CPC ROM Emulator using Pi Pico

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.

ROM Commands

• PICOLOAD - reboot the PICO into bootloader mode
• LED,n - Control the PICO LED n=1 for on, n=0 for off
• ROMDIR - list the available ROMS
• CPC464 - Load 464 ROMS
• CPC664 - Load 664 ROMS
• CPC6128 - Load 6128 ROMS
• FW31 - Load FW31 ROMS
• ROMIN,n,m - Load ROM m into a slot n
• ROMOUT,n - Remove ROM from a slot n
• ROMLIST - List ROM slots
• CFGLOAD,n - Load a ROM configuration from config slot n
• CFGSAVE,n - Save current ROM configuration into config slot n
• ROM7,n - Enable (n=1) or disable (n=0) ROM slot 7

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.