Attaching AVR with Z80 Machine Emulator

Overview

The project aims to emulate an old good Z80 machine on an AVR ATmega88 microcontroller to be able to run software that has to be CP/M machine emulated.

Details

A Turing-complete device with enough storage is all that is needed to emulate a certain machine. An AVR ATmega88 fulfills the function of the Turing-complete machine as it contains 8K of flash and a bit more than 1K of SRAM. An old 4-bit DRAM 128KB GM71C4256A chip was chosen since 1K is not enough for the emulated machine by a long shot. This makes more than enough to be stored by the 128K for CPM to run in. the minimum specifications for CP/M includes 20K of RAM, an 8080-type processor, and some kind of storage.

A MMC/SD card replaced the out of date floppy disks and the built-in UART was used since CP/M only needs a character-based terminal as its input and output. No level shifters are needed to talk to the SD card since the chip runs at 3.3V. The wiring shows that the I/O pins control the RAM, which are partially shared with MMC that is controlled over the built-in SPI hardware.