• Mechanical Vibration
• Load Capacitor
• Temperature
• Age

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. For resonance at the correct frequency, the crystal should be loaded with its specified load capacitance, which is the value of capacitance used in conjunction with the crystal unit. Load capacitance is a parameter specified by the crystal manufacturer; typically expressed in pF. A mismatched load capacitor can contribute to an error of up to almost 400 ppm. It is important to consider capacitor value due to parasitic capacitance of the PCB traces and other crystal leads.
Temperature affects crystal frequency and contributes significantly to crystal errors. Many crystals are designed to center the inflection in error near the room temperature. A typical crystal error doubles in as little as 20°C (degree Celsius) variation.
Temperature affects crystal frequency and contributes significantly to crystal errors. Many crystals are designed to center the inflection in error near the room temperature. A typical crystal error doubles in as little as 20°C (degree Celsius) variation.
The error due to temperature and aging presents a significant challenge to a system designer. Even though a high-quality crystal with properly matched capacitors may be used, along with the best layout practices, they do not account for temperature or aging. This is due to the fact that these factors are unknown during the design process, and hence, must be taken care of during its run-time execution.
Timing errors, due to aging or temperature variations, are typically very slow in nature and will not abruptly change the crystal frequency. By characterizing their effects, time could be adjusted in the software. This can, however, complicate the RTCC routines since large counters are needed to apply these adjustments at the correct time.
To counter the drift caused by the above sources, Microchip’s PIC24F RTCC has an automatic calibration feature. It features a software writable register, capable of compensating for up to 260 ppm crystal error, which is sufficient to counter typical crystal error due to mismatched load capacitor, change in temperature, etc., without adding a significant software overhead during run time. This is a unique feature since most off-the-shelf RTCC solutions do not support run-time calibration.

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