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

2. Schematic Checklist and PCB Layout Design
To avoid reboots caused by external interferences, the CHIP_PU trace should be as short as possible and routed
away from the clock lines. A pull-up resistor and a ground capacitor are highly recommended.
Notice:
CHIP_PU pin must not be left floating.
2.1.3 Flash
ESP32 can support up to four 16 MB external QSPI flash and SRAM chips. The demo flash used currently is an
SPI flash with 4 MB ROM, in an SOP8 (208 mil) package. The VDD_SDIO acts as the power supply pin. Make
sure to select the appropriate flash according to the power voltage on VDD_SDIO.
SCK/CLK
SCS/CMD
SHD/SD2 SWP/SD3
SDI/SD1
SDO/SD0
GPIO16
SDI/SD1
SDO/SD0
SCK/CLK
SCS/CMD
SWP/SD3
SHD/SD2
GPIO5
GPIO18
GPIO23
GPIO17
GPIO19
GND
VDD_SDIO
GND
VDD_SDIO
GND
VDD33
VDD33
C18
1uF
ESP32-D0WD
VDD_SDIO
26
GPIO16
25
GPIO17
27
SD_DATA_2
28
SD_DATA_3
29
SD_CMD
30
SD_CLK
31
SD_DATA_0
32
SD_DATA_1
33
GPIO5
34
GPIO18
35
GPIO19
38
GPIO23
36
VDD3P3_CPU
37
C4
0.1uF
U3 FLASH
/CS
1
DO
2
/WP
3
GND
4
DI
5
CLK
6
/HOLD
7
VCC
8
Figure 4: ESP32 Flash
2.1.4 Crystal Oscillator
There are two clock sources for the ESP32, that is, an external crystal oscillator clock source and an RTC clock
source.
2.1.4.1 External Clock Source (Compulsory)
Currently, the ESP32 Wi-Fi/BT firmware only supports 40 MHz crystal oscillator. In circuit design, capacitors C1 and
C2 which connect to the ground are added to the input and output terminals of the crystal oscillator respectively.
The specific capacitive values depend on further testing of, and adjustment to, the overall performance of the
whole circuit. It is recommended that users reserve a series resistor of 0 Ω on the XTAL_P clock trace to reduce
the drive strength of the crystal, as well as to minimize the impact of crystal harmonics on RF performance. Note
that the accuracy of the selected crystal is ±10 ppm.
Espressif Systems 5 ESP32 Hardware Design Guidelines V2.7