Communication with the User

Communication is done using the single LED (aka blinkenlight).

If everything is well, then the LED blinks once a second. If the voltage is at warn level, it only blinks every other second and if the voltage drops down to shutdown level, the LED is turned all off.

If the ATTiny sends the Raspberry a should-shutdown message, then the LED blinks 5 times every interval (e.g., 5 times every other second when at warn level). The same signal is given when the button is pressed while the Raspberry is running (i.e., no timeout has occurred).

If the Raspberry hasn‘t read an I2C register for longer than the timeout the LED blinks 10 times fast. After that the ATTiny resets the Raspberry. The same signal is sent when the button is pressed while the Raspberry is shut down before it is restarted.