Clock Monitor Frequency Check
Overview
This sample uses the clock monitor API in
CLOCK_MONITOR_MODE_WINDOW mode: the hardware continuously compares a clock against
programmable thresholds and reports every crossing through a callback, which logs whether the
clock ran too fast or too slow. The main loop only prints a periodic heartbeat.
The window is centred on CONFIG_SAMPLE_EXPECTED_HZ, widened by
CONFIG_SAMPLE_TOLERANCE_PPM and refreshed every CONFIG_SAMPLE_WINDOW_NS. Leaving the
expected frequency at 0 derives the centre from the clockâs actual rate through the
clock control API.
Requirements
A clock monitor exposed through the
clock-monitordevicetree alias.FREQME-based boards also need samples/drivers/clock_monitor/clock_monitor.overlay.
Building and Running
west build -b frdm_mcxe31b samples/drivers/clock_monitor/check_freq
west flash
On a board using the FREQME peripheral, add the shared overlay:
west build -b frdm_mcxn236 samples/drivers/clock_monitor/check_freq -- -DEXTRA_DTC_OVERLAY_FILE="../clock_monitor.overlay"
west flash
Sample Output
[00:00:00.000,000] <inf> sample: clock check running (expected ~0 Hz, tolerance +/-50000 ppm, window 1000000 ns)
[00:00:05.000,000] <inf> sample: heartbeat: clock check running
[00:00:10.000,000] <inf> sample: heartbeat: clock check running
A clock drifting outside the window, or stopping, is reported from the callback:
[00:00:12.345,000] <err> sample: [cmu-fc@2bc000] monitored clock below lower threshold (or lost)