Here’s a compact, practical ESP32 DevKit V1 pinout + the important “gotchas” and recommendations you’ll use most. I’ll call out the critical rules (boot/flash/ADC/touch/etc.) first so you don’t hit the usual traps.

Quick reference (useful defaults)

• Power pins: 3V3 (3.3 V regulator output), GND, EN (reset / enable). Many DevKit boards also expose a VIN/5V from USB → regulator, but use 3.3 V for peripherals.

• USB-UART / Programming: GPIO1 = TX0 and GPIO3 = RX0 are the serial console and used by the USB-to-UART for flashing/monitor. Avoid using them for critical hardware if you rely on serial prints/programming.

• I²C (common default): SDA = GPIO21, SCL = GPIO22 (you can remap if needed).

• SPI (typical VSPI hardware pins): MOSI = 23, MISO = 19, SCK = 18, SS = 5. You may remap SPI using the GPIO matrix but these are the common defaults. 

• DAC (analog out): GPIO25, GPIO26 — use these for true analog (DAC) output.

• PWM: Almost any GPIO can do PWM (LEDC) — except the input-only pins and flash pins (see next sections).

Pins you must not use / avoid or use carefully

1. SPI flash pins — GPIO6..GPIO11
   These are connected to the module’s integrated SPI flash on WROOM/WROVER modules and must not be used for general I/O on DevKit modules. Using them will break the module. 

2. Input-only pins: GPIO34, 35, 36, 37, 38, 39
   These pins are input only — they cannot be used as outputs and they don’t have internal pull-ups/pull-downs. Use external pull resistors if you need stable behavior. 

3. ADC constraints (ADC1 vs ADC2)

   • ADC1 channels are on GPIO32–GPIO39 and are safe to use while Wi-Fi is active.

   • ADC2 channels (GPIO0,2,4,12–15,25–27) are shared with Wi-Fi drivers — don’t rely on ADC2 pins when Wi-Fi is used (they can behave unpredictably). 

4. Boot / strapping pins — don’t pull them to the “wrong” level during reset
   On ESP32 some pins are sampled at reset to decide the boot configuration. The important ones are GPIO0, GPIO2, GPIO5, GPIO12 (MTDI), and GPIO15 (MTDO) (behaviour differs slightly across ESP32 variants).

   • GPIO0 low → enter UART download boot (used by flashing).

   • GPIO12 (MTDI) is special: if HIGH during reset it can change the flash power rail (VDD_SDIO) to 1.8 V, which will make most 3.3 V flash chips fail to boot (brown-out). Leave GPIO12 floating/pulled LOW unless you intentionally need 1.8 V.
     If you wire external hardware to these pins, make sure they do not force the wrong level at power-on/reset. 

Touch, RTC, and wake pins

• Touch pins (capacitive): the ESP32 has 10 touch inputs mapped to GPIOs. Common mapping (T0..T9):
  T0=GPIO4, T1=GPIO0, T2=GPIO2, T3=GPIO15, T4=GPIO13, T5=GPIO12, T6=GPIO14, T7=GPIO27, T8=GPIO33, T9=GPIO32. Touch pins are handy for wake-from-deep-sleep.

• RTC / deep-sleep wake pins: several GPIOs are routed to the RTC domain (e.g. GPIO36/39/34/35/25/26/33/32/4/0/2/15/13/12/14/27 depending on variant) — check the specific board docs for the exact set if you need ultra-low-power wake sources. 

Practical rules & tips (short, actionable)

• If in doubt, use these safe pins for general I/O: GPIO4, GPIO16, GPIO17, GPIO18, GPIO19, GPIO21, GPIO22, GPIO23, GPIO25, GPIO26, GPIO27, GPIO32, GPIO33. (Avoid 6–11, and be careful with 0/2/12/15.)

• Use ADC1 for analog reads (32–39) if you also need Wi-Fi. For ADC pins, add filtering and calibration — ESP32 ADCs are noisy and non-linear. 

• Avoid connecting strong pull-ups/pull-downs to strapping pins — they affect boot. If you must use a strapping pin as regular I/O, add series resistors or ensure your external circuit is high-impedance at reset. 

• If you use I²C/I²S/SPI for peripherals, prefer the default pins (I²C 21/22, VSPI 23/19/18/5) first — they’re well supported in libraries. You can remap using the GPIO matrix if necessary. 

• Buttons on input-only pins: remember those pins lack internal pull resistors — add an external pull-up/down. 

✅ Correct behavior of GPIO4

• GPIO4 is not actually a boot strapping pin on most ESP32 variants (e.g., ESP32-WROOM-32, ESP32-WROVER).

• It is sampled at boot on some very early or special ESP32 revisions (engineering samples, ESP32-D2WD), but in all common DevKit V1 and WROOM/WROVER modules, it behaves as a normal GPIO.

So in practice:
👉 GPIO4 is safe for general I/O, digital in/out, or peripheral functions.
It’s not like GPIO0, GPIO2, GPIO12, or GPIO15 — those truly affect boot mode.

🧩 Why confusion exists

Espressif’s original documentation listed GPIO4 among “strapping” candidates because:

• It can serve as MTDI in some alternate configurations, and

• It’s also one of the touch sensor pins (T0), which caused overlap in early datasheets.

But in current ESP32 DevKit V1 boards (and Arduino/ESP-IDF defaults):

• GPIO4 is not sampled for boot mode,

• it’s not reserved by flash or PSRAM,

• and it’s one of the safest general-purpose pins available.

✅ Summary Table (updated)

Quick cheat-sheet (condensed)

• Don’t touch: GPIO6..GPIO11 (flash)

• Input only: GPIO34..GPIO39

• Serial console: GPIO1 (TX), GPIO3 (RX)

• Default I2C: SDA=21, SCL=22

• Default SPI (VSPI): MOSI=23, MISO=19, SCK=18, SS=5

• DAC: GPIO25, GPIO26

• ADC1 (safe with Wi-Fi): GPIO32..GPIO39

• Strapping to watch: GPIO0, GPIO2, GPIO5, GPIO12, GPIO15 (don’t force levels at boot)

(References: Espressif datasheets / docs + community pinout references and tutorials.) (espressif.com)