CPC ROM Emulator using Pi Pico

Quickstart

To load the firmware image:

• Press and hold the bootsel button on the pico and plug the pico into a PC. It should appear as a USB drive alled RPI-RP2.
• Copy the firmware image firmware/cpc_rom_emulator.uf2 onto the Pico USB drive

You have now programed your Pico. It should reboot and appear a USB drive called PICOROM with about 1.5MB capacity.

Obtain ROM images

You will need some CPC ROM images. You will also need the picorom.rom file, which is in the firmware directory. The ROM images need to be plain ROM dumps, with no additional headers. They should be 16384 bytes in size. There are many ROM iamges available here https://www.cpcwiki.eu/index.php/ROM_List Be careful as some of these images contain an additional header, which you will need to remove first.

• Copy your ROM images onto the PICOROM USB drive

As a minimm you will need OS_6128.ROM and BASIC_1.1.ROM. You should also add picorom.rom

At this point, you can test your PICOROM by unplugging the USB cable and plugging it into your CPC. If all is well, the CPC shold boot into Basic 1.1

Create config files

You can create config files from the PC to define what ROMS to load. These files consist of one or more lines like this:

<SLOT>:<ROMFILE>

Where <SLOT> = L for lower ROM or 0-9 for upper ROM bank
and <ROMFILE> = the filename of the ROM to load

For example:

# Default config
L:OS_6128.ROM
0:BASIC_1.1.ROM
1:picorom.rom
2:maxam15.ROM
3:Protext.rom
4:Utopia_v1_25b.ROM
5:Manic_Miner.rom
6:Chuckie.rom

At startup, if the Pico finds a file called DEFAULT.CFG it will load that. Otherwise if will try to load OS_6128.ROM and BASIC_1.1.ROM

Status LED

The LED shows the status of the board:

• solid on - emulating ROMs for the CPC
• 1/2 second on/off - emulating USB drive for PC
• off - bootloader mode (or no power!)
• Repeating rapid flashes indicate an error:
  • 4 flashes = Failed to load OS ROM
  • 5 flashes = Failed to load Basic ROM

ROM Commands

If you have loaded the picorom.rom, the you get some new commands on the CPC which let you control the board:

• |PUSB - start emulating a USB drive. CPC will stop working.
• |LED,n - Control the PICO LED n=1 for on, n=0 for off
• |ROMSET,"<config file>" - load a new config from the Pico.
• |PDIR - list all available ROMS on the Pico
• |ROMS - List currently inserted ROMs
• |ROMOUT,n - remove a ROM from slot n
• |ROMIN,n,"<rom file>" - loads rom into slot n

More details

Signal	GPIO	CPC Pin	Pico Pin
A0-A13	GPIO0-GPIO13	18-5	1-2,4-7,9-12,14-17
D0-D7	GPIO14-GPIO21	26-19	19-22,24-27
~ROMEN	GPIO22	42	29
ROMDIS	-	43	-
A15	GPIO26	3	31
WRITE_LATCH	GPIO27	-	32
CPC RESET	GPIO28	40*	34
Pico RESET	-	-	30

* GPIO28 connected to CPC RESET via a diode

CPC RESET __|\_|___ GPIO28
            |/ |

CPC Reset also has a push button to 0V

WRITE_LATCH signal is created using a Diode-OR gate:

  A13 __|\|___
        |/|  |
             |
  ~WR __|\|__|____  WRITE_LATCH (GPIO27)
        |/|  |
             |
~IORQ __|\|__|
        |/|

All diodes IN4148 or similar. Pulldown provided by GPIO pin.

ROMDIS is connected to 5V

Pico is powered from 5V via a IN4148 diode to VSYS

Pico Reset is a push button to 0V

Notes

At startup, ROMs are loaded into RAM arrays, the the second core emulates all ROM The first core handles the ROM latch at 0xDFxx with the help of a PIO state machine. The same IO port is also used to send commands to the PICO. This is done by writing a series of bytes to the port, startign with a 0xfc (which I don't think is a valid ROM number). Format is as follows:

• 0xfc - cmd prefix
• cmd byte
• 0 to 4 parameter bytes

Data is sent from the PICO to the CPC via a 0xff byte area in the ROM at 0xC100. Format is as follows:

• sequence number - incremented when the PICO has completed the command
• status code. 0=OK
• data type. 1 = null terminated string
• data ( 0 or more bytes)

There is a CPC ROM which provides a control over the ROM emulator.

PCB

WARNING There is an error on the schematic and PCB silkscreen. D2 is reversed. So, if you are going to build this, make sure that you insert D2 with the cathode (stripe) at the bottom.

There is a schematic and PCB layout which includes an optional USB interface.

My PCBs were made by PCBWay. The gerbers I used are also avalilable.

Credits

FatFs - http://elm-chan.org/fsw/ff/

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USB flash drive code - https://github.com/oyama/pico-usb-flash-drive

Copyright 2024, Hiroyuki OYAMA. All rights reserved.

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