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Matt Callow’s CPC_PICOROM project turns a Raspberry Pi Pico into a ROM-slot emulator for a real Amstrad CPC computer. It can provide the CPC’s lower ROM and multiple upper ROM slots, with ROM images copied to the Pico through a USB drive interface. It is a DIY hardware-and-firmware build—not software for running CPC programs on a PC.
What the CPC_PICOROM project does
The project replaces ROM chips or cartridges for an Amstrad CPC with ROM images held by a Pico. The repository’s current README describes support for the lower ROM and up to 12 upper ROMs, with the actual number constrained by available Pico RAM. Its configuration instructions use upper-slot numbers 0–13. These figures describe the repository’s current documentation, not a universal capacity guarantee for every hardware or firmware version.
Callow’s design uses two Pico cores: one handles ROM access, while the other manages the ROM latch and CPC commands. A PIO state machine is also involved. At startup, ROM images are loaded into RAM arrays; the Pico additionally exposes its flash storage as a USB mass-storage drive so a computer can copy files to it. See the CPC_PICOROM repository for the firmware, project files, wiring, and setup details.
ROM capacity and files: what the sources say
The capacity descriptions differ by source and should not be combined into one specification. The repository’s current README says up to 12 upper ROMs, limited by Pico RAM. In an earlier Hackster.io report, Callow described a lower ROM plus eight upper ROMs and said the images were stored in the Pico’s 2 MB flash; that report also says the Pico was overclocked to 225 MHz. Those are earlier, source-specific design details, not confirmed requirements or limits for the repository’s later configuration. The counts and reported characteristics appear in Hackster.io’s project report.
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Callow’s repository says the project accepts plain ROM files and files with 128-byte headers, removing an AMSDOS header when it detects one. Its README lists OS_6128.ROM and BASIC_1.1.ROM as minimum system ROM files, and recommends adding picorom.rom for control from the CPC.
How configuration and CPC-side control work
Load a startup configuration
A file named DEFAULT.CFG can map ROM filenames to slots, with each line specifying a slot and a ROM filename. Consult the repository’s current README for the exact file format and supported slot notation before preparing the file.
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Manage ROMs from the CPC
The companion picorom.rom provides CPC-side commands to list available ROMs, inspect slots, load or remove a ROM, switch configurations, and enter USB-drive mode. In USB-drive mode, the CPC stops working while the Pico presents itself as a drive. This is a file-transfer mode, not a way to use the CPC normally while accessing the Pico’s USB storage.
Parts, wiring, and board notes
The repository documents Pico connections to the CPC address and data buses and control signals. Its described build uses passive components rather than additional active logic: a Pico, 1N4148 or similar diodes, a 2.2 kΩ pulldown resistor, and a suitable CPC edge connector. It powers the Pico from the CPC’s 5 V supply through a 1N4148 diode to VSYS. Follow the repository’s wiring information for the exact pin mapping and connector orientation; do not infer a connector fit from the parts list alone.
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- D2 orientation: The repository warns that D2 is reversed on both the schematic and PCB silkscreen. The cathode stripe should be at the bottom.
- Rev1 pulldown: Rev1 boards lack the pulldown resistor; the repository describes how to add it.
- PCB fabrication: The repository includes PCB design files. In a 2023 project log, Callow reported that PCBWay supplied boards for his build; this is a dated account, not a statement about current availability or terms.
Important electrical and compatibility caveats
5 V signals and Pico GPIO
In Gareth Halfacree’s Hackster.io report, Callow warned: “The Pico IO pads are not rated for 5V, and this circuit contains no level shifters.” He said he had seen comments suggesting the inputs might tolerate 5 V, but that this was not certified, and reported that his own build had worked so far. He concluded, “Use at you own risk.” His experience does not establish that connecting the CPC bus to the Pico is electrically safe or guaranteed. Assess this risk before building or connecting the circuit.
Optional USB interface and reset report
The repository describes the optional USB interface as only partially Albireo-compatible, specifically supporting USB drive use. In a project log dated June 14, 2023, Callow reported that his assembled optional interface worked with Albireo DOS or UniDOS. The same log says one PCB build had a CPC reset problem even though his original prototype worked. These are dated reports about his builds, not a blanket compatibility guarantee. See the Hackaday.io project logs for the maker’s account.
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