Power Consumption

Official Espruino boards come with one of two different types of processor:

• Nordic nRF52 - Puck.js, Pixl.js, MDBT42Q and Bangle.js
• ST STM32 - Original Espruino, Pico, and WiFi

nRF52

nRF52 devices are used in newer Espruino products. On these, Espruino is able to enter low power sleep modes automatically. Out of the box, power consumption is around 20uA (0.02mA) or over a year on a CR2032 battery.

Power usage varies slightly with the board used (based on voltage regulators and peripherals), so for detailed power usage stats see:

• Puck.js Power Consumption
• Pixl.js Power Consumption
• MDBT42Q Power Consumption
• Bangle.js Power Consumption
• Bangle.js 2 Power Consumption

STM32

Espruino on STM32 can run in one of 3 different modes.

Note: Standby mode is available on the STM32 chip (very low power, but all data is lost from RAM). It is not currently used in Espruino.

Sleep

This is the normal low-power mode for Espruino on STM32. You don't have to do anything to enter this at all, Espruino will enter this mode whenever it isn't doing anything.

Deep Sleep (Stop)

This is the best low-power mode in Espruino on STM32, which involves turning off the main clock (which stops all peripherals) and waking only when an external pin changes state or after a set amount of time. It is only enabled when you run setDeepSleep(1) (you can turn it off with setDeepSleep(0)). It's not enabled by default because:

• Espruino can't be woken by Serial/USART traffic (and will not receive data while in Deep Sleep). For instance you would be unable to use Deep Sleep if you were using the Bluetooth module and wanted to react to messages from it. To work around this, you'd need to implement RTS/CTS flow control in software (waking Espruino on RTS, and only setting CTS after setDeepSleep(0) has been called).
• All external peripherals (including timers for PWM) will stop. Espruino does not currently detect if any of these timers are in use before entering Deep Sleep.

Conditions for Deep Sleep

For deep sleep to work, you must:

• Have called setDeepSleep(1)
• Not be connected to USB
• Not have any data waiting to be sent down Serial or USB
• Have no pending callbacks from setIntervals/setTimeout that are less than 1.5 seconds away (this is less on the Pico). Espruino uses the real time clock for wakeups, and the RTC can only wake up on a second by second basis.
• Not have any Waveforms running, and not have a digitalPulse(...) command that is yet to finish

Note: Espruino won't enter deep sleep as soon as you execute setDeepSleep(1). It'll wait until all the bullet points above are satisfied, and then it'll enter it. For example you can type setDeepSleep(1) while you're connected to your PC via USB, and Espruino will only enter deep sleep mode once you unplug from USB.

setDeepSleep(...) sets a flag internally - so you only need to call it once (not each time you want to go to sleep).

Examples

You can wake after a set amount of time - for example to flash an LED every 60 seconds:

setInterval(function() {
  digitalWrite(LED1, 1);
  setTimeout(function () {
    digitalWrite(LED1, 0);
  }, 20);
}, 60000);
setDeepSleep(1);

or you can wake when an external pin changes state - for instance to flash an LED when pressing the button:

setWatch(function() {
  digitalWrite(LED1, 1);
  setTimeout(function () {
    digitalWrite(LED1, 0);
  }, 20);
}, BTN, true);
setDeepSleep(1);

Debugging Sleep

You can call functions to help with debugging:

• setSleepIndicator(LED1) will light LED1 whenever the device is not sleeping, allowing you to make sure it is sleeping as much as possible.
• setBusyIndicator(LED2) will light LED2 when Espruino is not sleeping and is busy executing JavaScript

There is currently no way to see when Espruino has entered Deep Sleep.

Note that Espruino won't go to sleep when it is connected via USB, as it knows it has ample power available.

If you don't have a dedicated Power Usage meter, the best way to measure power draw is to connect a multimeter in series with a battery and power Espruino from that. You'll then get a good idea of exactly how much power is being used.

If Espruino is not sleeping, check that you haven't called setInterval with a small timeout. For instance you may have been controlling model aircraft servos or something similar, but if they are already in the correct position you can call clearInterval(...) to ensure that Espruino is not having to wake up as often.

Saving Power

Bear in mind that lighting just one LED light can use more power than the entire CPU. Don't turn on LEDs when sleeping, and be sure that Espruino isn't powering anything externally. If you've used outputs, you may be able to set them to 'float' either by reading the value with digitalRead or by using pinMode(pin, "input") to change the state of the pin to input.