This application note provides guidelines for designing Pierce oscillators, selecting crystals, and optimizing PCB layouts for STM32 and STM8A/S…
This document introduces the basics of Pierce oscillators and explains how to determine the necessary external components for a stable design. It covers oscillator theory, the specific requirements for feedback resistors and load capacitors, and methods for calculating drive levels and external resistors.
The guide also includes an easy selection guideline for suitable crystals and external components, lists recommended crystal part numbers for specific applications, and offers practical hints for PCB layout to ensure reliable operation.
| Recommended feedback resistor range for 32.768 kHz | 10 to 25 MΩ |
|---|---|
| Recommended feedback resistor range for 1 MHz | 5 to 10 MΩ |
| Recommended feedback resistor range for 10 MHz | 1 to 5 MΩ |
| Recommended feedback resistor range for 20 MHz | 470 kΩ to 5 MΩ |
RF is embedded in the oscillator circuitry to make the internal inverter act as an amplifier by biasing it at Vout = Vin.
RExt is mandatory only if the dissipated power in the crystal exceeds the drive level specified by the crystal manufacturer.