Intro and Forward:

The HB63C09M was an idea I hatched back two years ago when I was finishing up my first home-brew computer the HB6809. But it goes back further than that. The beginning is really around 2020 when I started building kit computers as a means to pass the time during the pandemic. I had purchased a kit from McJohn in Italy called the Z80-MBC2, by Fabio Defabis – an excellent little computer kit with a single logic chip, atmega32, Z80 CPU and a RAM chip. I was hooked on the small form factor and the things that I could do with my little computer. I made a few hardware expansions for it, most notably a joystick interface for my Tetris clone that I had programmed in pascal. At the time there were people asking Fabio on the Facebook group for the project if he was going to do other processors. Before long he was working on the V20-MBC (which I built) and later he went on to make the 68K-MBC which is architecturally slightly different. After a while, no new MBC computers were made, and by then I was on to building my 6809 based system, and still quite a novice at this (I am still!)

Things happened life got busy I spent some time working on other projects and about a year ago started playing around with Arduino – and digging very deeply into the Z80-MBC2. I really wanted to understand how memory was loaded using a microcontroller and I wanted to know how Fabio had shared the bus with the Z80. Right around the same time I had also stumbled onto an even older work, Brad Rodreguez’s “Scroungemaster” and I fell in love. The scroungemaster is (well was) a concept for a 6809 multi-processing build Brad had built on wire wrap. He had used a rather simple circuit to do clock stretching in order to hand off cleanly to the other CPU’s into common ram. He detailed this in a paper entitled “Multiprocessing for the Impoverished” (found at https://www.bradrodriguez.com/papers/) I quietly started to play around with the design and build a small expansion for the HB6809 that allowed me to use an Arduino like a peripheral without a latch simply by stretching the clock.

In his paper, Brad discusses the parallels between the Z80 and the 6809. I found a lot of parallels in my Arduino sketch and the work that Fabio had done with the Z80-MBC2, and after a few months I had a working prototype that could stage itself from the Arduino (at the time using latches) and boot into a machine language monitor. After that I streamlined the design, making it slightly more like the Z80-MBC2’s little brother the Z80-MBC by opening up a few address lines to the Arduino and then additionally solving a very difficult timing problem by changing from a controller generated clock to a synchronous clock (both the 63C09’s internal clock and the ATMEGA32 are clocked by the same 20Mhz crystal) and the board fully stages from ROM files saved to the SD card. No Rom code exists on the AVR EEPROM (save a small less than 40 byte program that bootstraps from the SD card). This allows the programmer and user to switch between environments without having to Flash a ROM onto the computer!

After a few revisions of the board, I found I could further shrink the form factor to a smaller 10x10 PCB – albeit 4 layers. The Exact same size PCB as the Z80-MBC2! For me, it was like coming full circle, and now I think I am ready to share this design with others. What I have is a fully functioning – ready to go 63C09 playground. But there's still some work on the software, this is where you come in.

What I am calling this board revision is “The Prerelease” edition. Essentially the hardware is finished. From this point, any and all architecture changes will need to be approved by the “Community” -- what that means specifically will have to depend on the group of developers that go ahead and early adopt the design for the express purpose of porting software to it. For now there is enough (really) for people to start playing around with the platform in earnest, basic, machine language monitor are all there. I have started this document which is a rough outline of a manual for programmers and developers. But to really go forward I need somebody who’s more versed in operating systems than I am. I could possibly figure out how to continue developing software and add LBA device support to basic and possibly the monitor – but I would very much be learning as I go. So The main purpose of this release is to get the boards out there in to peoples hands, hopefully people with considerably more systems knowledge who have a passion for the 6x09 CPU and can really take the lead on developing things like FLEX, Nitros09, Forth and hopefully a lot more!

Resources, How to get started:

Some files are here in the files section, however most up - to date info is always going to be on the Github Everything including the BOM and Gerbers for assembly is available there.  If you are confused or need a hand with something you can try the Facebook page.  There is a brief programmers reference, that goes over accessing devices as well as the memory map.  A short step by step assembly guide can be found in the programmers reference, and here which includes programing the bootloader as well as programing the firmware onto the MCU.

What if I want to help:

Email me at Z80Dad@Gmail.com and we can talk, I'd be thrilled to help you get a board into your hands, and I can offer boards and kits cheaper to people willing to help develop software.   I have a very small number of boards and parts left over from development, and would like to put those into the hands of hobbyists or developers.  Right now I am trying to see how that looks.  I have a few that I promised to some of you already and have began sending those out.  In the next few days I'll be putting together a Tindie storefront and ill make some boards available there once I've verified how many sets of parts I have.   Honestly - building the board up from scratch isn't so bad.  Its about 44 dollars worth of parts shipped (not including the CPU and the modules ), the PCB itself is approximately the same price from PCBWay (for 5 PCB's - the down side of this is then you have 5). I want to find a way that makes sense for people to do a group buy ... that might be the best way to get this started, so in the next few days Ill post something up on the Facebook group to all who are interested in a group buy to see what that looks like.