Power and Low-Power Firmware

Average current and the energy budget

A battery holds a fixed amount of charge, measured in milliamp-hours (mAh). The life of a device is that capacity divided by the average current it draws, so a device that sleeps most of the time and wakes briefly can last for years on a small cell, and the average is dominated by whichever of the two states uses more charge.

I_avg = Σ (I_state × t_state) ÷ T    life (h) = usable capacity (mAh) ÷ I_avg (mA)

Sleep modes

Platform	Typical sleep modes and currents	Wake sources
ATmega328P (3 V)	Idle ≈ 0.4 mA (CPU off, peripherals on); power-save ≈ 1 µA with a timer; power-down ≈ 0.1 µA (≈ 4 to 6 µA with the watchdog running)	Pin change, external interrupt, watchdog, timer 2
STM32L4 / L0	Sleep ≈ 1 to 3 mA; stop mode ≈ 1 to 3 µA with RAM retained; standby ≈ 0.3 µA	RTC alarm, pin, UART, low-power timer
nRF52 (BLE)	System ON idle ≈ 2 µA; System OFF ≈ 0.3 µA	RTC, GPIO, radio events
ESP32	Light sleep ≈ 0.8 mA; deep sleep ≈ 10 µA (RTC timer only); hibernation ≈ 5 µA	RTC timer, touch, EXT0 / EXT1 pins, ULP coprocessor
RP2040	Dormant ≈ 0.8 mA board-level (more in practice on Pico boards because of the regulator and flash); dormant on bare chip tens of µA	GPIO, RTC

These are chip figures from datasheets and vary with voltage, temperature and exactly what is left on. The board matters: a development board’s USB chip, power LED and regulator often draw milliamps and hide the chip’s microamps. Measure your own hardware.

Duty cycling: the biggest lever

The average current falls almost in proportion to the fraction of time the device is awake, until the sleep current becomes the floor. Using the node above (20 mA for 100.0 ms, 10 µA asleep, 1 600 mAh usable):

Wake interval	Average current	Calculated life
1 s	2.009 mA	0.1 years
10 s	0.210 mA	0.9 years
60 s	0.043 mA	4.2 years
600 s	0.013 mA	13.7 years
3 600 s	0.011 mA	17.3 years

Beyond about ten years the battery’s own self-discharge (1 to 3 % a year for lithium primary cells, much more for rechargeables) and aging set the real limit, so there is no point in saving the last microamp: design to meet the life you need, then stop.

Wake-up design in firmware

• Sleep as soon as the work is done and wake only on a timer (an RTC or the watchdog timer) or an event (a pin change). Never poll in a busy loop.
• Do the work in the wake handler or the main loop, then go back to sleep: a state machine whose idle state sleeps.
• Turn off what you do not use before sleeping: the ADC, the radio and unused serial ports; switch off sensors with a MOSFET or their own enable pin.
• Wake the slow clock domain: a low-power 32.768 kHz crystal and RTC keep time for microamps; the fast oscillator takes time to restart (up to milliseconds for a crystal), which is energy too.
• Keep state across sleep: deep sleep modes lose RAM (except a small retained area); store what you need in retained memory or non-volatile memory (see Memory, Number Formats and Reliability).
• Handle the wake race: an event that arrives just before you sleep must still wake you; check the flag with interrupts disabled before the sleep instruction.

// ATmega328P: sleep until a pin or the watchdog wakes us (avr/sleep.h)
#include <avr/sleep.h>
#include <avr/power.h>

void sleep_until_event(void) {
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  cli();
  if (!event_pending) {              // the race check: only sleep if nothing arrived meanwhile
    power_adc_disable();             // switch off unused peripherals
    sleep_enable();
    sei();                           // the instruction after sei() still runs before the interrupt
    sleep_cpu();                     // ~0.1 uA here; wakes on the interrupt
    sleep_disable();
    power_adc_enable();
  } else {
    sei();
  }
}

Regulators and quiescent current

A linear regulator (LDO) drops the extra voltage as heat; a buck converter switches and wastes much less. But in sleep the regulator’s own quiescent current (I_q) can exceed the MCU’s.

Clock speed, voltage and “race to sleep”

A CMOS chip’s dynamic power is about P = C V² f, so it scales with the clock frequency and the square of the voltage. But power is only half the story: the energy for a fixed task is power times time, and a faster clock finishes sooner. If the chip can then sleep deeply, running fast and sleeping longer (race to sleep) is usually the better choice.

Hidden drains: pins, pull-ups and peripherals

• Floating inputs draw extra current as they hover between thresholds: configure unused pins as outputs low or as inputs with a pull-up (consult your chip’s low-power guidance).
• Pins connected to something powered: an output driving an LED, a sensor, or another chip that is off can leak current through its protection diodes; set pins to inputs or low before turning a peripheral’s power off.
• Peripherals left on: the ADC, brown-out detector, watchdog, internal reference, analog comparator and timers each draw microamps; disable them for deep sleep (the brown-out detector is the one you may need to keep, see below).
• External parts: voltage dividers (see the battery divider in Analog Input and Signal Processing), sensors, level shifters, the USB-serial chip and LEDs: each may draw more than the MCU.
• Leakage in capacitors and boards: electrolytic capacitors leak microamps, and a dirty board adds more. Use low-leakage ceramic or film capacitors in a microamp design.

Radios: Wi-Fi, BLE and LoRa

Radios dominate the energy budget of a wireless node: transmitting costs tens to hundreds of milliamps. Minimize how long the radio is on, how often, and how many bytes it sends.

Radio	Typical active current	Notes
Wi-Fi (ESP32)	Transmit 120 to 240 mA; receive 80 to 100 mA; connection takes 0.5 to 3 s	Connecting is the expensive part: use a fixed channel and static IP, keep the BSSID, or send one UDP packet
Bluetooth LE	Peak 5 to 15 mA, ≈ 0.01 to 0.05 mA averaged at slow advertising	Built for coin cells; use long connection intervals and slave latency
LoRa / LoRaWAN	Transmit 40 to 120 mA for 0.05 to 2 s per message; receive 10 mA	Miles of range at a few messages an hour; watch the duty-cycle limits
Cellular (LTE-M, NB-IoT)	Hundreds of mA peaks; seconds per connection	PSM and eDRX modes sleep for hours at microamps
Zigbee, Thread, Z-Wave	10 to 40 mA transmit	Mesh networks with sleepy end devices

Batteries, brown-out and low-battery handling

Cell	Voltage range	Notes
Alkaline AA/AAA	1.6 V fresh to 0.9 V	Two cells give 3.2 to 1.8 V: check the MCU’s minimum voltage; cheap; capacity falls at high current
Lithium primary (CR2032, AA lithium)	3.0 V flat; 1.5 V per AA lithium	Low self-discharge (about 1 % a year), long shelf life; coin cells cannot supply high pulses: add a capacitor
Li-ion / LiPo	4.2 V full, about 3.7 V nominal, 3.0 V empty	Rechargeable; needs a protection circuit and a charger; never discharge below the cut-off
LiFePO4	3.65 V full, 3.2 V nominal, 2.5 V empty	Safer chemistry; flat curve; direct 3.3 V supply possible
NiMH	1.4 V full, 1.2 V nominal, 1.0 V empty	Self-discharge of 1 to 2 % per day for standard cells (low-self-discharge cells are better)

• Brown-out detection: the MCU resets (or interrupts) when the supply falls below a threshold (here 2.7 V), instead of running erratically; it draws a microamp or so, which is a small price. Set the threshold above the voltage at which the chip’s flash and clock stop being reliable at your speed.
• Low-battery warning: measure the battery voltage (the internal reference trick in Analog Input and Signal Processing), average it (a sag during a radio transmission is normal), and warn well above the cut-off. A Li-ion cell is nearly empty at 3.0 V, but its voltage curve is flat between about 3.7 and 3.9 V, so the voltage is only a rough gauge of charge.
• Pulse loads: a coin cell’s internal resistance (10 to 30 Ω) makes the voltage sag under a radio burst: a 100 µF capacitor or a hybrid capacitor supplies the pulse.
• Safe shutdown: on a low battery, save the data, switch off the loads, and sleep until the supply recovers or is replaced.

Measuring low currents

Related

• Embedded Systems: the overview of this guide.
• Wireless, Networking and Updates: Wi-Fi, BLE and MQTT with minimum on-time.
• Firmware Architecture, State Machines and RTOS: state machines and tickless RTOS operation.
• Batteries and Power Supplies and Battery Runtime and Pack Calculator.
• Microcontroller Electronics: the hardware side of low power.

FAQ

How long will my project last on a battery?

Divide the usable capacity (derated to about 80 %) by the average current. Measure the average over a complete wake-and-sleep cycle; the sleep current and the awake time are usually the two numbers that matter.

Why does my “10 µA” device draw 5 mA?

The development board’s other parts: the USB-serial chip, power LED, regulator quiescent current, and any pull-ups, sensors or peripherals still powered. Measure the bare design, or remove those parts.

Is it better to run slowly or to sleep?

Usually race to sleep: finish the work quickly and spend the rest in a deep sleep. But if the CPU must wait for a slow peripheral, a slower clock saves energy. Measure both on your own hardware.

Can a coin cell power a Wi-Fi or Bluetooth device?

BLE yes, if you advertise slowly and use a capacitor for the pulses. Wi-Fi draws too much current for a coin cell: use a larger lithium or alkaline pack, or an energy-harvesting design.

References

• Microcontroller datasheets and low-power application notes (Microchip AVR, STMicroelectronics STM32L, Nordic nRF52, Espressif ESP32 sleep-mode current tables).
• Battery makers’ datasheets (Energizer, Panasonic, Saft) for capacity, pulse performance and self-discharge.
• Nordic Semiconductor, Online Power Profiler; Joulescope documentation (measuring dynamic currents).

Last reviewed: October 2026. Currents are typical datasheet values for the parts named: they vary with voltage, temperature and configuration, so measure your own board.