from ESP documentation
3.3 System Clocks
3.3.1 CPU Clock
Upon reset, an external crystal clock source (2 MHz ~ 60 MHz), is selected as the default CPU clock. The external
crystal clock source also connects to a PLL to generate a high frequency clock (typically 160 MHz).
In addition to this, ESP32 has an internal 8 MHz oscillator, of which the accuracy is guaranteed by design and
is stable over temperature (within 1% accuracy). Hence, the application can then select from the external crystal
clock source, the PLL clock or the internal 8 MHz oscillator. The selected clock source drives the CPU clock,
directly or after division, depending on the application.