Key facts
- Brand
- STMicroelectronics
- Model
- AN2867
- Industry
- Industrial
- Documents
- 2
- Languages
- EN · ZH
- Last added
- 2012-08-07
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STMicroelectronics AN2867 Oscillator Design Guide for STM8S, STM8A and STM32F1 Microcontrollers
This document introduces the basics of the Pierce oscillator topology and explains how to properly design an oscillator circuit to avoid deployment delays caused by non-functional crystals. It details the theoretical principles required for stable operation, including feedback resistor roles, load capacitance calculations, drive level measurements, and startup time considerations.
The guide offers practical advice on selecting suitable external components and provides a list of recommended crystal oscillators and ceramic resonators specifically for STM32F1 and STM8A/S series microcontrollers. Additionally, it includes critical PCB layout hints to minimize stray capacitance and inductance, ensuring reliable oscillator performance.
All STMicroelectronics AN2867 documents by type
How to use the product for one specific task, with worked examples.
Questions answered in these manuals
What is the role of the feedback resistor RF in ST microcontrollers?
In most cases, RF is embedded to make the internal inverter act as an amplifier by biasing it at Vout = Vin and forcing it to operate in the linear region.
From: STMicroelectronics AN2867 Oscillator Design Guide for STM8S, STM8A and STM32F1 Microcontrollers
When is the external resistor RExt mandatory?
RExt is only mandatory if the dissipated power in the crystal exceeds the drive level specified by the crystal manufacturer; otherwise, its value is 0 Ω.
From: STMicroelectronics AN2867 Oscillator Design Guide for STM8S, STM8A and STM32F1 Microcontrollers
How do I calculate the external resistor RExt for an 8 MHz crystal?
Use the crystal’s load capacitance (CL2) to compute its reactance; for CL2 = 15 pF the calculated RExt is about 1326 Ω.
From: STMicroelectronics AN2867 Application Note Crystal Oscillator Circuit Design Guide
What feedback resistor values are recommended for a 32.768 kHz crystal?
A feedback resistor between 10 MΩ and 25 MΩ is recommended for 32.768 kHz crystals.
From: STMicroelectronics AN2867 Application Note Crystal Oscillator Circuit Design Guide