Debugging power usage

In a previous blog post I mentioned how a small timer I made kept eating batteries unexpectedly. With no way to analyzer the power consumption, the problem is almost impossile to debug. That is why I started working on a self-made power analyzer. The self-made power analyzer is now fully functional.

Using PulseView its now trivial to inspect the power usage of any low-power device. Just hook it up and press the Start button! So I attached the timer which mysteriously ate batteries.

The provided 3.3 Voltage supply makes it extremely easy to analyze most microcontrollers. I just connected it to the battery terminal of the timer and opened PulseView. I selected the Nori power analyzer, now there was only one thing left to do, press start and use the device for a bit. What I saw completely explained why my batteries kept running out in a few weeks.

Its using a whopping 1.3 mA while idle! A typical 9V battery only contains 500mAh. Lets calculate the battery life.

idle_consumption = 1.3mA
battery_capacity = 500mAh
idle_hours = battery_capacity /  idle_consumption = 384.6 hours
idle_days = idle_hours / 24 = 16 days

Only 16 days!? Thats is extremely disappointing. The idle power consumption is way too high.

Lets check out the code (simplified example) that the microcontroller is running:

clock.SetMode(Nori::TimerMode::CTC)
    ->SetClockSource(Nori::ClockSource::Div256)
    ->SetLimit((uint16_t)31250)
    ->AttachInterrupt([]() { timer.Tick(); });

set_sleep_mode(SLEEP_MODE_IDLE);
while (true) {
    sleep_mode();
    set_sleep_mode(SLEEP_MODE_IDLE);
}

Wait a minute! maybe SLEEP_MODE_IDLE is not as efficient as expected. However SLEEP_MODE_IDLE is needed to wake the atmega using the timer. What happens if we only use the SLEEP_MODE_IDLE while the timer is running, and SLEEP_MODE_PWR_DOWN otherwise.

while (true) {
    if (timer.CanShutdown()) {
        set_sleep_mode(SLEEP_MODE_PWR_DOWN);
    } else {
        set_sleep_mode(SLEEP_MODE_IDLE);
    }
    sleep_mode();
}

Lets check the new power usage using the power analyzer.

Wow! only 0.011mA is used while the timer is idle now! Lets calculate the battery life using the new power consumption.

idle_consumption = 0.011mA
battery_capacity = 500mAh
idle_hours = battery_capacity /  idle_consumption = 45454.5 hours
idle_days = idle_hours / 24 = 1893.94 days
idle_years = idle_days / 365 = 5.18 years

That is an improvement of nearly 120 times the original battery life! (If the timer is idle at all times). To improve the battery life even further, I also disabled the backlight from the display.

Ofcourse everything is free and published online. The source files of the power analyzer can be found here.