# Clocking

The following oscillators are available as sources for the system clock used for active operation of the system:

- 48 MHz crystal oscillator (XTALCLK), typically used in Run Mode when RF operation is required. Prescalers exist to provide divided clocks (including system clock) to other parts of the system.
- A fast RC oscillator (RCCLK) can provide an alternative to the 48 MHz crystal oscillator. However, RF operation is not possible using the fast RC Oscillator.

The following oscillators are available as sources for the standby clock used for system timing:

- A 32 kHz crystal oscillator (XTAL32K) typically used in Sleep and Standby Modes for precision timing and to maintain the real-time clock (RTC)
- A standby RC oscillator (RC32) that meets the Bluetooth Low Energy sleep clock accuracy requirements, which can be an alternative to the 32 kHz crystal oscillator; see RC Oscillator

A built−in clock detector ensures a proper system reset in case the system clock goes below 2 kHz.

Flexible clock management allows the different clock sources to be used in power−efficient ways, and minimizes the use of external components. Internal RC oscillators can be used for fast startup, and then easily be switched out for crystal oscillators when precision timing is required. The RC32 32 kHz RC oscillator can meet the Bluetooth Low Energy Sleep Clock accuracy requirements, which can alleviate the need for an external 32 kHz crystal.

The system clock can also be sourced from external sources.

For more information on clock generation and distribution in the RSL15 SoC, see Clock Generation.

For HAL firmware information related to clocking, see the RSL15 Firmware Reference: Clock Configuration.

## RTC

The RTC timer consists of a 32-bit free-running up-counter, clocked by the standby clock.

For HAL firmware information related to the RTC, see the RSL15 Firmware Reference: Real-Time Clock Function Documentation.