Board that intercepts signals from the Acorn Electron or BBC Micro's 6502 CPU, and provides a very flexible interface into the heart of the machine. (Good enough to implement something like the Slogger MRB, or to use a soft CPU in an FPGA, or to do a 3.3V flash/RAM/IO board.)
I've assembled one of these and tried it out with no daughterboard connected, in an Electron, with very basic HDL that just copies A15:A13 from the CPU to the motherboard. It boots fine, as expected.
I've also tried it out with the the miniSpartan + Pi Zero daughterboard, and the OS ROM removed, and the XC6SLX25 implementing some sideways ROM and RAM.
pcb/ - PCB design
cpld/ - CPLD design, which assumes you have a 6502 in the top socket (i.e. the address buffers are always pointed away from the CPU).
Initial tests have been really promising, and I've realized that soft 6502 CPU cores are very small, so I can implement them in small nonvolatile FPGAs like the Lattice MachXO/MachXO2 series.
The next version of this board will not include a CPU socket, just a pair of 74LVC16245 buffers, to handle the soft 6502's address and data bus, as well as all control inputs.
