WIlliam Herr's RPDot Is Just About the Smallest Raspberry Pi RP2040 Development Board Possible

Measuring barely larger than the RP2040 chip itself, the RPDot makes all other development boards look like cumbersome giants.

Gareth Halfacree
9 months ago β€’ HW101

Electrical engineer William Herr has built what is likely to be the smallest functional Raspberry Pi RP2040 development board ever, fitting entirely on the tip of one finger: the RPDot.

"I got the idea from Solder Party's RP2040 Stamp, but I thought that it could be designed smaller, granted the RP2040 is only 7Γ—7mm," Herr explains, referring to the surface-mountable square RP2040 module which has proven popular for a range of projects ill-suited to a bare RP2040 chip. "I settled on 10Γ—10mm because of the ability to route traces around the RP2040 and have some room for castellated edges."

Despite being barely larger than the chip which drives it, the RPDot packs in the features. First is, of course, a full Raspberry Pi RP2040 microcontroller β€” meaning two Arm Cortex-M0+ cores running at a stock 133MHz, 264kB of static RAM (SRAM), and a clever programmable input/output (PIO) block which can be used to create state machines running independently of the main CPU cores.

To this, Herr has added a selection of 0201-footprint surface-mount devices (SMDs) plus an ultra-compact 3.3V low drop-out regulator, and a nearly-as-small quad-SPI flash device for program storage. Perhaps the most impressive inclusion: physical buttons for boot mode selection and reset, going above and beyond the features built in to Raspberry Pi's own considerably larger Pico development board with its lack of reset button.

"When designing this board, I knew it would have to be at least four layers (two for GND and power (split voltage plane) and two for signals)," Herr says. "In hindsight it would have been much better as a six layer board, but the price was too hard for a test run, so four layers it was. Choosing what pins I wanted to go on the castellated edges took me a long time, too; I did SWD [Serial Wire Debug], all power lines, USB, and all ADC [Analog to Digital Converter] pins, and the rest are [the] digital pins closest to the edge."

The result is a surface-mountable stamp-format module which makes the already compact RP2040 Stamp look positively gargantuan. It's also assembled entirely by hand, using a stencil, "very, very expired" lead-free solder paste, and a hot plate, plus a 3D-printed jig and a hot-air station for the RP2040 chip.

More details on the build, design files for which have not been made public, are available on Herr's Hackaday.io page; the engineer has also designed a carrier board for the part, which he promises to show off "in the next few days."

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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