ESP32 and ESP32-WROOM-32E: pinout, strapping pins, ratings and a minimal circuit

"ESP32" is printed on three different things. The chip is a 5 × 5 mm QFN with 48 pins and a ground pad; the current one is the ESP32-D0WD-V3: dual core, no flash inside, chip revision v3.0 or v3.1. The module is that chip under a metal shield with its 40 MHz crystal, the flash and an antenna, on a small board with 38 castellated pads; the one to buy is the ESP32-WROOM-32E. The dev board is a module plus a USB-serial chip, a regulator and two buttons, with pin headers. Its header labels are the module's GPIO numbers, so the module pad map below is the one to read.

The suffix on a WROOM-32E is the memory: -N4, -N8 and -N16 carry 4, 8 or 16 MB of flash, an R2 adds 2 MB of PSRAM (and then GPIO16 belongs to the PSRAM), and -H4 or -H8 are rated to 105 °C instead of 85 °C. The WROOM-32UE is the same module with an antenna connector. The older ESP32-WROOM-32 has the same pads but an older chip, and Espressif no longer recommends it for new designs. Espressif makes all of them; there is no second maker of the ESP32 to compare.

The module has 38 pads at 1.27 mm pitch plus a ground pad underneath, on a board 18.0 × 25.5 mm. The pitch is half that of a breadboard, so the module needs a breakout or its own PCB. Pads 17 to 22 and 32 are not connected: inside the module, GPIO6 to GPIO11 go to the flash and are not brought out (module datasheet p. 12).

Of the 26 GPIOs on the pads, 4 are inputs only (pads 4 to 7), 5 are strapping pins, and GPIO1 and GPIO3 are the serial port that loads the firmware and prints the boot log. That leaves 15 pins with no job at reset (Figure 4). Pad 14, GPIO12, is the one to treat with care: the reference schematic on p. 39 prints "IO12 should be kept low when the module is powered on".

The bare chip is for your own RF board, with a crystal, flash and a matched antenna line; most projects never touch it. Its 48 pins count anticlockwise from pin 1 at the top left, 14 down each side and 10 across the top and bottom, with the ground pad as pin 49. Here GPIO6 to GPIO11 come out as the SD_ pins 28 to 33, and they are the bus to the external flash.

The chip has four supply domains: VDDA and VDD3P3 for the analog and radio parts, VDD3P3_RTC for the RTC pins, VDD3P3_CPU for the digital pins, and VDD_SDIO, an output that feeds the flash at either 1.8 V or the RTC supply voltage (pp. 17 to 19). On the module the only supply pad is 3V3.

At reset the chip latches the level on five pins and keeps it until the next reset. After that the pins are ordinary GPIOs again. Each has an internal pull-up or pull-down that sets its default, so a pin left unconnected gives the normal start-up. Anything you wire to these pins must not overpower that default while EN rises and for 1 ms after (the hold time t_H, p. 23).

GPIO0 picks the boot mode. Held high, which its pull-up does, the chip runs the program in flash. Held low at reset, with GPIO2 also low, it waits for new firmware over the serial port; that is all the BOOT button on a dev board does. GPIO12 (MTDI) sets the voltage of the flash supply: low gives 3.3 V, high gives 1.8 V. The WROOM-32E's flash runs on 3.3 V, so a sensor or pull-up that holds GPIO12 high at power-up starves the flash and the module will not boot. GPIO15 (MTDO) low only silences the boot messages, and GPIO5 matters only to SDIO slave timing.

GPIO34 to GPIO39 are inputs only. They have no output driver and no internal pull-up or pull-down (ESP32 Series Datasheet p. 61), so a button on one of them needs its own resistor. They are fine for analog sensors and for signals you only read.

ADC2 shares its hardware with the Wi-Fi driver. Espressif's ESP-IDF guide (v6.1, ADC oneshot driver) says ADC2 is also used by Wi-Fi and that a read can fail with a timeout while another driver holds it. So if the project uses Wi-Fi, read analog only on the 6 ADC1 pads: GPIO32 to GPIO36 and GPIO39 (GPIO37 and GPIO38 exist only on the bare chip). Of those, GPIO32 and GPIO33 are also free outputs; the other four are the input-only pins. The 10 ADC2 pads still work as digital pins.

The ADC itself reads 150 to 2450 mV at its widest attenuation, with up to ±60 mV of error after eFuse calibration (p. 44). A 3.3 V signal needs a divider to fit; see the voltage divider calculator.

With one maker, the comparison here is the chip against the module. They agree on the one that matters: 3.6 V is the ceiling on every supply pin. The module's storage limit is lower (105 °C against 150 °C) because of the flash and the board, and the module datasheet does not repeat the chip's 1200 mA limit on total I/O current. Design to the module's figures when you use the module.

There is no 5 V tolerance anywhere. An input may sit at most 0.3 V above the I/O supply (Table 5-3, p. 52), which on a 3.3 V module is 3.6 V. A 5 V Arduino output or a 5 V sensor wired straight to a GPIO is outside the rating (the logic levels lesson explains the thresholds). Use a divider or a level shifter; the 3.3 V to 5 V level shifter guide covers both directions.

The pin figures are typical, not limits: Espressif gives 40 mA source and 28 mA sink at maximum drive strength, and the source figure falls toward 29 mA per pin as more pins source current at once (p. 52, note 2). At 3.3 V an input reads high above 2.47 V and low below 0.82 V, and a high output is guaranteed only down to 2.64 V.

The radio is what sizes the power supply. Transmitting 802.11b at full power, the module averages 239 mA and peaks at 379 mA, 140 mA above its own average, in bursts as packets go out. That is why both datasheets ask for a supply that can deliver 500 mA, and why the reference schematic puts 22 µF on the 3V3 pad. The chip table (p. 52) gives 240 mA for the same mode, but measured at 50% duty; the module table is the one to size a supply from.

• Anything that needs Wi-Fi or Bluetooth with a real processor behind it: sensor nodes that report to a server, web-controlled relays, data loggers, small displays.
• Battery sensors that wake, send and sleep. Deep sleep with the RTC timer running is 10 µA (chip p. 30), but the regulator and anything else on the board are added to that.
• Jobs needing more than an ATmega328P has: two cores at up to 240 MHz, three UARTs, touch inputs on ten pins, and two 8-bit DACs on GPIO25 and GPIO26.

It is a poor fit for a project that must run from 5 V logic without level shifting, needs more than a handful of analog inputs alongside Wi-Fi, or must run for months on coin cells while transmitting often. For a 5 V AVR with simpler power needs, see the ATmega328P page.

The smallest circuit that runs a bare WROOM-32E from a 5 V USB supply, programs over serial and lights an LED: an AMS1117-3.3 regulator, two capacitors on the rail, an RC delay on EN with a reset button, a BOOT button on GPIO0, and an LED on GPIO23, a pin with no role at reset.

To load firmware, hold BOOT, tap RESET, then let go of BOOT: the chip starts with GPIO0 low and waits on the serial port. For powering a board from USB, a wall adapter or a battery, see how to power an ESP32 or Arduino, and the RC time constant calculator for other EN values.

• Put the 22 µF capacitor at the 3V3 pad. Resets the moment Wi-Fi connects usually mean the supply or its wiring cannot follow the 379 mA bursts.
• Keep copper, ground pour and metal away from the antenna end of a WROOM-32E. The module datasheet marks a keep-out zone (p. 10); the 32UE has none because its antenna is off the board.
• Use GPIO34 to 39 for buttons only with an external pull-up or pull-down: they have none inside.
• Pick analog inputs from ADC1 (GPIO32 to 36 and GPIO39 on the module) if Wi-Fi is on, and expect ±60 mV even after calibration.
• For loads above a few milliamps, relays or motors, switch through a transistor; the MOSFET guide for Arduino and ESP32 picks one that turns on fully from 3.3 V.
• Never connect anything to GPIO6 to 11 on a bare chip design unless it is the flash. On the module they are not brought out, which settles it.

The WROOM-32UE drops into a WROOM-32E footprint. The older WROOM-32 fits too, as long as nothing on the board uses pads 17 to 22. Newer chips such as the ESP32-S3 and ESP32-C3 are different parts with different pinouts and are not substitutes.

1. With power on, measure 3.3 V on pad 2 against ground, and 3.0 V or more there while Wi-Fi is transmitting. A rail that dips below 3.0 V needs a bigger supply or the bulk capacitor moved closer.
2. Measure EN. It must sit above 2.47 V; anything near 0.6 V or below holds the chip off.
3. Connect a USB-serial adapter to TXD0 and RXD0 and press RESET. With GPIO15 left unconnected the chip prints its boot log on TXD0; no output at all means it is not starting.
4. If it boots without your circuit but not with it, measure GPIO0, 2 and 12 during reset. GPIO12 above 2.47 V at that moment is the fault.

Use the ESP32-WROOM-32E (or the 32UE if the board sits in a metal box) rather than the bare chip. Feed it from a regulator that can deliver 500 mA with 22 µF at the 3V3 pad, put 10 kΩ and 1 µF on EN, and keep GPIO12 low at power-up. Read analog on ADC1 if Wi-Fi is on, level-shift anything 5 V, and plan outputs on the 15 pins with no role at reset before touching the strapping pins.

Indian shops sell far more ESP32 dev boards than bare modules, and many are listed only as "ESP32" with a photo. Read the print on the metal shield: ESP32-WROOM-32E (or 32UE) is the current module; plain ESP32-WROOM-32 is the older one. A listing that says "30-pin" or "38-pin" describes the dev board's headers, not the module. For a board of your own, buy the bare module with its flash size in the part number (-N4 is plenty for most projects). Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.