PicoCPC custom board

In the computer playground wars of the 1980s, children would spend hours extolling the virtues of either the ZX Spectrum or Commodore 64, depending on which side of the fence they sat. As the arguments raged, those who owned an Amstrad CPC machine tended to watch from the sidelines; but, deep down, they knew their chosen machine could very much hold its own.

The PicoCPC ROM is used for small data transfers from the add-on to the CPC; the final PicoCPC model’s PCB is going to have an RP2350B microcontroller soldered directly on to it

As time has gone on, there’s been a growing appreciation for the 8-bit computers created by Lord Sugar’s company. In recent years, a small but nonetheless loyal community has been creating a string of games that push the machines to their limits (check out Pinball Dreams as proof). They’ve also been producing hardware to take the CPC to the next level. A good example of this is the M4 board, which not only enables wireless LAN but also allows an SD card to be used for storage.

Joining the hardware roster is the PicoCPC, a new multi-purpose add-on that can be used across the CPC range, including the Plus machines launched in 1990. It’s impossible to sum up its capabilities in one sentence. Rather, this neat little device — built around a Raspberry Pi Pico 2 — provides a whole host of benefits for machines that maxed out at 128kB of memory. 

The prototype PicoCPC fitted with a Raspberry Pi Pico 2 and a small display

For starters, the PicoCPC extends the memory up to 1024kB. It also emulates a floppy disk controller, allows the use of up to 16 emulated ROMs (for instant access to the likes of the Protext word processor and alternative operating systems such as SymbOS and FutureOS), and enables cartridge-loading of software produced for the GX4000 console. It adds six-voice audio, courtesy of the PlayCity sound card emulation, and there’s also a clock. For CPC enthusiasts, it’s fast becoming an essential upgrade.

Key to success

The idea emerged after Stéphane Plantard noticed a problem with many previous CPC add-ons. “I discovered there were a lot of expansions for Amstrad’s early computers, but they were expensive, rare, and made with deprecated chips,” he says. Over time, he became very familiar with the inner workings of the CPC. He produced an external Gotek drive for the CPC 664 and the CPC 6128, as well as a device that powers the CPC and allows it to be connected to a TV instead of the bundled monitor.

The device has gone through a few revisions; Stéphane is also working on a cartridge with an SD card reader for Amstrad’s GX4000 console

“Then my friend Freddy started a project called PicoMEM for old PCs,” he says, of a device based on the RP2040 microcontroller that runs emulated 8-bit ISA boards on a real PC. “I thought I could do a similar card for the CPC, so I started to write some CPC-related code on the PicoMEM to test the SD card readings, which allowed me to become familiar with Raspberry Pi Pico. Freddy then pushed me to create my own card, and I’ve produced three prototypes so far.”

Stéphane says he approached the project according to the agile principles, which essentially means he sought to be flexible, open to change, and willing to work closely with the CPC community (there are twelve principles in total, and they’re part of the Agile Manifesto published in 2001). “It means I maintain a backlog of ideas and set up my epics and my sprints,” he explains. “Having a strategy is a requirement when you work alone on such a big project.”

The Amstrad CPC 464 made its debut in 1984; the PicoCPC plugs into the back
(Credit: Bill Bertram, Wikimedia Commons, CC BY-SA 2.5)

In essence, Stéphane has been working in short development cycle bursts, or “sprints”, that tend to stretch to a week. “It has allowed me to achieve some progress and reach goals; it’s often enough to stay motivated,” he notes. Given the project has taken a year so far, motivation has proven important. Stéphane has certainly seen the many benefits of basing the project around a Raspberry Pi Pico 2, and he says it’s preferable to the alternatives he considered.

“The CPC generates a lot of signals that the card has to act upon, so an STM32 microcontroller would not be fast enough,” he says, of one potential choice. He could have opted for an FPGA solution instead, but this, too, posed problems: “It would have required expensive chips, and I don’t know how to work with them,” he adds. “At the end of the day, the Pico 2 is cheap and fast. Its two cores and the PIOs make it a lot more suitable for this kind of task than any other available microcontroller. It uses one core to manage CPC I/O, a second core to run the emulations, and the PIO pilots the multiplexors to get addresses and pull/push data from/to the CPC.”

Quick upgrade

Some of Stéphane’s ideas are still being worked on. “The PicoCPC does not support USB mice and joysticks yet. It’s in the backlog but not done,” he says. But, as it stands, the PicoCPC makes a big difference to the experience of using an Amstrad CPC, and it also addresses a few practical issues.

“An ordinary CPC user would want to use an original computer to play old games from back in the day,” Stéphane says. Since the tape-based CPC 464 is easier to find than the disk-based CPC 6128, the PicoCPC can be used to transform a 464 into a 6128 by adding an emulated floppy drive, more RAM, a later version of BASIC (v1.1), and the C3 mode which allows access to the extra RAM beyond the base 64kB. “All the CPC 6128 games and demos will then work on a CPC 464.”

You can use PicoCPC to boost the amount of memory for any one of Amstrad’s five CPC computers

Likewise, on a CPC 6128, PicoCPC can override the internal floppy. “Users won’t have to take care of deteriorated, old floppy disks,” Stéphane says. But that’s just touching the surface, he adds. “Many new games developed in recent years require 512kB of memory,” he explains. “The Pico does not offer enough memory for that, which is why I added an SRAM chip on the card.”

He hopes the availability of PlayCity will push future software developers to use the expanded audio capabilities, and he’s excited about other possibilities. “I plan to add proper hard-disk emulation, AdLib music card emulation, and new blank floppy disk creation,” he reveals. “But the first plan for the PicoCPC is to make it available. I’m working with retailers in France, the UK, and Spain, and hope it’ll be on sale very soon.”

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