Esp32 Wroom 36pin

ESP32-WROOM-32 devkit — 36-pin

The original DOIT ESP32 DevKit V1, before the flash pins were dropped to make the shorter 30-pin board everyone sells now. Eighteen pins a side: the 30-pin layout plus six pins at the USB end that go straight to the module's internal flash die.

Those six are the only functional difference from the 30-pin board, and they are a liability rather than a feature. Everything else — the usable GPIO set, the strapping pins, the analog budget — is identical.

• reference/board-hardware.md — the board: header pin map with every pin's alternate functions, the six flash pins, a 20-second procedure to confirm which board size you actually have, the USB bridge and auto-reset circuit, power tree, flash partitions — plus a development guide (§10 toolchain, §11 sdkconfig, §12 flashing and recovery, §13 peripheral cookbook, §14 symptom → cause → fix table).
• reference/esp32-soc.md — the silicon: pins that do not exist, strapping semantics and the GPIO_STRAP_REG bit order, ADC/DAC/touch maps and accuracy, IO_MUX vs GPIO Matrix and the SPI speed rule, power modes, memory map, deep-sleep wake pins.
• reference/esp32-family.md — the rest of the family, for "should this be a different ESP32?" questions: what does and does not port between chips, radio and USB capability per chip, the RMT generation table (WS2812 under Wi-Fi load), deep-sleep memory and ULP/LP-core availability, and a chip-selection table.
• reference/recipes.md — code that compiles: platformio.ini, sdkconfig.defaults, ADC1 with calibration, LEDC LED and servo, debounced input, the new I2C master driver, 80 MHz SPI, UART2, Wi-Fi scan and station connect, NVS, deep sleep, RMT WS2812.
• template/ — a project that builds clean, in two variants, plus a scaffold script. See template/README.md.

Confirm the board first

"36-pin ESP32" is the least standardised of the three sizes, and Espressif never made one, so there is no vendor pin table for it. Twenty seconds settles it:

If the count does not match, switch skills — the pin map is the whole point of having three. reference/board-hardware.md §1 and §2.1 have the detail, including which parts of the row order are reconstructed rather than transcribed from a vendor document.

Orientation

Rules that prevent the expensive mistakes

Each of these produces a failure that looks like something else.

1. SD2 SD3 CMD SD1 SD0 CLK are the flash bus. Wire nothing to them, ever. Those six pins at the USB end are GPIO9, 10, 11, 8, 7, 6, and they go to the module's internal 4 MB flash die — which the CPU is executing out of. A pull-up, an LED, or even a scope probe with too much capacitance gives you one of: a hang just after the bootloader banner, a boot loop with invalid header, or — worst — a board that runs and corrupts flash sectors under load. They exist on the header for factory bring-up, not for you. This is the only thing that makes a 36-pin board different from a 30-pin board, and it is a hazard, not a feature.

2. CONFIG_ESP_CONSOLE_UART_DEFAULT=y, never ..._USB_SERIAL_JTAG. The original ESP32 has no USB peripheral at all — every byte comes out of UART0 through a separate bridge chip. Configuration copied from an ESP32-C3/C6/S3 project produces a firmware that runs perfectly and says nothing, and you debug blind.

3. GPIO12 must be LOW at reset. It is MTDI, and high at reset straps VDD_SDIO to 1.8 V while the module's flash is a 3.3 V part. The board then boots intermittently and the symptom looks like flash corruption, not like a pin problem. A pull-up resistor, an LED to 3V3, or a peripheral idling a line high on GPIO12 all do it. Fine after boot; add a 10 kΩ pull-down if you must use it.

4. ADC2 is dead while Wi-Fi is running. The Wi-Fi PHY arbitrates for the same hardware and wins: adc_oneshot_read() on any ADC2 channel returns ESP_ERR_TIMEOUT until esp_wifi_stop(). There is no workaround. That leaves ADC1 — GPIO32, 33, 34, 35, 36, 39 — as the entire analog budget of a connected application. Plan the pinout around it before wiring, not after.

5. SPI3_HOST on GPIO18/19/23/5, or you get 26.67 MHz. Those four are SPI3's IO_MUX pads and bypass the GPIO Matrix. Any other pin set — or the same pins on SPI2_HOST — is matrix-routed, and the driver silently clamps full-duplex transfers to 80 MHz / 3 because of the matrix's 25 ns delay. No error is reported; spi_device_get_actual_freq() is how you find out.

6. GPIO34/35/36/39 are input-only and have no internal pull-up or pull-down. gpio_config() accepts GPIO_PULLUP_ENABLE on them and returns ESP_OK. Nothing happens. A button on GPIO34 needs a physical 10 kΩ resistor, and a floating one reads as noise rather than as a stable level.

7. A reset the moment Wi-Fi starts is the power supply, not your code. The TX burst pulls ~300 mA; the datasheet asks for a 500 mA source. A laptop USB-2 port, a thin cable or a lying hub all produce the same signature — everything works until the first packet. esp_reset_reason() returns ESP_RST_BROWNOUT; say so rather than reading the stack trace.

8. Never power VIN and USB at the same time. There is no ORing diode. With USB connected, VIN is an output at ~5 V and a second supply back-feeds the host.

9. UART1's default pins are GPIO9 and GPIO10 — two of the six from rule 1. Using UART1 without uart_set_pin() first hangs the board. Use UART2 on GPIO17/GPIO16 instead, and note that uart_set_pin() must come before uart_driver_install() — the reverse order compiles, runs, and leaves the peripheral on its defaults.

10. Only RTC GPIOs wake the chip from deep sleep: 2, 4, 12–15, 25–27, 32–36, 39 (GPIO0 is RTC-capable but not on this header). GPIO5, 16, 17, 18, 19, 21, 22, 23 cannot, however you configure the interrupt. The wake pin's pull must be set with rtc_gpio_pullup_en(), not gpio_set_pull_mode() — the digital pull registers are powered down in sleep.

11. Deep sleep on this board is 8–20 mA, not 10 µA. The LDO's quiescent draw, the USB bridge chip and the power LED are all still on. The datasheet's 10 µA is the bare chip. Never quote it for a devkit.

12. Do not set CONFIG_SPIRAM=y. A WROOM-32 has no PSRAM; the driver fails to find it and aborts before app_main(), which reads as a bricked board. That option belongs to WROVER modules.

13. CONFIG_ESP32_XTAL_FREQ_40=y. The module's crystal is 40 MHz. The 26 MHz setting exists for other modules and garbles the console at every baud rate, which reads as a broken cable.

14. GPIO0 is not on this header — it is wired only to the BOOT button and the auto-reset transistor. That is a quiet advantage over the 38-pin board: nothing you wire can hold the boot strap and drop the board into download mode, so uploads are more reliable. Do not look for a D0/IO0 pin; there isn't one.

15. The internal I2C pull-ups are 45 kΩ. Fine for two devices on a short jumper, not for a metre of ribbon or 400 kHz. A bus that scans clean at 100 kHz and fails at 400 kHz needs external 4.7 kΩ resistors, not a driver change.

When the task is choosing pins

The 36-pin board exposes 31 GPIOs but has the same 25 usable ones as the 30-pin board. Work outward from the constraints:

1. Analog first. If Wi-Fi is in the design, every analog input must land on GPIO32, 33, 34, 35, 36 or 39 (rule 4). Four of those six are input-only, which suits sensors.
2. Then the fast bus. If anything needs SPI above 26 MHz, GPIO18/19/23/5 are spoken for (rule 5).
3. Then outputs that must be quiet at boot. GPIO2 is driven briefly by the bootloader and carries the LED; GPIO1 emits the ROM log at 115200 baud. A relay or a MOSFET gate on either clicks on every reset.
4. Everything else goes on 4, 13, 14, 16, 17, 19, 21, 22, 23, 25, 26, 27.

The six extra pins over a 30-pin board are not spare capacity (rule 1). If you run out of GPIOs, reach for an I2C expander.

When the task is "the board does not work"

Diagnose in this order — each stage only depends on the ones before it:

1. Is anything wired to the six pins by the USB connector? Unplug them first (rule 1). On this board size that is the failure the 30-pin owners never see.
2. Does the port enumerate? No port ⇒ cable (try a known data cable) or bridge driver.
3. Does anything print at 115200? The ROM bootloader's rst:0x1 (POWERON_RESET) banner comes out before any of your configuration applies. If that is absent but the port exists, suspect the cable's data lines or a wedged bridge chip.
4. Does it print but say nothing after? Rule 2 — the console is misconfigured, or MTDO/GPIO15 is strapped low.
5. Does it boot-loop? Read rst:. ESP_RST_BROWNOUT ⇒ rule 7. RTCWDT_RTC_RESET or invalid header ⇒ esptool.py erase_flash and reflash the minimal variant.
6. Does it reach app_main() and then misbehave? Flash --minimal to establish a baseline, then add one subsystem at a time.

template/src/board_report.c runs steps 4–6 for you and is worth pasting into any project that is misbehaving — it prints the strapping latches, the reset reason, the flash size and the partition table.

Starting a new project

Do not hand-assemble one. template/ builds clean; scaffold from it:

~/.claude/skills/esp32-wroom-36pin/template/variants/new-project.sh <target-dir> [--full|--minimal]
cd <target-dir> && pio run

• --minimal — LED blink on GPIO2 + console heartbeat. 144,493 B flash, 13,380 B RAM. Flash this first on a board you have not used before: it touches only the toolchain, the flashing route, UART0 and one GPIO, and its two outputs fail independently — LED but no console means the serial side, console but no LED means this board has no LED on GPIO2. It also prints BOARD_NAME, so a board-size mismatch shows up immediately.
• --full (default) — board self-test: chip/reset/strapping report, partition dump, ADC1 with calibration, Wi-Fi scan, LEDC heartbeat, BOOT button to re-run. 784,417 B flash, 37,036 B RAM. The jump is the Wi-Fi stack.

Both build as-is with ESP-IDF 6.0.1 (verified, zero warnings). Nothing is generated and no paths are embedded, so copying template/ by hand works identically. template/README.md maps files to subsystems so a --full scaffold can be stripped back.

When the user already has a project, prefer bringing it in line with the template's platformio.ini, sdkconfig.defaults and include/board.h over rewriting their code.

Arduino (framework = arduino) is a perfectly reasonable choice for this board and every pin rule above applies unchanged — but the Arduino core's analogRead() hides the ADC2/Wi-Fi conflict of rule 4 instead of reporting it.

Flashing

pio run -t upload -t monitor

The bridge chip's DTR/RTS drive EN and GPIO0 through the two-transistor auto-reset circuit, so esptool handles the whole cycle with no buttons — and because GPIO0 is not on this header (rule 14), nothing of yours can interfere. The board appears as /dev/cu.usbserial-* or /dev/cu.SLAB_USBtoUART (macOS), /dev/ttyUSB0 (Linux, needs dialout membership), or a COM port (Windows, WCH or Silicon Labs driver).

When it fails with Wrong boot mode detected or No serial data received, the manual sequence is: hold BOOT → tap EN → keep BOOT a second longer → release BOOT → upload. Some clones need BOOT held for the whole upload. If 921600 baud is unreliable, drop upload_speed to 460800.

Bad firmware cannot brick this board. The ROM bootloader is in mask ROM and always answers that sequence:

esptool.py --chip esp32 -p <port> erase_flash    # ~10 s for 4 MB

There is no debug header and no USB-JTAG. External JTAG means wiring a probe to GPIO12/13/14/15 — and a probe that idles MTDI (GPIO12) high stops the board booting (rule 3). In practice you debug this board over UART0.

Reporting

Say what is verified and what is derived — this skill has more derived content than its siblings, because Espressif never made a 36-pin board.

Certain: the GPIO set on the header, and everything each GPIO can do. Those come from the ESP32-WROOM-32 and ESP32 datasheets, not from the board.

⚠︎ Derived: the physical row order in reference/board-hardware.md §2 is reconstructed from the confirmed 30-pin layout, the Espressif-documented 38-pin layout, and vendor descriptions of the DOIT board's six extra pins. It is marked as such in that file. Nothing in the code depends on it — only on which GPIO a silkscreen label means, which is certain. Point the user at the check in §2.1 rather than asserting the row order.

Verified on this machine: both template variants build clean with PlatformIO 6.1.19 / platform-espressif32 7.0.1 / ESP-IDF 6.0.1, zero warnings, at the flash and RAM figures quoted above. Every recipe in reference/recipes.md was compiled in the same project. The 26.67 MHz SPI figure is computed from _GPIO_MATRIX_DELAY_NS in the IDF HAL.

Not verified on hardware: none of this was run on a physical board in this session. Vendor-dependent details — the user LED's pin and polarity, which bridge chip is fitted, whether the auto-reset transistors are present — are stated for the DOIT/CH340 mainstream and should be confirmed with the --minimal variant.