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--- 20b:software-battery_level [2008/07/25 20:18]
newell created
+++ 20b:software-battery_level [2008/07/28 19:49] (current)
newell Updated source to fix startup switch to Thumb mode.
@@ Line 1: / Line 1: @@
+===== Proof of concept: battery level measurement =====
+==== Description ====
+Demo program showing that it is possible to measure the battery voltage with the onboard brownout detector.
+==== Usage ====
+Boot calc into test mode (to allow stock firmware to configure and init the chip),
+connect a debugger, halt the chip, download the image, and let 'er rip.
+It'll probably corrupt anything you had in ram before.
+==== Download link ====
+ {{20B:battery_level.tgz}}
+==== Readme.txt ====
+Another sample program for the HP20B.  This is a proof of concept to
+illustrate that the brownout detector can be used to probe for the
+battery voltage.
+The idea is that by ramping down the brownout threshold, we can find the
+point at which the unit goes out of brownout mode.  When the threshold
+is above battery voltage, the brownout bit will be high.  As the
+threshold drops, eventually it will be less than the battery.  The last
+theshold at which the brownout bit was set can give us an approximate
+measure of the battery voltage.  While it seems to work ok, it's not
+exactly linear.  Not that it needs to be, for this application.
+The first few hex digits on the display are the last value of the power
+supply controller status register.  Mine shows 0x1d00 when not in
+brownout (threshold under battery), and 0x1d40 when in brownout (threshold
+above battery).  The next hex digit is the current threshold voltage setting
+(refer to the book for conversion to volts), and the rightmost digit is
+the edge of brownout to no brownout.
+Finally, the measured battery voltage is converted to decimal, scaled by
+, and placed in the exponent area of the display.
+The probe loop is intentionally slow, so that you can see the probe
+process and brownout detector in action.  Hook your calc up to a
+variable power supply and see the voltages change.  (Note, the exponent
+display is only updated at the end of the probe loop, so it'll be
+several seconds before you see it change.  Adjust your supply *slowly*.)
+The keys are not scanned.
+Runs from ram, Thumb mode, trashes memory, and requires some sort of JTAG
+debugger to load and start the program.  OCD Commander reports a starting
+address of 0x002FF000, in case your debugger can't figure it out.
+Under OpenOCD, I do a “soft_reset_halt”, then a “load_image
+battery_level.hex”, followed by a “resume 0x2ff000”.  The startup code
+wants to run in ARM mode, so be sure the core is set accordingly.  Once
+it jumps to main, it should switch cleanly into Thumb mode.  (Or at
+least it seems to for me, single-stepping through it.)
+Disclaimer: Yes, the code is ugly.  Fix it at your own risk.  Consider it
+nothing more than a toy to demonstrate that we know a little about the hardware.
+Happy HP Hacking!

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