[34S] battery warning indicator / question

[matthiaspaul]

For those who can modify the calculator hardware, there are several options to at least improve the situation:

- A low-leakage low-ESR capacitor (perhaps 10-47uF) should be added in parallel to C17/C16/C15 as close to the CPU as possible. This is particularly important for those who have added the USB extension, because then VBAT is routed through circuitry on the USB board and via long and thin wires, which will increase the voltage drops under load. Low leakage and clean soldering is important here because otherwise this will have the reverse effect.

- If an IR-LED is present, its anode should be decoupled from VBAT by a small resistor (perhaps 10R) and bypassed to GND with another low-leakage capacitor (ca. 10uF).

This should help to stabilize the weak VBAT plane considerably and even extend the battery life somewhat. See also:

http://www.ti.com/lit/wp/swra349/swra349.pdf

If the USB board is installed, it is easy to derive an "EXTERNAL_POWER_GOOD" signal from it. It requires just one more wire between the cathode (which is *not* connected to R4) of the common-anode dual-diode D1 and one of the controller's free I/Os. (It might be possible to make the presence of this feature auto-detectable by the firmware without increasing power consumption, otherwise it would require a special firmware image. The firmware overhead wouldn't be more than a couple of bytes, so it wouldn't harm. We should just "standardize" on the I/O to be used for this purpose.) If the signal indicates that an external power supply is present, high speed is allowed (or even automatically enabled regardless of the measured battery voltage). If no external power supply is present, the firmware should be much more conservative about allowing the calculator to run at high speed, so that it will not reset before the battery is actually exhausted. After all, speed is hardly important on a calculator, whereas high reliability and long battery life are.

See also: http://www.hpmuseum.org/forum/thread-294...l#pid28498

Adding yet another wire, we could also enable one of the A/D converters to measure the battery voltage, however, this has a number of disadvantages:

- not applicable if the calculator should not be opened
- increased power consumption compared to the BOD solution
- extra code necessary to handle A/D conversions (so far none of the A/Ds were used, so no code dealing with the A/D converter is present at all right now)

I guess it would be possible to further tweak the existing power management algorithm somewhat (beyond just setting the thresholds to more conservative values), but it is a bit too early (for me) to make actual suggestions, since I haven't run into such low battery conditions myself with this calculator and this would certainly help to get a better sense for the underlying problem and possible solutions.

Greetings,

Matthias

EDIT: For easier reference more threads on similar topics:

http://www.hpmuseum.org/forum/thread-3453.html
http://www.hpmuseum.org/forum/thread-3078.html
http://www.hpmuseum.org/forum/thread-3077.html