SWRU626 December 2025 CC3501E , CC3551E
The Slow Clock is the initiator clock of the SoC system, and the only active clock in the sleep modes for power saving optimization. The Slow Clock supports several options (modes) of clock sources for different solutions according to relevant use cases and cost wishes.
To enable those modes Slow Clock uses the two Analog IPs: LFXT for the external clock options, and LFOSC for the internal clock option. The default Slow Clock mode is the Internal Slow Clock (generated by LFOSC IP), and in the boot sequence one of the modes is selected for the continuation operation.
The 4 slow clock modes (sources):
Generates 256kHz clock by dedicated analog IP.
32kHz clock generated by SOC PLL (320MHz) from fast clock (see Section 7.3.1). Used for accuracy improvement during active power mode.
LFXT analog IP generated 32kHz clock using an external crystal (XTAL) connected to LFXT_P and LFXT_N pins.
An external device or dedicated one-pin oscilator generates a 32kHz clock, connected to LFXT_P pin.