The Mighty Power Reduction Register

A project log for Snail Mail Notifier

Low power, wireless device that informs user of regular mail.

SolenoidSolenoid 01/30/2016 at 20:310 Comments

The ATtiny85 is a really wonderful chip, it presents an array of features (timers, interrupts, watchdog, ADC, PWM...), but makes it challenging to use with a relatively low pin count, flash and SRAM. The fun doesn't have to end there though, once the code is running another challenge is to reduce the power the project is drawing. On the emitter I got it down to the lowest spec in the datasheet: 0.1uA, pretty good, but what about the receiver? It has to run all the time in order to catch the emitter messages...

The ATtiny has a couple of ways to reduce its power usage: some peripherals like the ADC and timers can be disabled or turned off completely. The Power Reduction Register (PRR) allows to turn off the timers 1 and 0, Universal Serial Interface (USI) and the Analog to Digital Converter (ADC) - datasheet p.38.

I tried to see the effects on the current when disabling the unneeded peripherals on the receiver (ADC and USI), the difference was a notable 2mA, lowest consumption achieved was 19.5mA at 5V, a full 10%! Just for kicks: this is about 850Wh per year when run constantly, at 0.2CHF/kWh it costs 0.17CHF to run the receiver for a year. Disabling the unused peripherals saves 0.017CHF per year.

My current measurement setup involved some crude, self-made tools.