Electronic – the best low-power color indicator

To the primary question, as to how to measure the voltage of a battery, the most simple method is to use one of the ADC pins on the arduino.

If you're providing 5v from the battery (unlikely), then you can run the + straight to one of the analog in pins, and use analogRead() - each increment in value (from 0-1023) will represent 4.9mV. So multiply the read value by .0049 to get the volts read.

If you're running higher than 5v (more likely), you can use a voltage divider circuit (see: http://en.wikipedia.org/wiki/Voltage_divider) to bring the voltage you're sending to the analog input to <= 5v. If you're running 12V, you'll want to divide the voltage into roughly 1/3. Then, after multiplying the ADC value, multiply by 3 to get actual voltage.

Thus, if using 12V, and a 1/3rd voltage divider:

#define BAT_PIN 14

float read_batt_volts(void) {

  int val = analogRead(BAT_PIN);

  float volts = (float) val * (float) 0.0049 * (float) 3;

  return(volts)
}

!c

I think you can fix it as follows:

Put a pulldown on sysoff, maybe 100k. Just make sure it is strong enough to overpower the internal 5M pullup. Also keep sysoff connected to your IO expander. It should be configured as an output driven low. But when you want to power down, have the expander drive the output high, which will overpower the 100k pulldown, and turn the system off.

Once the system is off, your rails will all collapse, so the IO expander will be dead, and the 100k pulldown will ensure that sysoff is now low so that startup will be possible the next time charger is attached. You will need to make sure that the IO never goes high during bootup or normal operation. For example, program it to be low before you program it to be an output.