low-power-embedded

Low-Power Embedded

Purpose

Guide agents through MCU low-power modes: sleep vs deep sleep (stop/standby), peripheral and bus clock gating, wake-up source configuration, and practical current measurement — with Cortex-M WFI/WFE and vendor PWR examples (STM32-focused patterns apply broadly).

When to Use

• Battery-powered firmware missing power budget
• Wake-up latency vs consumption tradeoffs
• Debugging "device won't wake" or "current still mA in sleep"
• Before shipping RTOS idle hook or bare-metal main loop sleep
• Cross-linking with skills/baremetal/interrupts-and-exceptions-baremetal

Workflow

1. Power mode hierarchy (STM32-style)

Mode	CPU	Peripherals	RAM	Wake source	Relative current
Run	on	on	on	—	highest
Sleep	off	on	on	any IRQ	medium
Stop	off	most off	on	EXTI, RTC, UART	low
Standby	off	off	lost*	WKUP pins, RTC	lowest

* Standby clears most SRAM; use backup domain or external EEPROM for state.

2. Enter Sleep (WFI)

/* Cortex-M — sleep until interrupt */
__disable_irq();
/* configure wake source (e.g. EXTI, RTC alarm) */
__enable_irq();
__WFI(); /* or __WFE() for event-based wake */

Ensure pending interrupts are cleared before WFI or wake may be immediate.

3. STM32 Stop mode pattern

#include "stm32f4xx.h"

void enter_stop_mode(void)
{
    /* Gate clocks you do not need */
    RCC->AHB1ENR &= ~RCC_AHB1ENR_GPIOAEN; /* example — only if safe */

    PWR->CR |= PWR_CR_CWUF;   /* clear wake flags */
    PWR->CR |= PWR_CR_PDDS;   /* deep sleep = Stop */
    SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;

    __WFI();

    /* After wake: re-enable HSE/PLL — clocks lost in Stop on many parts */
    SystemInit();
}

After Stop, re-init clocks and peripherals that lost their registers.

4. Clock gating checklist

Before sleep
├── Disable unused peripheral clocks (RCC xENR)
├── Disable ADC/DAC continuous modes
├── Stop DMA channels
├── Enter peripheral low-power (UART mute, SPI off)
└── Configure lowest viable regulator scale (Voltage Scale 2/3)