Why do STM32 have two crystal oscillators?

The STM32 microcontrollers typically include two crystal oscillator circuits (one high-speed external oscillator (HSE) and one low-speed external oscillator (LSE)) for several key reasons related to power efficiency, precision, and system flexibility:

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1. Different Clock Requirements for Different Functions

STM32 MCUs serve multiple roles that require different clock frequencies:

• High-Speed Clock (HSE - 4-48MHz)

  • Used for the CPU core, peripherals (USB, Ethernet, high-speed timers)

  • Provides high accuracy for time-critical tasks (e.g., USB communication, motor control PWM).

  • Often connected to an 8-25MHz crystal (e.g., 8MHz for STM32F4).

• Low-Speed Clock (LSE - 32.768kHz)

  • Used for the Real-Time Clock (RTC) and low-power modes.

  • Optimized for battery-backed operation (e.g., keeping time in sleep mode).

  • The 32.768kHz frequency allows easy division to 1Hz for RTC seconds counting.

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2. Power Efficiency

• HSE can be disabled in low-power modes (e.g., STOP or STANDBY) to save energy.

• LSE remains active in deep sleep (consuming <1µA) to maintain RTC functionality.

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3. Redundancy & Reliability

• If the HSE fails, the STM32 can switch to the internal oscillator (HSI) for fault tolerance.

• The LSE ensures the RTC keeps running even if the main system clock fails.

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4. Peripheral Flexibility

• Some peripherals require independent clocks:

  • USB OTG needs a 48MHz (±0.25% accuracy) clock (often derived from HSE via PLL).

  • CAN bus requires a precise clock for reliable communication.

  • RTC needs a stable 32.768kHz source for calendar/timekeeping.

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5. Internal vs. External Oscillators

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6. When Can You Skip One?

• LSE is optional if:

  • You don’t need an accurate RTC (use LSI instead).

  • The application runs only on internal oscillators (HSI/LSI).

• HSE is optional if:

  • The application doesn’t need USB/Ethernet or high-precision timing.

  • The internal HSI is sufficient (but less accurate).

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7. Example STM32 Clock Configuration

A typical setup in STM32CubeIDE might:

1. Use HSE (8MHz) → PLL → 72MHz CPU clock.

2. Use LSE (32.768kHz) → RTC for timekeeping.

3. Enable HSI (16MHz) as a backup.

c

// STM32 HAL Clock Initialization Example
RCC_OscInitTypeDef osc = {0};
osc.OscillatorType = RCC_OSCILLATORTYPE_HSE | RCC_OSCILLATORTYPE_LSE;
osc.HSEState = RCC_HSE_ON;       // Enable 8MHz crystal
osc.LSEState = RCC_LSE_ON;       // Enable 32.768kHz crystal
HAL_RCC_OscConfig(&osc);

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8. Key Takeaways

• HSE = High performance, precision (for core and fast peripherals).

• LSE = Low-power, RTC accuracy (for battery-backed timekeeping).

• Using both ensures optimal power/performance tradeoffs.

• If precision isn’t critical, you can rely on internal oscillators (HSI/LSI).

For USB, Ethernet, or CAN, the HSE is mandatory due to tight timing requirements. For wearables or battery-powered devices, the LSE is essential for low-power RTC operation.