The Seeed Studio XIAO RP2040

The Pico's RP2040 on a thumbnail: fourteen pads, a USB-C socket, a red, green and blue user LED and a NeoPixel of its own.

What it is

The same RP2040 as the Raspberry Pi Pico, a Cortex-M0+, on the XIAO family's outline: 21 by 17.5 mm, with seven castellated pads down each long edge, 0.6 inch apart, so it pushes into a breadboard across the channel. The board also carries the BOOT and RESET buttons, a three-color user LED and a WS2812 NeoPixel. Mokxi runs the chip, its pads and both lights.

The pin numbers are the XIAO's

The silkscreen numbers are the XIAO family's, not the chip's. Mokxi builds this board the way the arduino-pico core's "Seeed XIAO RP2040" does: a bare number is a GPIO, and D0 to D10 and A0 to A3 are defined as GPIO numbers. So write D0, not 0.

Pad	GPIO	Also
D0	26	A0
D1	27	A1
D2	28	A2
D3	29	A3
D4	6	I2C SDA
D5	7	I2C SCL
D6	0	UART TX, see below
D7	1	UART RX, see below
D8	2	SPI SCK
D9	4	SPI MISO
D10	3	SPI MOSI

With the USB at the top, D0 to D6 run down the left and 5V, GND, 3V3, D10, D9, D8 and D7 down the right. 5V and 3V3 are supplies for the breadboard, at 5 V and 3.3 V.

A0 to A3 read the volts on the net, ten bits against 3.3 V unless analogReadResolution() asks for more. Every pad can do PWM with analogWrite(), and every pad can take attachInterrupt().

Wire uses D4 and D5. SPI uses D8, D9 and D10, with no chip select pad of its own, so pick any free pad for CS. Both buses run in software on those pads. Serial reaches the serial monitor, and Serial1 is the same port. D6 and D7 are UART0's pins on the chip, but here they stay ordinary pins. There is no Serial2 on this board.

The user LED

The user LED is three LEDs in one, each wired from 3.3 V to its pin, so each lights when its pin is LOW:

Color	GPIO	Name
Red	17	LED_BUILTIN, PIN_LED, PIN_LED_R
Green	16	PIN_LED_G
Blue	25	PIN_LED_B

So digitalWrite(LED_BUILTIN, LOW) turns the red one on and HIGH turns it off. The three pins are inside the board, on no pad.

The NeoPixel

The NeoPixel is a WS2812 on GPIO 12 (PIN_NEOPIXEL), and its supply comes from GPIO 11 (NEOPIXEL_POWER). Drive NEOPIXEL_POWER HIGH first, as Seeed's examples do. Until then the NeoPixel stays dark however much data it is sent, as on the real board.

It is a full WS2812 model of its own, so Adafruit_NeoPixel works as it does on a desk:

#include <Adafruit_NeoPixel.h>

Adafruit_NeoPixel pixel(1, PIN_NEOPIXEL, NEO_GRB + NEO_KHZ800);

void setup() {
  pinMode(NEOPIXEL_POWER, OUTPUT);
  digitalWrite(NEOPIXEL_POWER, HIGH);
  pixel.begin();
}

The board draws the NeoPixel in the color it was last sent.

What is modeled

The Pico model's chip, whole: the Cortex-M0+ core, GPIO and the pads, PWM on every pin, the TIMER behind millis() and delay(), UART0 wired to the serial monitor, pin interrupts, the watchdog and the ADC on GPIO 26 to 29, at the real RP2040 addresses. The three user LEDs and the NeoPixel are on the board.

What is not modeled

The pads on the back (the SWD pair, RUN and the battery pads) and the power LED. RESET and BOOT are buttons with no pin.

From the RP2040 itself, as on the Pico: the second core, PIO, DMA, the chip's own I2C and SPI blocks (Wire and SPI run in software instead), the USB controller (Serial reaches the monitor directly), the RTC and the clock tree.

Common mistakes

digitalWrite(0, HIGH) to drive D0. Here 0 is GPIO 0, which is D6. Write digitalWrite(D0, HIGH).

digitalWrite(LED_BUILTIN, HIGH) to light the LED. On this board HIGH is off. Write LOW.

A NeoPixel sketch from another board that stays dark. Add pinMode(NEOPIXEL_POWER, OUTPUT) and digitalWrite(NEOPIXEL_POWER, HIGH) to setup().

Starter projects

XIAO RP2040 NeoPixel is the board across a breadboard, turning on the NeoPixel's power with GPIO 11 and fading it around the color wheel on GPIO 12 with Adafruit_NeoPixel.

XIAO RP2040 blink is the board across a breadboard, blinking the red user LED on GPIO 17, which lights when the pin is LOW.