Crystal Oscillator; External Clock Source (Compulsory); Esp32 Flash; Esp32 Crystal Oscillator - Espressif Systems ESP32 Series Manual

3.1 Schematics 3 SCHEMATICS AND PCB LAYOUT DESIGN
select the appropriate flash according to the power voltage on VDD_SDIO. Users can add a 0402 serial resistor
to Pin21 SD_CLK and connect it to the Flash CLK pin. The resistor can reduce driving current, eliminate crosstalk
and external interference, and adjust sequence.
Figure 6: ESP32 Flash

3.1.4 Crystal Oscillator

There are two clock sources for ESP32, that is, an external crystal oscillator clock source and an RTC clock
source.

3.1.4.1 External Clock Source (compulsory)

ESP32 can support 40 MHz, 26 MHz, and 24 MHz crystal oscillators. When using ESP32, please make sure you
select the right crystal oscillator frequency in the flash download tool. In circuit design, capacitors C1 and C2 are
added to the input and output terminals of the crystal oscillator, respectively. The values of the two capacitors can
be flexible, ranging from 6 pF to 22 pF. However, the specific capacitive values depend on further testing of, and
adjustment to, the overall performance of the whole circuit. Normally, the capacitive values of C1 and C2 are within
10 pF for the 26 MHz crystal oscillator. For 40 MHz crystal oscillator, C1 ⩾ 10pF , C2 ⩽ 22pF . The accuracy of
the crystal oscillator should be ±10 PPM. The operating temperature range should be from -20°C to 85°C.
Figure 7: ESP32 Crystal Oscillator
Espressif Systems 16 ESP32 Hardware Design Guideline V1.0