The Real Time Clock (RTC) in Cypress nvSRAM is driven by a crystal oscillator with an external crystal having a nominal frequency of 32.768 kHz. Most 32.768 kHz crystals have a frequency tolerance of ±20 ppm at +25°C. This frequency tolerance will result in timing error over a long period of time. For instance, a crystal with 20 ppm error in nominal frequency would drift the clock accuracy by 52 seconds in a month. This drift can be compensated by using the calibration feature available in the RTC circuit in the nvSRAM. Another crystal parameter which affects crystal clock accuracy is the parabolic curvature constant, k, which changes the crystal frequency with temperature. The parabolic curvature constant, k, is typically –0.036 ppm/°C2. If the temperature of operation is known, the RTC calibration can be done to compensate the RTC clock frequency due to temperature drift.
