBay or the like. Then they arrive at your door and the price is still twenty five cents including shipping.

At least this is what I do for button cells.

- Pauli


it only takes for those button cells to be not quite tip-top, and there's a problem, either flashing or operational brown out (try Q^-1). you'll all be crying into your beers when you're hacking at 3am and run out of 'em :-)

but more seriously, now ive made my paper overlay and have the 34s here on my desk, the angle incline is rather nice. doesn't rock about either!


PS. how do i enter more than 12 digits in DEC 10 mode?



how do i enter more than 12 digits in DEC 10 mode?

?? FIX, SCI, ENG? :-)



PS. how do i enter more than 12 digits in DEC 10 mode?

You can't directly.

- Pauli


Just buy a pack of 100 of the coin cells off ebay. That will last quite a while.

Cost? About $10-15 or so?

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