ESP32 Forum

The chip I am using is esp32s3.
I've checked the idf-framework and technical reference, and I found a register named SYSTEM_SOC_CLK_SEL. SYSTEM_SOC_CLK_SEL is XTAL_CLK by default. So would external 40MHz crystal oscillator stop running if I change this register to RC_FAST_CLK or PLL_CLK?

No. A bunch of other stuff (PLL, radios, APB bus, ...) other than the CPU runs off the Xtal (directly or indirectly), so it will have to keep running.