MC6800 computer

Retro computer using the basic M6800 chip set - MC6800, MCM6810, MC6821, MC6850, powered and communicating with PC via USB cable.

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The board is a showcase for the chipset I found one day. Motorola MC6800 was the second serious 8-bit microprocessor, competing with Intel 8080. The computer has 256 B of RAM, implemented with two MCM6810 128 byte chips, programmable IO provided by MC6821 PIA and serial console interface using MC6850 ACIA. Single 7400 is used as basic glue logic. To save on the other components, the other necessary functions (Reset and clock generation, USB-UART bridge) are implemented with STM32F103 "BluePill" board hardware and firmware, serving as system controller. The BluePill firmware is based on SDC_One concept - when connected to a PC, the board appears as two VCOMs. One is used for setting the parameters of system controller, the other connects to MC6850 ACIA and provides console interface for MC6800.
All the passive components are SMD, mostly 0603 size. They are all mounted on the bottom side of the PCB, to avoid spoiling the perfect view of 40+ years old ceramic case chips. ;)

The STM32F103 BluePill board serves as system controller, providing user-configurable clock and reset generator for MC6800, MC6850 transfer clock generator, periodic interrupt timer for the MC6800 and USB-to-UART bridge for ACIA. The computer is powered from PC via the USB cable.

At the PC side it appears as two VCOM ports: one for system controller configuration, another for ACIA communication.

The controller's firmware allows for setting the computer parameters: clock frequency of MC6800 and MC6850 (independently), SysTick timer period and optional PIA buffer directions. The settings may be stored in controller’s Flash. It may also reset the MC6800 or issue an NMI request to it based on a command entered from the terminal.

With proper BluePill firmware modifications, the board may support MC6802 in place of MC6800. This is the next step planned.

  • 1 × MC6800 Microprocessors, Microcontrollers, DSPs / Microprocessors (MPUs)
  • 1 × MCM6810 Memory ICs / Static RAM (SRAM)
  • 1 × MC6821 Microprocessors, Microcontrollers, DSPs / IO Controllers
  • 1 × MC6850 Microprocessors, Microcontrollers, DSPs / IO Controllers
  • 1 × 2732 EPROM memory

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  • First runs

    gbm04/23/2022 at 08:11 0 comments

    Originally the board was designed to use 28C256 EEPROM. When I realized the chips I have don't work, I added 2732 EPROM support as an alternative.

    System controller firmware and M6800 hardware operation verified with a simple demo program running on MC6800. Now working on some more robust demo, maybe a tiny monitor firmware for loading and executing small programs from RAM.

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