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.
- 8peripheral areas modeled
- 4projects ready to run
- Real timeon your own machine
What is modeled
Taken from the board's own contract document and put in plain words, with what is missing beside it.
Modeled
- 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 732 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 modeled
- 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.
Open one and press Run
Each of these opens in the editor exactly as it is drawn here.
Every built-in project on the XIAO SAMD21
Each has its own page with the circuit running, the code and a parts list.
Arduino libraries that compile as they are
Paste a tutorial that includes one of these and it builds. For any other library, the build names the Mokxi header that does the same job.
And 14 more, 22 in all: Adafruit_BME280.h, Adafruit_BMP280.h and SparkFunBME280.h; RTClib.h; Adafruit_SHT31.h; Adafruit_PCF8591.h; Adafruit_ADXL345_U.h; LedControl.h; Adafruit_LEDBackpack.h; Adafruit_TCS34725.h; Adafruit_SGP30.h; Adafruit_INA219.h; Adafruit_ADS1X15.h; Adafruit_MCP23X17.h; Bounce2.h; HX711.h.
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.
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.
A RISC-V chip at 160 MHz, on the real memory map.
The classic Arduino, with an ATmega328P running at 16 MHz.
The Uno’s chip on a small board that fits a breadboard.
The big Arduino, with 54 digital pins and four serial ports.
An Uno-shaped board whose chip does its own USB serial.
A second RISC-V board, with 31 pins and simulated WiFi.
The classic ESP32 at 240 MHz, with simulated WiFi.
The ESP32-S3 at 240 MHz, with USB serial and an RGB LED.
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.