Real-Time Clock (RTC) Drift Management#

Drift Characteristics with Internal Resistor-Capacitor (RC) and External 32 kHz Crystal (XTAL)#

RTC accuracy depends on the low-frequency clock (LFCLK) source:

  • RC 32 kHz oscillator

    • As specified in the data sheet, the typical frequency variation is about ±1.2% across temperature and voltage.

    • This is ~12,000 parts per million (ppm) (that is, up to ~1,030 seconds/day (approximately 17 minutes)).

    • Strongly temperature dependent and the drift may be in the order of tens of ppm of per °C.

    • Acceptable for watchdogs, coarse wake-up timers, or applications that can frequently resync.

  • XTAL 32.768 kHz oscillator

    • Combined accuracy (tolerance + error due to temperature variation + load error): ±250 ppm maximum.

    • Typical high-quality crystal (per AN1335 recommended parts): ±20 ppm → ~1.7 s/day drift.

    • Much better stability across Sleep cycles and temperature changes.

    • Recommended for real-time clocks, wireless sync, and low-drift applications.

Numerical Error Examples#

Clock Source

Accuracy (ppm)

Error per Hour

Error per Day

RC 32 kHz

±12,000 ppm (±1.2%)

~43 seconds

~17 minutes

XTAL 32.768 kHz

±250 ppm

~0.9 seconds

~21.6 seconds

XTAL 32.768 kHz (High-quality)

±20 ppm

~72 milliseconds

~1.7 seconds

Techniques for Managing RTC Drift#

  • Sleep Clock Calibrator (SCC) through the Calendar Calibration componentOnce enabled through the Calendar calibration service, the SCC hardware automatically and repeatedly measures the selected low-frequency (LF) clock (RC 32 kHz) against the 40 MHz reference and updates the RTC time base accordingly.

Note: The XTAL 32.768 kHz path does not require calibration. Use RC calibration only when operating without an external crystal.

  • Application-level resynchronization

    • Periodically synchronize the Calendar time to a trusted source such as the Network Time Protocol (NTP) server or a wireless beacon or time synchronization function (TSF).

    • Recommended for wirelessly connected devices that require accurate long-term timekeeping.

Drift Behavior across Sleep-Wake Cycles#

  • RTC continues in the Ultra-Ultra-Low-Power (UULP) domain across sleep.

  • With RC sources, drift increases if temperature or voltage changes while sleeping.

  • With XTAL, drift is stable and predictable (ppm level).

  • After Wake-Up, the calibrator can be run to re-adjust RC before resuming.