When Piers Rocks teased his One ROM project months ago, many hoped Raspberry Pi’s RP2350 microcontroller could eventually power it. The wait is over. With the release of the RP2350 A4 stepping—bringing 5V-tolerant GPIOs and fixes to key limitations—he demonstrates the RP2350 retro ROM breakthrough in action, running directly inside a Commodore 64 without level shifters.
This video captures the first successful boot, where a single One ROM replaces the system’s BASIC, KERNAL, and character ROMs. From that emotional milestone, Piers takes viewers through the challenges of designing a PCB just 17x14mm in size, balancing layout restrictions, heat dissipation, and firmware optimization to deliver cycle-accurate results.
Design Goals and PCB Challenges
One ROM needed to fit the footprint of original ROM chips, pushing the RP2350 into tight quarters. While the STM32 variant offered built-in flash and oscillator support, the RP2350 required external components. This added complexity, cost, and routing hurdles, but its smaller package and overclocking potential made the effort worthwhile. Piers shows how careful assembly and smart pin assignment allowed him to eliminate cycle-wasting operations, keeping performance sharp.
Price and Performance Insights
The narrative that the RP2350 is cheaper doesn’t hold up once extra parts and assembly costs are included. In fact, the STM32 solution can be slightly more economical. Still, the RP2350 has advantages—more cores, more memory, and overclocking headroom up to a staggering 540MHz. Piers compares startup times, performance quirks, and even explores advanced features like XIP cache pinning and GPIO co-processors, offering honest perspective rather than hype.
Real-World Application for Retro Systems
This isn’t just theory. The RP2350 retro ROM breakthrough shows a working Commodore 64 powered by a single replacement module, compatible with multiple ROM types and capable of swapping images at runtime. It represents a practical tool for vintage computing enthusiasts, proving that modern microcontrollers can blend with classic hardware in innovative ways.
There is a companion (more detailed) technical article to go alongside this episode:
https://piers.rocks/2025/08/25/rp2350-5v-gpios.html
For those eager for deeper technical notes—price comparisons, schematic details, and more—Piers has a companion blog post linked in the video description.
