This document provides guidance on how to choose a resonator or quartz crystal in order to achieve reliable oscillation with the ST62 Microcontroller. The document first provides the major resonator parameters useful for a design. It then proposes measurement methods to ensure a safe oscillation.
This document provides guidelines for choosing resonators or quartz crystals to ensure reliable oscillation with the ST62 Microcontroller. It details the major resonator parameters required for design and proposes specific measurement methods to verify safe oscillation.
The note covers the Barkhausen criteria, start-up conditions, loop gain measurement, and frequency stability. It includes a specific test case for a CSA Murata crystal resonator with an ST6210xx microcontroller, providing calculated values and start-up time measurements.
| Resonator series resistance (rs) | < 60 ohms |
|---|---|
| Minimum amplifier gain | > 500 |
| Recommended load capacitance (4 MHz) | 2x30 pF |
| Calculated oscillation frequency (4 MHz target) | 4.03 MHz |
| Minimum safety margin (gain) | +13.6 dB |
| Maximum start-up time | < 1 ms |
The oscillator gain (BxG) must be greater than 1, and the amplifier gain must provide a sufficient gain margin of greater than 3 dB.
The ratio V4/Vo must be between +3 and +10 dB (1.5 to 3) to ensure the oscillator start is safe.
The start-up time can be accelerated by dividing the Cin capacitance in two.