Build an FPGA Microbee in Three (Not so) Easy Steps

Microbee,_Melbourne_Museum

[Brad Robinson] was feeling a bit nostalgic for his Microbee, so he rebuilt it in an FPGA. Not once, but three times. For the uninitiated, the Applied Technology Microbee was a Z80 based computer 1980’s. Designed in Australia, the Microbee did not see much popularity outside its home continent. Even so, the introduction to home computers many Australians was on a Microbee. [Brad] actually wrote several programs for the Microbee, including some games sold by Applied Technology themselves.

Fast forward to 2012, [Brad] is learning FPGAs, and wants to build a Microbee in VHDL. The FPGAbee was born. The first iteration of the FPGAbee began with the CPU, which came from the T80 open source VHDL Z80 core. Around this core [Brad] added the video controller, keyboard, and sound. When he started adding disk functionality, [Brad] ran into some problems. He wanted to use a FAT formatted SD card for cassette and hard disk emulation.

The relative complexities of the FAT format meant he would have to use some custom software to make this work. [Brad] decided to run this software on a second Z80 core. Both cores would need access to memory, and this is where [Brad] learned what he calls “a hard lesson in cross domain clocks” on FPGAs. Multiple clock nets can cause major propagation delay issues. [Brad] was able to work through the problems, but it caused him to step back and re-evaluate the entire design. This was the start of FPGABee2.

Brad eliminated the dual processor setup, and redesigned a few other parts of the system. This second iteration of the design proceeded smoothly to completion. At the end of the project, [Brad] had a Microbee emulated in a Xilinx Spartan6 on a Digilent Nexys-3 board. While great for prototyping, the Nexys-3 is not very practical as a final system. [Brad] wasn’t about to roll a BGA based PCB himself, so he tried out a Xess Xula2 board. The Xula2 did require some porting work, but [Brad] was able to get the system up and running. A custom through hole PCB carrier board resulted in a slick finished project, the XulaBee.

[Thanks Dave!]

Comments

  1. ..the memories..

    The Microbee was actually quite popular as a school-computer in Sweden. I remember as a student changing the BASIC code of “Worm” to get more score. It was actually running in some kind of network (serial?) with central storage too..

  2. Matt says:

    Wow brings back memories! We had a network of these at our primary school and I changed the BASIC code of Taipan to get more money :)

    Ours had a central file server as well. If I remember right, you’d type NETDIR to see what programs you could load off the server and NETLOAD “program” to load it.

    The Bee was the first computer I ever saw that made me want to get a computer. I ended up with a Commodore 16 for a week or so, then we took it back to Grace Bros and bought a Commodore 64. Started my career :)

  3. xobmo says:

    Please check your posts for grammatical errors. [Brad] as we know him can be called “he” after mentioning his name once or twice in the article. You wrote [Brad] 9 times and Brad once. This makes for a hard to read article. Thanks!

  4. Paul Pounds says:

    I have good memories of the old Microbee. One of three prototype Microbee Gammas sits on the desk of my colleague at University of Queensland. I can’t wait until he gets it up and running again.

  5. MarkS says:

    wow – pretty nice. What’s involved with porting to Papillio ? big enough ?

  6. Andrew says:

    Ah, hundreds of hours spent after school learning programming on a MicroBee.

    in#5 anyone?

    We had a serial network made by one of the mothers at our school. It used an Apple ][e as a file server. You selected on an RS232 multiplexer box which computers would receive the program.
    First it downloaded a short bootloader and then loaded the actual program you requested (requested verbally, that is, to whoever was operating the file server. It was more correctly a “file pusher”).

    Good times!

  7. Cosmic R says:

    Microbee brings back memories. I remember impressing my grade 4 teacher that I knew BASIC, which I had learned on my Tandy 1000. This must have been about 1990.

    My school had about 10 microbees, whilst the ‘computer class’ teacher had an Amiga 1000.

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