mohitmishra786/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)

5. Wake-up sources

| Source | Config | |--------|--------| | EXTI GPIO | SYSCFG + EXTI IMR, edge trigger | | RTC alarm | RTC WUT/WAKEUP | | UART | Start bit detection (some MCUs) | | IWDG | Not a wake source — resets device |

6. Measurement tips

• Use ammeter in series with VDDA/VDD; short sampling window for uA
• DWT cycle counter for wake latency benchmarking
• Compare run vs sleep current with same clock tree documented

7. RTOS note

FreeRTOS configUSE_TICKLESS_IDLE maps to WFI/Stop — coordinate with skills/embedded/freertos.

8. Agent usage

/low-power-embedded Configure STM32 Stop mode with EXTI0 wake and PLL restore

Common Problems

| Symptom | Cause | Fix | |---------|-------|-----| | Still mA in "sleep" | Debug interface active (SWD) | Disconnect debugger; disable DBGMCU | | Immediate wake from WFI | Pending NVIC IRQ | Clear flags before WFI | | Lost state after wake | Entered Standby not Stop | Use Stop if RAM must persist | | UART dead after wake | Clock tree not restored | Call SystemInit() path | | Higher than datasheet uA | Floating GPIO | Set unused pins analog or pull |

Related Skills

• skills/baremetal/interrupts-and-exceptions-baremetal — EXTI wake IRQs
• skills/baremetal/gpio-baremetal — pin mode for leakage control
• skills/baremetal/stm32-baremetal — RCC and PWR registers
• skills/baremetal/timers-pwm-baremetal — stop timers before sleep
• skills/embedded/freertos — tickless idle integration