The RTC primarily matches a 32.768kHz tuning fork crystal. The frequency stability of tuning fork crystals is significantly affected by temperature. Using 25℃ as a baseline, the further the temperature deviates, the greater the frequency drift. For example, at -10℃ or 60℃, the temperature drift can reach -49ppm, leading to increased timing errors.
Specific effects include:
Frequency-Temperature Characteristics: The frequency-temperature curve of a tuning fork crystal (e.g., 32.768kHz) is a negative quadratic parabola, centered at 25℃. Increases or decreases in temperature cause a decrease (slower) in frequency. For example, when deviating from 25℃, the frequency deviation increases quadratically.
Temperature Drift Quantification: Within the range of -10℃ to 60℃, the typical temperature drift reaches -49ppm. At industrial-grade temperatures (-40℃ to 85℃), the temperature drift can reach as high as 151ppm, potentially increasing timing errors from 10 minutes to 1.3 hours per year.
Cut Influence: The temperature drift coefficient of common XY-cut tuning fork crystals is -0.035ppm/℃², and that of surface-mount type is -0.04ppm/℃². Different cuts (such as NT-cut) can optimize temperature drift, but the tuning fork structure itself has poor adaptability to a wide temperature range.
To reduce the impact of temperature, temperature compensation (such as TCXO) or a cut with smaller temperature drift (such as AT-cut crossover to achieve 32.768kHz) should be used. However, the inherent characteristics of tuning fork crystals limit their application in wide temperature ranges.
