What is the difference between using an internal vs. external oscillator in STM32?

 Using an internal vs. external oscillator in an STM32 microcontroller affects clock accuracy, stability, cost, power consumption, and design complexity. Here's a comprehensive breakdown of the differences:



1. Internal Oscillator (e.g., HSI, MSI, LSI)

What it is:

  • Built-in oscillator in the STM32 chip.

  • Common types:

    • HSI (High-Speed Internal): Typically 16 MHz

    • MSI (Multi-Speed Internal): Used in STM32L series (100 kHz to 48 MHz)

    • LSI (Low-Speed Internal): ~32 kHz, for watchdogs or low-power timing

Advantages:

FeatureDescription
No external componentsReduces BOM cost and PCB complexity
Lower powerEspecially MSI for low-power modes
Faster startup timeIdeal for wake-up from sleep modes
Space savingNo external crystal required

Disadvantages:

FeatureDescription
Lower accuracy±1% to ±3% frequency variation (temperature, voltage dependent)
Not suitable for USBUSB and precise UART require exact timing
Limited stabilityMay drift over time and temperature

2. External Oscillator (HSE, LSE)

What it is:

  • Requires connecting a crystal oscillator or resonator externally.

  • Types:

    • HSE (High-Speed External): 4–26 MHz, for system clock

    • LSE (Low-Speed External): 32.768 kHz, for RTC

Advantages:

FeatureDescription
High accuracyTypical crystals offer ±20 to ±50 ppm accuracy
Stable frequencyStable across temperature/voltage changes
Required for USB / RTCUSB, CAN, and real-time clock need accurate timing
Better for communicationUART, SPI, I2C require consistent baud rates

Disadvantages:

FeatureDescription
More expensiveExternal crystal and load capacitors required
Longer startup timeEspecially for LSE (slow crystal oscillation)
Board space and layoutRequires careful PCB design to minimize noise
Power consumptionSlightly higher compared to internal sources

Comparison Summary

FeatureInternal OscillatorExternal Oscillator
AccuracyLow (±1%–3%)High (±20–50 ppm)
CostLow (no components)Higher (crystal, capacitors)
StabilityVaries with temp/voltageVery stable
Startup TimeFastSlower (LSE especially)
USB Compatibility❌ Not recommended✅ Required
RTC CompatibilityLSI is usable but not accurate✅ LSE is preferred
Use in Low PowerMSI is optimizedLSE good for low-power RTC

When to Use Each

Use CaseRecommendation
General-purpose timing, low costInternal oscillator
USB, precise UART, or real-time clockExternal oscillator
Low-power wearable with RTCExternal (LSE)
Prototyping and early developmentStart with internal
Final product needing precise timingUse external

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