ESP32-S31 pinout: every ESP32-S31-WROOM-3 pin, the boot pins, and which USB port is which

The WROOM-3 is 22.0 × 30.0 × 3.5 mm with pads on all four sides of a 0.85 mm grid. Pads 1 to 25 run down the left, 26 to 47 along the bottom, 48 to 72 up the right, and 73 to 94 across the top, just below the antenna. That top row is all ground except pad 91. Pad 95 in the middle is a ground pad made of nine squares, and pads 96 to 99 are the four corner grounds.

Names and functions from Table 3-1 of the module datasheet. I²C and UART1 to UART3 have no fixed pins: the GPIO matrix routes them to any free GPIO. The fixed functions below matter for Ethernet, SDIO, the camera and LCD interfaces, the ADC and touch.

Four pins are read when EN goes high and latched until power-down. Espressif asks for them to hold their level for at least 3 ms after EN rises; after that they work as normal GPIOs.

The S31 has two USB controllers on separate pins. Pick by what the port is for:

• IO33 (D−) and IO34 (D+), pads 42 and 43: the USB Serial/JTAG controller, connected by default. It is full speed (12 Mbit/s) and gives you a serial console, flashing and JTAG debugging with no extra chip. This is the pair to bring to a USB-C connector on a development board.
• USB_DP and USB_DM, pads 40 and 41: the USB 2.0 high-speed (480 Mbit/s) OTG controller on dedicated pins. Use it when the product itself is a USB device or host. It can also download firmware in download mode.

Espressif’s layout rules for both: route each pair as a 90 Ω ±10% differential pair at equal lengths, over a continuous ground plane, with as few vias as possible, and surrounded by ground copper. Leave room for 22 to 33 Ω series resistors close to the module on IO33 and IO34. A USB-C receptacle also needs its own 5.1 kΩ resistor on CC1 and on CC2: USB-C CC resistors explains why.

The bare ESP32-S31 is a QFN80, 8 × 8 mm, with 60 GPIOs numbered GPIO0 to GPIO61; there is no GPIO29 and no GPIO41. The flash interface (SPICS, SPIQ, SPIWP, SPIHD, SPICLK, SPID) is GPIO26, 27, 28, 30, 31 and 32, and Espressif ranks those six as already allocated. In the WROOM-3 they connect to the module’s quad SPI flash, which is why the module stops at 54. The PSRAM is a different story from the ESP32-S3: it is octal 1.8 V memory inside the chip package and its pins are not bonded out, so no variant loses a GPIO to it.

All are rated −40 to 85 °C ambient. The WROOM-3U is 22.0 × 24 × 3.5 mm with a connector for an external antenna in place of the PCB antenna, and the same pads, except that pad 91 is a second antenna pin (ANT2), disabled by default.

• Power: 3.3 V on pads 3 and 4 from a regulator that can deliver at least 0.6 A, with 22 µF and 0.1 µF at the module as in Espressif’s reference schematic.
• EN: never floating. An RC delay of 10 kΩ to 3V3 and 1 µF to ground lets the supply settle before the chip starts; add a reset button from EN to ground. If the supply rises slowly, for example from a battery charger, Espressif recommends a supervisor that resets the chip below about 3.0 V.
• Boot: a button from IO61 to ground. Leave IO60 and IO36 alone at reset, and do not put a circuit on IO37 that could float if you ever burn the JTAG eFuse.
• Flashing: USB-C to IO33 and IO34, or a USB-to-UART bridge to TX0 and RX0 (pads 68 and 69). The JTAG pins are IO54 to IO57 (pads 64 to 67), shared with the camera interface.
• Antenna: put the antenna end at the edge of the board, ideally hanging past it, with no copper on any layer under it. Espressif asks for at least 15 mm of clearance around the antenna inside the enclosure, and dense ground vias on the board next to it. Keep USB, the UART lines and crystals away from the antenna.
• Ground: pad 95 and the row of pads under the antenna are all ground; solder them to a solid plane.

The ESP32 PCB design guide walks through the same support circuit on a finished ESP32-S3 board, from the regulator to the antenna keep-out.

PCBEditor’s part library carries the ESP32-S31-WROOM-3 with Espressif’s own KiCad footprint, the same pads drawn in the figure above. The footprint’s antenna keep-out becomes a keep-out on your board that moves with the module: the copper pour, the router and the rules check keep copper out of it, and the KiCad export writes it as a rule area. The strapping, USB and power choices on this page are yours to make in the schematic; check them against the current datasheet revision before ordering.