###Disclaimer This code is a proof-of-concept, very little effort has been put in to ensuring it is bug free.
Reading VBAT on the Edison
It is possible to read the Lipo battery voltage on an Intel Edison without using the Yocto battery_voltage. This method of reading battery voltage works on devices running Ubilinux.
There is a system device, bcove_adc, that is connected to various voltage and temperature input lines. These can be read using
/sys/devices/platform/bcove_adc/basincove_gpadc/channelreads a bitmask of channels to report. Channel 0 is the battery voltage.
/sys/devices/platform/bcove_adc/basincove_gpadc/sampletriggers an ADC conversion to take place.
/sys/devices/platform/bcove_adc/basincove_gpadc/resultcontains the result of the ADC conversion. It is an integer between 0 and 1023. A result of 1023 is roughly equivalent to 4.5v.
$ echo 0x1 | sudo tee /sys/devices/platform/bcove_adc/basincove_gpadc/channel 1 $ echo 1 | sudo tee /sys/devices/platform/bcove_adc/basincove_gpadc/sample 1 $ cat /sys/devices/platform/bcove_adc/basincove_gpadc/result sample_result = 980 sample_result = 0 sample_result = 0 sample_result = 0 sample_result = 0 sample_result = 0 sample_result = 0 sample_result = 0 sample_result = 0
In this case, the result is 980. 980 is about 96% of 1023, so the voltage is about 96% of 4.5v. This is 4.3v. In this case, the battery is connected to the charger, so this value is to be expected.
There are two different methods of reading voltage provided. One, voltage.c, requires root access (must be run with
sudo). Build it with
make voltage. voltage.c takes 1 argument when run, a number. The number dictates the output format.
./voltage: Full text output
./voltage csvCSV output
./voltage shortproduces results in the format
./voltage percentagereports only the battery percentage
./voltage jsongives JSON output of the voltage and percentage.
The other method, voltage_server.c, is a service which watches for accesses to files much like sysfs. These files are set up to be accessable by all users, so once the server process has started, no further root access is required. To start the server, run :
make voltage_server sudo ./voltage_server &
The interface for this method is located in
/tmp. To trigger an ADC read, write
1 to /tmp/battery_trigger
$ echo 1 > /tmp/battery_trigger
After some length of time (0.1 seconds is sufficient), the results are available in 3 files:
/tmp/battery_voltagecontains the voltage in mV
/tmp/battery_percentagecontains the battery percentage.
/tmp/battery_rawcontains the average of 5 ADC readings.
The python script, report_voltages.py, gives an example of how to use this interface.
sudo make install will install
voltage_server as a service and start automatically when the edison starts. This is thoroughly untested.
For OpenAPS users that want to log voltage to nightscout, I use the following two files: https://gist.github.com/cjo20/42ce5227cc1412513448da17f5f1ab84
Percentages generated by this tool are calibrated to hit 0% at 2.95v. 100% is 4.18v. At 0% the edison will shut down completely. Wireless communications will usually cut out before this (around 3.2v).
The voltage/percentage relationship is not linear, as there is a large plateau in battery voltage as capacity is used up. I hope to improve this given some more data.