Espressif Systems ESP32 Technical Reference Manual page 34

4.7 Pad Hold Feature
4.7 Pad Hold Feature
Each IO pad (including the RTC pads) has an individual hold function controlled by a RTC register. When the pad
is set to hold, the state is latched at that moment and will not change no matter how the internal signals change
or how the IO_MUX configuration or GPIO configuration is modified. Users can use the hold function for the pads
to retain the pad state through a core reset and system reset triggered by watchdog time-out or Deep-sleep
events.
4.8 I/O Pad Power Supply
IO pad power supply is shown in Figure 9.
• Pads marked blue are RTC pads that have their individual analog function and can also act as normal
digital IO pads. For details, please refer to Section 4.11.
• Pads marked pink and green only have digital functions.
• Pads marked green can be powered externally or internally via VDD_SDIO (see below).
4.8.1 VDD_SDIO Power Domain
VDD_SDIO can source or sink current, allowing this power domain to be powered externally or internally. To
power VDD_SDIO externally, apply the same power supply of VDD3P3_RTC to the VDD_SDIO pad.
Without an external power supply, the internal regulator will supply VDD_SDIO. The VDD_SDIO voltage can be
configured to be either 1.8V or 3.3V (the same as that at VRTC), depending on the state of the MTDI pad at reset
- a high level configures 1.8V and a low level configures 3.3V. Setting the efuse bit decides the default voltage of
the VDD_SDIO. In addition, software can change the voltage of the VDD_SDIO by configuring register bits.
Espressif Systems
Figure 9: ESP32 I/O Pad Power Sources
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4 IO_MUX AND GPIO MATRIX
ESP32 Technical Reference Manual V1.0