Crystal Calibration with GPS Redux

Stuart revisits an article from 2020 with a different approach, exploring the calibration of microcontroller clock crystals using GPS technology, aimed at enhancing timing accuracy in various applications. Key findings highlight the importance of calibration for long-term timing precision, detailing methods to achieve significant improvements in accuracy.

- By Stuart Ball

I described a method of calibrating a 32kHz clock crystal in a TM4C1233D5 microcontroller (MCU) with a GPS module in my article, “Calibrating an MCU’s RTC Using GPS: Put Time on Your Side” (Circuit Cellar 360, July 2020) [1]. The Texas Instruments (TI) TM4C parts have an internal 32KHz oscillator for implementing a real-time clock (RTC). This new approach does the same thing but with a more generalized method that applies to MCUs without a dedicated RTC oscillator and calibration register.

CRYSTAL ACCURACY

A typical crystal, as used in an MCU, has some tolerance, measured in parts per million (ppm). So the frequency of a 14.7456MHz crystal, as I’m using in this design, will vary. It’s exactly like the tolerance in a resistor or capacitor, but the tolerance of resistors and capacitors is measured in percent, or parts per hundred. Crystals are more accurate, so their tolerance is measured in ppm; 1ppm is equivalent to 0.0001%. Just like resistor/capacitor tolerance, the crystal frequency can be higher or lower than the nominal frequency.

To convert this to practical numbers, if the 14.7456MHz crystal I used in this design has a tolerance of ±30ppm, then the frequency variation is ±442Hz (14.7456MHz x 30/1,000,000). Therefore, the frequency of any given part may vary from 14.745158MHz (14,7456,000 - 442) to 14.746042MHz (14.7456,000 + 442).

WHY CALIBRATE?