Vedhathiri
Author
The 68k-MBC is a homebrew single-board computer built around the Motorola 68008 processor, running at 8 MHz. What makes it interesting is that it can be built in two versions, Lite and Full. The Lite version keeps the hardware very simple, using three main ICs: the 68008 CPU, a PIC18F47Q10 microcontroller, and SRAM. It supports 128 KB or 512 KB of RAM and can run CP/M-68K. The Full version adds more hardware and supports 128 KB, 512 KB or 1 MB of RAM, two serial ports, RS232 support and CPU interrupts. The computer is not just a processor experiment either. It can run CP/M-68K as a standalone computer, run Enhanced 68K BASIC programs, load and run custom 68000 programs through its serial port, and automatically start a program when powered on. Optional modules also add microSD storage, an RTC, GPIO and I²C expansion.
The most interesting part is the PIC18F47Q10. Instead of using separate chips for several support functions, the PIC works as the computer's I/O system, EEPROM, reset controller and 8 MHz clock generator. Its firmware, called IOS (Input Output Subsystem), connects directly to the 68008 system bus and handles the I/O operations needed by the CPU. During startup, the PIC also feeds instructions to the 68008 and loads the required code into RAM. The firmware even handles the 68008's two-word instruction prefetch during this process. This makes it possible to build a working 68008 computer without adding separate chips for all of these jobs.
The two versions mainly differ in how much hardware and how many features they provide. The Full version uses more address-decoding hardware and can provide up to 1 MB of RAM, two serial ports, RS232 support and CPU interrupts. The Lite version removes much of this hardware and uses a much simpler address-decoding method, allowing the computer to be built with only three main ICs. In return, Lite is limited to 128 KB or 512 KB RAM, one usable serial port, I²C on the AUX connector and no CPU interrupts. It can still run all the provided software, including CP/M-68K. With optional microSD storage acting like a hard drive, GPIO expansion, an RTC and add-on boards such as uTerm and uCom, the 68k-MBC becomes more than a simple retro-computer project it is a working 68008 computer that can run software, accept programs from a modern PC, use external storage and connect to other hardware.The 68k-MBC is also an open-source project, with its hardware and software files released by the maker, making it possible for others to study, build and modify the computer.
