Seeed XIAO ESP32C3

Fig. 1. Seeed XIAO ESP32C3, 2x7 2.54 mm, 21 x 17.5 mm. Hover or tap a signal to light up every pin that carries it.

• The footprint is the point. D0 to D10, 5V, GND and 3V3 sit in the same places on the XIAO SAMD21, RP2040, ESP32C3, ESP32S3 and nRF52840, so a carrier board outlives whichever chip you started with. Only the peripheral mapping behind each pin changes.
• The silkscreen D numbers are not GPIO numbers. D0 is GPIO2, D6 is GPIO21, D7 is GPIO20. Arduino code uses D0 to D10; anything touching the ESP-IDF or raw registers needs the GPIO column.
• Three strapping pins are on the header: D0 (GPIO2), D8 (GPIO8) and D9 (GPIO9). D9 is also the BOOT button. Hang a pull-down on any of them and uploads start failing.
• Four analog inputs, D0 to D3, but only D0 to D2 are on ADC1. D3 is ADC2, which the radio takes over, so an analog read there stops working once WiFi is up.
• There is a lithium battery charger on board, with pads on the underside rather than a connector. The charge LED is on its own pin, not a GPIO.
• 5V is VBUS, straight from the USB connector. It is an input when you power the board that way and an output when USB is connected, and it is not regulated.
• Only D0 to D3 can wake the chip from deep sleep.
• There is a U.FL connector for an external antenna next to the PCB trace antenna. A solder jumper selects between them; moving it is worth several dB.
• Adafruit's QT Py uses this same 14-pin footprint, so the two are interchangeable in a carrier. The larger Adafruit Feather footprint is a different thing entirely and FeatherWings will not fit.

• Drive a pin and read an analog input

  ESP-IDF v5

  #include "driver/gpio.h"
  #include "esp_adc/adc_oneshot.h"
  
  /* Silkscreen D numbers are not GPIO numbers: D0 is GPIO2. */
  gpio_reset_pin(GPIO_NUM_2);
  gpio_set_direction(GPIO_NUM_2, GPIO_MODE_OUTPUT);
  gpio_set_level(GPIO_NUM_2, 1);
  
  adc_oneshot_unit_handle_t adc1;
  adc_oneshot_unit_init_cfg_t unit = { .unit_id = ADC_UNIT_1 };
  adc_oneshot_new_unit(&unit, &adc1);
  
  adc_oneshot_chan_cfg_t ch = {
      .atten    = ADC_ATTEN_DB_12,   /* full 0 to ~3.1 V range */
      .bitwidth = ADC_BITWIDTH_DEFAULT,
  };
  adc_oneshot_config_channel(adc1, ADC_CHANNEL_2, &ch);   /* GPIO2 = D0 */
  
  int raw;
  adc_oneshot_read(adc1, ADC_CHANNEL_2, &raw);

  Only D0 to D2 are on ADC1. D3 is ADC2, which the WiFi driver takes over, so a read there starts failing the moment the radio connects.

Table 1. Pin assignments.