Run-Time Calibration of Watch Crystals
For watch and timekeeping applications, 32.768 kHz crystals with an accuracy close to 20 ppm are common, but 20 ppm translates to a ±0.65536 Hz frequency deviation, or a whopping 51.8 seconds error per month. This error only accounts for variation in crystal properties. Other significant sources include temperature, aging, component selection and layout.
In this application note, we discuss errors associated with low-cost watch crystals used in Real-Time Clock and Calendar (RTCC) applications and methods to overcome these errors. We also discuss a unique built-in calibration feature in Microchip Technology’s Real-Time Clock and Calendar circuits, which minimizes these errors during run time.
SOURCES OF CRYSTAL ERROR
Cross cut (X-Cut) crystals are the most common type of crystal used in (RTCC) circuits. These crystals are inexpensive, readily available and reasonably accurate.
The following are the most common factors leading to oscillator errors in crystal sources:
- Mechanical Vibration
- Load Capacitor
Mechanical vibration should be avoided to minimize crystal errors. If possible, we need to move all vibration sources away from the crystal. Potential vibration sources include buzzers, speakers, motors and so on.