XIAO Plus Boards Add 16 GPIOs and Battery Charging

Two Familiar Boards, Rebuilt Around Their Own Limitations

Seeed Studio has released Plus versions of two of its longest-serving XIAO microcontroller boards, and the changes read like a list of every complaint people have filed about the originals. The XIAO SAMD21 Plus and XIAO RP2040 Plus keep the thumbnail-sized footprint that made the series popular, then use redesigned castellated edges and backside pads to expose far more I/O than the originals ever did.

• XIAO SAMD21 Plus: Microchip SAMD21G18 Cortex-M0+ at 48 MHz, 256 KB flash, 32 KB SRAM, 23.5 x 17.5 mm, $5.90
• XIAO RP2040 Plus: Raspberry Pi RP2040 dual-core Cortex-M0+ at 133 MHz, 264 KB SRAM, 2 MB SPI flash, 21 x 17.8 mm, $4.90
• 16 extra digital I/Os plus I2C and I2S on the SAMD21 Plus; 15 extra digital I/O and PWM pins plus I2C on the RP2040 Plus
• Onboard power management IC on both, with battery pads, charge status LED and — on the RP2040 Plus — software battery voltage monitoring

The pricing is the part that keeps this in reach for hobby projects. Under six dollars for a board with proper Li-ion charging built in is roughly what a bare charging module costs on its own.

Why the Castellated Pads Are the Real Upgrade

The original XIAO boards traded I/O for size, and that trade was fine right up to the moment your project needed one more pin. The Plus revisions attack this by moving pins to castellated holes along the edges and to pads on the underside, which does two things at once.

First, it exposes signals that were physically present on the microcontroller but had nowhere to go on a board this small. Second, castellated edges make the board reflowable as a system-on-module — you can solder it directly onto your own carrier PCB instead of socketing it. Seeed positions the SAMD21 Plus explicitly for production-ready SoM designs, which is a meaningful step up from the usual hobby-board framing.

For anyone who has built a wearable or a battery-powered sensor node, the on-board PMIC matters just as much. Charging lithium cells safely means a dedicated charge controller, and bolting one on externally costs you board area, a connector and a soldering session. Having it integrated with a charge LED and voltage monitoring removes an entire subassembly from a project.

What Can You Build With a Board This Size?

The niche these fill is projects where the enclosure, not the electronics, sets the size limit — wearables, handheld instruments, sensors that live inside something else. A 21 by 17.8 mm board with battery management and 15 usable GPIOs covers a surprising amount of that ground.

The RP2040 Plus is the more capable of the two on paper: dual cores at 133 MHz, 264 KB of SRAM, and the RP2040's programmable I/O blocks, which let you bit-bang protocols the silicon has no dedicated peripheral for. The SAMD21 Plus is the lower-power option and carries I2S, which makes it the pick if audio is involved. Both boards come with an onboard programmable RGB LED, which sounds trivial until you are debugging a device with no display.

Software support is broad on both. The SAMD21 Plus runs Arduino, PlatformIO, MicroPython, CircuitPython and Zephyr. The RP2040 Plus adds TinyGo and Rust to that list. That breadth is not a small thing — it means the board fits whatever toolchain you already know rather than forcing a new one.

Where These Fit in the XIAO Line

Seeed has been steadily filling out this series rather than replacing it, which is the right call for a form factor whose value is that accessories and carrier boards keep working. We looked at the wireless end of the range when the XIAO nRF54LM20A Sense arrived with Bluetooth 6.0, and the RP2040 in particular keeps showing up in compact builds — our write-up of the ChronoWatch X2040 round-screen DIY watch is a good example of the kind of project these Plus boards are aimed at.

The takeaway for makers is simple: if you previously ruled out a XIAO because you ran out of pins or needed external charging circuitry, both of those objections just got answered without the board getting any bigger. More small-board news is in our mini computers section.

Sources: CNX Software — September 7, 2026; Seeed Studio product page — XIAO SAMD21 Plus — accessed September 7, 2026; Seeed Studio product page — XIAO RP2040 Plus — accessed September 7, 2026.