Source: https://mokxi.com/boards/xiao-samd21 Updated: 2026-09-27 Board # Seeed XIAO SAMD21 simulator, all eleven pads analog The XIAO SAMD21 is the smallest board here: 21 by 17.8 mm, an ATSAMD21G18A at 48 MHz, and eleven castellated pads of which every single one is an analog input. Serial is the chip’s own USB; Serial1 is a real USART on D6 and D7. Sign up free - 8 peripheral areas modeled - 4 projects ready to run - Real time on your own machine Click to open it in the editor The board is running real firmware right now. Click it to open the circuit in the editor. The model ## What is modeled Taken from the board's own contract document and put in plain words, with what is missing listed underneath. Core Cortex-M0+ (ARMv6-M) at 48 MHz, with the datasheet cycle count of every instruction PORT Both groups, with DIR, OUT, IN, PINCFG, PMUX and WRCONFIG, including the rule that OUT picks an input’s pull direction SysTick A one-millisecond reload under millis(), and a delay() that parks the core in WFI between ticks SERCOM USART mode on all six, shifting real 8N1 frames a bit at a time; SERCOM4 on D6 and D7 is Serial1 TCC and TC TCC0-2 and TC3-7 as far as PWM, so analogWrite works on ten of the eleven pads at 735 Hz ADC Twelve bits against the 3.3 V rail on all twenty channels, so every pad reads a voltage EIC Sixteen external interrupt lines with edge and level sensing, which is what attachInterrupt uses USB A byte pipe at the real endpoint registers and the real descriptor table: enough for a serial port, and nothing above it ### Not in the model - The USB stack. No enumeration, no descriptors, no SETUP. `while (!Serial)` returns at once here and waits on a desk, and Keyboard and Mouse are not available at all. - The clock tree. The core runs at 48 MHz from the first instruction; GCLK, PM and SYSCTRL exist so a real startup runs to the end, and nothing is gated on them. - SPI and I2C on the SERCOMs, so no Wire and no SPI. The DAC, the analog comparators, the touch controller and I2S are absent too. - DMA, the event system, the NVM controller, the watchdog and the RTC. Their blocks read zero rather than faulting. - Two things the contract marks believed rather than verified: the TC block’s register offsets, and the peripheral multiplexing table that says which timer each pad can reach. Section 7 names both. Start here ## Open one and press Run Each of these opens in the editor exactly as it is drawn here. Projects ## Every built-in project on the XIAO SAMD21 Each has its own page with the circuit running, the code and a parts list. - XIAO SAMD21 serial Libraries ## Arduino libraries that compile as they are Paste a tutorial that includes one of these and it builds: each is written for Mokxi, on its own drivers, under the name the tutorials use. For any other library the build names the Mokxi header that does the same job. Servo.h attach, write, writeMicroseconds, read, detach, on any pin Wire.h I2C as a master; Wire.begin() puts it on D4 (SDA) and D5 (SCL) SPI.h transfer and transactions, on the board’s usual SPI pins EEPROM.h read, write, update, get, put and EEPROM[i], kept in 4 KB of RAM, which a reset clears: this board has no EEPROM to write LiquidCrystal_I2C.h the 16x2 LCD behind its I2C backpack Adafruit_GFX.h and Adafruit_SSD1306.h the 128x64 I2C OLED: text, pixels, lines, rectangles, circles, triangles and bitmaps DHT.h the DHT11 and DHT22: temperature, humidity and the heat index, as floats you can print OneWire.h and DallasTemperature.h DS18B20 thermometers, any number on one pin, found by the 1-Wire search Adafruit_BME280.h, Adafruit_BMP280.h and SparkFunBME280.h the Bosch pressure sensors: temperature, pressure, humidity and altitude RTClib.h the DS3231 and DS1307 clocks, with DateTime and TimeSpan Adafruit_SHT31.h the SHT31’s temperature and humidity, CRC checked LedControl.h MAX7219 eight-digit displays and 8x8 matrices: digits, characters, rows and single LEDs Adafruit_LEDBackpack.h the HT16K33 four-digit backpack: numbers, raw segments, the colon, brightness and blink Adafruit_TCS34725.h the TCS34725’s red, green, blue and clear counts, lux and color temperature Adafruit_SGP30.h the SGP30’s CO2eq and TVOC, its baseline and humidity compensation Adafruit_INA219.h current, bus voltage and power through the INA219’s shunt Adafruit_ADS1X15.h the ADS1115’s four sixteen-bit inputs, single or differential Adafruit_MCP23X17.h the MCP23017’s sixteen pins, one at a time or a port at once HX711.h the HX711 load cell amplifier: tare, set_scale and get_units, the units as floats you can print ## Questions Is it free? Yes. Every board and every part is on the free plan, with no account needed. Is every pad really an analog input? Yes. A0 to A10 are the same pads as D0 to D10 and the silkscreen prints both, so analogRead(A3) and analogRead(3) are the same pin. Why does analogWrite do nothing on D0? Because D0 is PA02, the DAC’s pin, and it reaches no timer at all. That is the chip, not the model: the runtime drives the pad on or off at the halfway mark rather than pretending to fade. Can it act as a keyboard? Not here. Keyboard and Mouse need the USB stack above the byte pipe, and the model says plainly that it does not have one. Will my firmware run on a real XIAO? The core, the PORT, the SERCOMs, the ADC and the EIC will. Two things are believed rather than verified (the TC register offsets and the pad multiplexing table), and section 7 of the contract is the short list to check. You will also need to relink at 0x2000 to go in over the UF2 bootloader. Also here ## The other boards Every one of these runs today, and every one is free. Looking for the shorter introduction? See the XIAO SAMD21 simulator page. ESP32-C3-DevKitM-1 runs now A RISC-V core at 160 MHz with GPIO, UART, the SYSTIMER, a twelve-bit ADC on GPIO 0 to 4 and LEDC PWM on any pin, on the real memory map. Firmware for the modeled peripherals uses the board’s memory map. The contract also marks register addresses inside the SAR ADC and LEDC that are still unverified. Open the board page Arduino Uno R3 runs now A complete ATmega328P, with three timers in every mode, USART0, the ADC, interrupts and sleep. Blink, serial, a held button and a hardware PWM fade, at 16 MHz and real time. Open the board page Arduino Nano runs now The Uno's ATmega328P on a board 45 by 18 mm, so it pushes into the breadboard instead of standing beside it. Two more analog inputs than an Uno, and the same firmware runs on both. Open the board page Arduino Mega 2560 runs now The big Arduino: fifty-four digital pins, sixteen analog inputs and four serial ports, on the ATmega2560. Everything an Uno sketch knows how to do, with enough pins left over to do it to twelve things at once. Open the board page Arduino Leonardo runs now An Uno-shaped board whose USB goes to the chip itself: Serial is a USB CDC endpoint and Serial1 is pins 0 and 1. The ATmega32U4 with its ten-bit timer, fourteen ADC channels and five external interrupts, at 16 MHz. Open the board page ESP32-C6-DevKitC-1 runs now A second RISC-V board: thirty-one GPIOs, 512 KB of SRAM in one window, UART0, the SYSTIMER, a twelve-bit ADC on GPIO 0 to 6 and LEDC PWM on any pin. The contract says plainly which addresses come from the manual, which are believed rather than verified, and what is not modeled at all. Open the board page ESP32 DevKit V1 runs now The classic ESP32 on an Xtensa LX6 at 240 MHz: the GPIO matrix, UART0, timer group 0, a twelve-bit ADC1 and the eight LEDC PWM channels, on the real memory map. One core where the chip has two, no radio and no FPU. Your own sketch compiles in the tab. Open the board page ESP32-S3-DevKitC-1 runs now The ESP32-S3 on an Xtensa LX7 at 240 MHz: GPIO matrix, UART0, USB serial, ADC1, LEDC PWM and the RMT driving the RGB LED, on the real memory map. One core where the chip has two, no radio and no FPU. Your own sketch compiles in the tab. Open the board page ESP8266 NodeMCU V1.0 runs now The Xtensa LX106 at 80 MHz in the call0 ABI, with D0 to D8 that are not GPIO numbers, two active-low LEDs, and PWM done in software because the chip has none. WiFi uses a simulated network, with no radio or real internet. The ADC is the one block whose registers are this model’s own, because Espressif never published the chip’s. Open the board page Raspberry Pi Pico runs now RP2040 on our own ARMv6-M core, with SIO GPIO, the 1 MHz timer and alarms, UART0 and eight PWM slices. Blink, a button, serial and a fade at real time, with sleep parked on WFI. Open the board page Raspberry Pi Pico W runs now The same RP2040 and the same forty pins as the Pico, so the same ELF runs on it. The CYW43439 is not modeled. WiFi uses a simulated network, and the on-board LED is driven through the board runtime rather than GP25. Open the board page BBC micro:bit V2 runs now The nRF52833 on our own Cortex-M4 core at 64 MHz, with GPIO, GPIOTE, three TIMERs, RTC0 and UARTE0 on the real register map. The 5x5 LED matrix scanned as the hardware scans it, buttons A and B, the speaker and three rings on the edge connector. Open the board page ATtiny85 runs now A whole microcontroller in an 8-pin DIP, straight into the breadboard: six I/O, two timers, an ADC and a USI. Blink, a button on INT0, a knob on the ADC and a serial port it bit-bangs for itself. Open the board page STM32F411 Black Pill runs now Cortex-M4 with Thumb-2, NVIC and SysTick at 84 MHz, with GPIO, TIM2 to TIM5 and USART1 on the real register map. Blink on PC13, a button, serial and a PWM fade, all at real time. Open the board page STM32 Blue Pill (F103) runs now Cortex-M3 at 72 MHz, with GPIO on the F1’s CRL and CRH, TIM1 to TIM4, USART1 and a twelve-bit ADC. Blink on PC13, a button, serial with a live reading, and a PWM fade, all at real time. Open the board page Arduino Uno R4 Minima runs now Renesas RA4M1, Arm Cortex-M4 at 48 MHz, in an Uno’s shape and on an Uno’s 5 V pins. Modeled: the PFS port block, GPT PWM on the six tilde pins, SCI2 as Serial1 on D0 and D1, and a fourteen-bit ADC that analogRead reads ten bits of. Not modeled: a USB stack (Serial is a byte pipe with no enumeration behind it), the ICU, so there is no attachInterrupt, and the AGT. The contract says plainly which addresses come from the manual and which are believed rather than verified. Open the board page Arduino Pro Mini runs now The Uno’s ATmega328P on the smallest Arduino there is, with no USB on it, in both builds: 5 V at 16 MHz and 3.3 V at 8 MHz. The 3.3 V board really runs at 8 MHz, so a sketch built for 16 MHz keeps half time on it, as it does on the bench. Open the board page Arduino Micro runs now The Leonardo’s ATmega32U4 on a board that pushes into a breadboard, with the L LED on pin 13 and the SPI pins on holes of their own. Serial is the chip’s own USB and Serial1 is pins 0 and 1, and the Leonardo’s programs run on it unchanged. Open the board page SparkFun Pro Micro runs now The ATmega32U4 in a Pro Mini’s footprint with a USB socket on the end, running the Leonardo’s programs. No LED on pin 13: its lamps are RX and TX, lit by pulling their pins low. Open the board page Wemos D1 mini runs now The NodeMCU’s ESP8266 on a board a third the size, with the same D0 to D8 names and only the holes that do something. WiFi on a simulated network, and your own sketch compiles in the tab. Open the board page ESP-01S runs now An ESP8266 with eight pins, four of them GPIO, and a blue LED that lights when its pin is low. Wired with jumpers, because its two rows will not sit in a breadboard. WiFi is simulated. Open the board page Seeed XIAO ESP32-C3 runs now The ESP32-C3 at 160 MHz on the XIAO’s thumbnail outline: fourteen pads, and D0 to D10 that are the XIAO’s numbers, not GPIOs. No user LED to blink, WiFi on a simulated network, and sketches that compile in the tab. Open the board page ESP32-C3 SuperMini runs now The smallest ESP32-C3 board in common use, sixteen pins named by GPIO, at 160 MHz. Its blue LED on GPIO 8 lights when the pin is LOW, the other way around from the DevKit’s. Open the board page Arduino Uno R4 WiFi runs now The Uno R4’s RA4M1 with a 12 x 8 LED matrix to draw on through Arduino_LED_Matrix.h. WiFi through WiFiS3.h on a simulated network: no radio, no ESP32-S3 in the model, no real internet. Open the board page ## Try it on your own firmware What you build here runs on the board on your desk, unchanged. Nothing to install and no account needed. Start building Open the editor