Emulate a small size ROM with a RAM and Arduino with Intel-HEX interface
ROMemu is a combination of software and hardware that allows the usage of modern software development tools for simple, old style single board computer systems. It emulates an (EP)ROM programmer to the development software and an in-circuit ROM/RAM on the board.
Here a diagram showing how the ROM-Emulator works. In operational mode, the RAM chip is connected through the buffers and POD converter to the socket of the target board. All the Arduino pins connected to the RAM are tri-stated, so the processor on the board sees the contents of the RAM.
When the Arduino accesses the RAM, the buffers are tri-stated, isolating the RAM from the board. The processor would see FFh in the memory region addressed by the socket. The Arduino can now fill the RAM with data received from the PC, or change it with its internal commands.
A feature planned for the coming version is a relay allowing the Arduino to set the target board processor in RESET mode during the transfer of data from the PC to the RAM. This prevents the processor from 'panicking' when the ROM disappears from its memory.
The software (Arduino sketch) is version 0.10.
Current command set of the Arduino sketch:
-- ROM emulator v0.10.3 --
Operational commands:
Cssss-eeee-tttt - Copy data in range from ssss-eeee to tttt
D[ssss[-eeee]]- Dump memory from ssss to eeee
Fhhhh - AddressOffset; subtracted from hex intel addresses
H - This help text
:ssaaaatthhhh...hhcc - accepts hex intel record
;ssss-eeee - Generate hex intel data records
E - Generate hex intel end record
Kssss-eeee - Generate checksums for address range
Maaaa-dd - Modify memory
O - Toggle echo
R[0|1] - Switch the RESET relay
Test commands:
A - test 32 kByte RAM with 00h, 55h. AAh and FFh patterns
Bpp - blink pin p (in hex)
Sssss-eeee:v - fill a memory range with a value
Tp - exercise port p
V - view ports C, L, A, CS, OE, WR, ARDUINOONLINE
Wpp v - Write pin (in hex) values 0, 1
? - This help text
The current hardware version 1.1 uses an Arduino Mega 2560 with a shield containing a 32 kByte RAM-chip which is (partially) readable by the target board processor. The Arduino USB-connection is available as serial port to the development system. This allows to write Intel-hex data to the RAM.
The problems with the 1.1 version are probably solved with the 1.3 version. This version awaits production and testing.
The KiCAD-files contain the projects for the shield, adapter and socket- PCBs. The first two have fixes from testing the previous produced boards.
For downloading hex-Intel files the romEmuFeed.py is provided. It supports three arguments; the hex file, the serial port (optional, '/dev/ttyACM0' is default) and an optional offset address (in hex) for data that doesn't start at 0000h. The actual address in the CPU memory map is determined by the PCB circuit, not the emulator. So data usually needs to be starting at 0000h (of the emulator RAM).
Usage: python3 romEmuFeed.py <hexFile> [<ttyPort>] [<hexOffset>]
The script could very well work for Windows and MacOS, but is untested.
A new and experimental download tool is the 'wxRerminalRE.py' application, a GUI replacement for the romEmuFeed.py program. It can select and configure the serial port, download Hex-intel files and monitor and control the ROM Emulator. It is developed and somewhat tested on Linux, but should work on Windows and MacOS to (courtesy of the wxTerminal.py it is based on).
There is a Hackaday page at: https://hackaday.io/project/175610-rom-emulator
F.J. Kraan, 2024-08-08