Jonathan Clark's RP3A0 Pico Is a Reverse-Engineered Raspberry Pi Zero 2 W in a New Form Factor

Firmware engineer Jonathan Clark has been hard at work unlocking the secrets of Raspberry Pi's RP3A0 system-on-chip, found on the Raspberry Pi Zero 2 W single-board computer — with a view to using the part in his own board designs.

"This [repository] contains my attempts at reverse-engineering the RP3A0 SoC from the Raspberry Pi Zero 2 W so I can place it on my own PCB," Clark explains. "I plan to eventually use this in another project, but for now I need a proof of concept. I decided to make the most cursed thing I could think of, [a Raspberry] Pi Pico sized board, but with a full Raspberry Pi processor on it. It has basically the same pinout as a [Raspberry] Pi Pico, all the GPIOs [General-Purpose Input/Output pins] that match up are in the same position. HDMI is sketchy, there's no Wi-Fi, and USB [Type]-C is a bit weird, but it boots and I can get it online."

Clark's mission began after seeing an X-ray image of the Raspberry Pi Zero 2 W and its RP3A0 system-on-chip, a compact 0.65mm-pitch developed by and exclusively available to Raspberry Pi. It's not a part you can purchase without it coming attached to one of Raspberry Pi's single-board computers, but that didn't stop Clark wanting to put one on his own board.

"There's another project I've been working on for years (that I'll probably document at some point in the future), but I wanted to add a Linux[-capable] processor to it," Clark explains. "However it's pretty space constrained and I didn't want to have to do traces for DDR [memory], so I was looking for an SoC that had a decent amount of RAM baked in, or a decent SoM [System-on-Module]. Unfortunately, most SoMs I found have parts on the other side as well, meaning I'd have to have a cutout in whatever board I was putting it on."

For the very same reason of space saving, Raspberry Pi's RP3A0 includes both the processor and RAM in a single part — so Clark snagged a Raspberry Pi Zero 2 W and desoldered its components before sanding the board down to capture imagery of each of its six layers. This provided a way to see exactly how each pin of the chip was wired up — which, in turn, allowed Clark to design the RP3A0 Pico, which is effectively a Raspberry Pi Zero 2 in a Raspberry Pi Pico form factor powered by a desoldered and reballed RP3A0.

While Clark has provided a wealth of material for those who would like to follow in his footsteps, he also has one key bit of advice: "This was a stupid idea and you probably shouldn't do it," he warns. "There are heaps of other Linux SoCs you can use, an especially ones that aren't a 21×21 BGA device with no public documentation. It's also not super easy to solder, so you might trash a couple parts trying to get it right."

For those who aren't put off by that warning, the project is documented in full on GitHub along with Altium project files for the RP3A0 Pico board and a part library for the chip itself.