Espressif Systems ESP32 Technical Reference Manual page 28

3.2 System Clock 3 RESET AND CLOCK
3.2.4.6 Clock Source Considerations
Most peripherals will operate using the APB_CLK frequency as a reference. When this frequency changes, the
peripherals will need to update their clock configuration to operate at the same frequency after the change.
Peripherals accessing REF_TICK can continue operating normally when switching clock sources, without
changing clock source. Please refer to Table 11 for details.
The LED PWM module can use RTC8M_CLK as a clock source, meaning that it can be used when APB_CLK is
disabled. In other words, when the system is in low power consumption mode (refer to power manager module),
normal peripherals will be halted (APB_CLK is turned off), but the LED PWM can work normally via
RTC8M_CLK.
3.2.5 Wi-Fi BT Clock
Wi-Fi and BT can only operate if APB_CLK has PLL_CLK as its clock source. Suspending PLL_CLK requires
Wi-Fi and BT to both have entered low power consumption mode first.
For LOW_POWER_CLK, one of RTC_CLK, SLOW_CLK, RTC8M_CLK or XTL_CLK can be selected as the low
power consumption mode clock source for Wi-Fi and BT.
3.2.6 RTC Clock
The clock sources of SLOW_CLK and FAST_CLK are low frequency clocks. The RTC module can operate when
most other clocks are stopped.
SLOW_CLK is used to clock the Power Management module. It can be sourced from RTC_CLK, XTL32K_CLK
or RTC8M_D256_CLK
FAST_CLK is used to clock the On-chip Sensor module. It can be sourced from a divided XTL_CLK or from
RTC8M_CLK.
Espressif Systems 27 ESP32 Technical Reference Manual V1.0