ESP32 power saving: Light Sleep during busy wait

[mvoss96]

Hi,
When using an E-paper Display with an esp32 it uses a lot of power while waiting for the busy pin to go low.

Currently the code in _waitWhileBusy() is more or less the following:

while(1)
{
 if (digitalRead(_busy) != _busy_level) break;
 else yield;
}

i propose to change it to something like this:

// Go to light_sleep until the busy pin goes low
gpio_wakeup_enable((gpio_num_t)_busy, GPIO_INTR_LOW_LEVEL);
esp_sleep_enable_gpio_wakeup(); 
esp_light_sleep_start();
esp_sleep_disable_wakeup_source(ESP_SLEEP_WAKEUP_GPIO);

I tested this with an esp32-c6 and a WeAct 4.2 E-paper Display and it reduced the average power consuption while waiting for the busy signal from 50mA to <10mA (tested using a Nordic Power Profiler Kit II)

[G6EJD]

Your battery must be really small if saving ~0.0083mAh is significant;assuming 10-secs of wait!

It would be more energy saving to slow the CPU down to 10MHz while it waits.

[mvoss96]

Thanks for your answer,
I tried reducing the CPU clock to 10 MHz during wait; however, it still consumes significantly more power than light sleep.

In my application, I am using a 1200 mAh LiPo battery with one partial screen refresh per minute. The ESP enters deep sleep for 60 seconds and then remains awake for about 1.5 seconds for a fast refresh, of which approximately 0.9 seconds are spent waiting for the busy signal.

I measured the average consumption over the entire sleep/wake cycle: With the current library code, the device consumes 1.2 mA on average, while with the light sleep modification it consumes only 0.4 mA—effectively tripling the battery runtime.

[G6EJD]

Understood. You can monitor the busy pin from within your code whilst checking to status periodically from light-sleep, so really no library amendment is required.