Question 1

Why doesn't ADC2 work when Wi-Fi is on with the ESP32?

Accepted Answer

On the classic ESP32, ADC2 is used by the Wi-Fi driver, so any analogRead() on an ADC2 pin (GPIO0, 2, 4, 12-15, 25-27) returns an error while Wi-Fi is active. Use an ADC1 pin (GPIO32-39) for analog readings when Wi-Fi is on. The ESP32-S3, C3, C5, C6 and H2 do not have this conflict.

Question 2

Can I use GPIO12 on the ESP32?

Accepted Answer

GPIO12 (MTDI) is a strapping pin on the classic ESP32. It is sampled at boot to select the flash voltage and must be LOW at boot for 3.3 V flash. If an external pull-up or device drives GPIO12 HIGH at boot, the chip can fail to access flash and will not start. You can use GPIO12 as a normal GPIO after boot, but never hold it HIGH during reset.

Question 3

Which ESP32 pins are safe to use?

Accepted Answer

On the classic ESP32, avoid GPIO6-11 (wired to internal SPI flash - using them crashes the chip), treat GPIO0, 2, 12 and 15 as strapping pins (avoid driving them at boot), and remember GPIO34, 35, 36 and 39 are input-only (no output, no pull-up/down). The generally safe general-purpose output pins are GPIO4, 5, 13, 14, 16-19, 21-23, 25-27, 32 and 33. If Wi-Fi is on, use ADC1 (GPIO32-39) for analog reads, not ADC2.

Question 4

Which ESP32 pins are input-only?

Accepted Answer

On the classic ESP32, GPIO34, 35, 36 and 39 are input-only. They have no output driver and no internal pull-up or pull-down resistor, so they can only be used as digital or analog inputs.

Question 5

Why can't I use GPIO6 to GPIO11 on the ESP32?

Accepted Answer

GPIO6-11 are connected to the ESP32's internal SPI flash chip. Using them for anything else interferes with code execution and crashes the ESP32. On modules with embedded flash they are usually not broken out; never repurpose them.