Quick Answer
Choose the XIAO RP2040 Plus for faster code, much more memory and PIO-driven timing. Choose the XIAO SAMD21 Plus when analogue inputs, a true DAC and the established SAMD21 Arduino ecosystem matter more. Both keep the 21 × 17.8mm XIAO footprint and add battery management plus hidden I/O pads.
The Same Footprint Hides the Real Choice
The word “Plus” does not mean a larger board. Seeed kept both models at 21 × 17.8mm and retained the familiar 14-pin XIAO edge layout. If you already use XIAO shields or have read our introduction to the XIAO family, the top-side connections remain familiar.
The useful change is underneath. RP2040 Plus adds 15 SMD pads, while SAMD21 Plus adds 16. Those pads lift the boards to 26 and 27 digital I/O respectively, but they do not magically give a breadboard another row of pins. To use them cleanly, design a carrier PCB or be prepared for fine soldering. That is why we see these as compact production modules, not merely nicer breadboard boards.
| Specification | XIAO RP2040 Plus | XIAO SAMD21 Plus |
|---|---|---|
| Processor | Dual Cortex-M0+, up to 133MHz | Cortex-M0+, up to 48MHz |
| Memory | 264KB SRAM, 2MB flash | 32KB SRAM, 256KB flash |
| Exposed digital I/O | 26 | 27 |
| Analogue inputs | 4 | 11 |
| Distinctive hardware | PIO, second I²C bus, USB D+/D− pads | DAC, I²S, second I²C bus |
| Battery support | Onboard PMIC and voltage monitoring | Onboard PMIC for a 3.7V single-cell pack |
Why We Would Pick the RP2040 Plus
The RP2040 is the easy recommendation for most new robots, LED controllers and timing-sensitive interfaces. Its two 133MHz cores, 264KB SRAM and 2MB flash leave far more headroom than the SAMD21. Programmable I/O, or PIO, can handle protocols and precise waveforms without tying up the main processor.
It also has broad support across Arduino, MicroPython, CircuitPython, PlatformIO and lower-level toolchains. Unless a SAMD21-specific library or peripheral decides the project, this is the board we would start with from our Arduino and microcontroller range.
When the SAMD21 Plus Is the Better Tool
The SAMD21 Plus wins when the signals matter more than raw compute. Eleven analogue-capable pins and a DAC make it a cleaner fit for sensor-heavy instruments, control voltages and compact USB devices. Its 32KB SRAM and 256KB flash are restrictive for large Python projects, displays or feature-heavy firmware, so we would not buy it merely because the GPIO count is one higher.
Both Plus boards add proper Reset and Boot buttons and onboard single-cell lithium battery management. That removes an external charging board from many portable builds, but it does not make every battery safe by default: use a suitable protected cell and follow the board’s polarity and voltage documentation.
What We Would Actually Buy
For a fresh design, we would buy RP2040 Plus unless the requirement list explicitly says DAC, more than four analogue inputs or an existing SAMD21 codebase. If a XIAO display is the centre of the project, check our XIAO round-display setup guide before assigning pins; the expansion hardware can consume connections your carrier board also expects.
FAQ
Q: Are XIAO RP2040 Plus and SAMD21 Plus pin-compatible?
A: Their standard 14-pin XIAO footprint is physically familiar, but peripheral functions and the additional underside pads differ. Verify the pin map before moving firmware or a custom carrier between them.
Q: Can either board run from a LiPo battery?
A: Yes. Both Plus variants include onboard battery management. SAMD21 Plus specifies a 3.7V single-cell Li-ion or LiPo input; use the documented battery pads and never feed battery voltage into a GPIO.
Q: Which is better for MicroPython or CircuitPython?
A: RP2040 Plus. Its 2MB flash and 264KB SRAM provide substantially more room than the SAMD21’s 256KB flash and 32KB SRAM.