HB6809 Homebrew computer

This is my 6809 Homebrew Computer

Similar projects worth following
I'm Building an expansion computer, based on Grant Searle's / Jeff Tranter's Minimal 6809 computers. The computer has 4 expansion slots, an expanded memory map, upgraded power distribution and expanded experimentation area.

I wanted a low cost kit like computer based around the 6809, with a very simple expansion bus.

Design goals:

  1. Cost - try to keep board footprints small (this was hard, further minimization with surface mount parts would be the way to go even lower)
  2. Minimal - keep the focus on simplicity, very few chips and very simple memory map
  3. Extendable - break out as many test points as practical and in locations that make them easy to bodge to.
  4. Runs existing firmware - Since I plan to use this as a learning platform I want the computer to be built around a reference design 
    1. Grant Searles Page :
    2. Jeff Tranter's page :

ACIA Serial Adapter PCB TOP.svg

Top Serial adapter Render

svg+xml - 982.77 kB - 08/15/2022 at 16:20


ACIA Serial Adapter PCB BOTTOM.svg

Bottom Serial adapter render

svg+xml - 982.82 kB - 08/15/2022 at 16:20


Schematic_HB6809 ACIA ADAPTER_2022-08-15.pdf

Here's the schematic for the minimal serial adapter.

Adobe Portable Document Format - 66.40 kB - 08/15/2022 at 16:18


HB6809 Rev 2.1 TOP.svg

Top of the main PCB render

svg+xml - 13.54 MB - 08/15/2022 at 16:17


HB6809 Rev 2.1 BOTTOM.svg

Bottom of the PCB render

svg+xml - 13.54 MB - 08/15/2022 at 16:17


View all 6 files

  • False starts are sometimes the best starts....

    Dave Collins4 days ago 2 comments

    So it has been a long time since the last update, but here goes....

    So I built up the Grant Searle / Jeff Tranter Single board 6809 computer on a breadboard: 

    If you haven't checked out the minimal 6809 on their sites its a good read both of them, and definitely more educational than anything you'll get from me!

    I put together the reset / clock divider we talked about in my last update.   Consequently, a lot of the schematics were poorly  drawn in that update and so it's best to ignore them. It worked like a charm but like so many things I wound up simplifying further and just hooking the crystal up to the CPU directly (its built in to the chip and though I don't think anybody has de-capped a 6809 to check (that I know of?)   It is VERY likely the clock is normalized with an inverter heading into clock generation inside the chip so why bother?    In the end, being able to scale the chip clock was fun to be able to run the CPU at 1 MHz, but I was always going to put the vintage chip in my collection and simply use the early 2000's era Thompson / SGS Version any how; and it runs at 2 MHz.

    Lets take a trip on down to feature creep city:

    It really is the city where good projects go to die...

    I knew about this issue (lets face it no good project is immune).  I had an idea to learn a little bit about address decoding, and expansion busses.   I decided I wanted a few things out of this computer build:

    1. I wanted it to be low cost ( not made of money, retro kits are expensive lets be real somebody has to pay for the time and energy that goes into making these things into kits, documenting them, distributing them etc.) 
      1. This means I wanted to start with a open or very simple design and expand
      2. I wanted to add very few parts, again to keep the costs down.
      3. I wanted to learn about address decode, and busses the way I learn things by doing and making mistakes.
      4. I knew, and accepted it wasn't going to be perfect but I wanted it to at least work; and be smart and minimal so that as many people could try it out or expand on it like I expanded on the last two projects.
    2. I wanted the chip count to be low, and I wanted the chips to mainly be HCT again for cost reasons as well as availability (you can still get through hole HCT chips new, at generally reasonable prices.)
    3. I wanted there to be a PCB, and example expansion card PCB I could release as open hardware.  I didn't feel right taking somebody else's design and hard work, and selling it as a kit to make money off of.  (even if i was modifying it in a big way It still runs Jeff's firmware, witch is kind of grants firmware, Which is KIND OF Microsoft's firmware.)  This is a clone, with bells and whistles and I am lucky to just have the community engagement in kind based off of the work of these two people.
    4. I wanted to focus on the 6809 and pin compatible 6309, I had no plans to directly support the external clock chip versions.

    I set out to build a back plane. I had no idea what a rabbit hole this was going to be.  First off lets be real, The world doesn't need another back plane standard for retro computers: 

    • STEbus
    • Euro card
    • S-100
    • Z50 Bus
    • even RC2014 (don't even get me started!)

    this was a huge mistake. full stop.

    I released several ideas for sub-par backplane designs, similar to RC-2014 and Z50 bus on some of the groups i frequent on Facebook and prepared for worst.  The internet is a great place to get feedback, but its also frustrating for people like me who are learning, and might not get everything right all the time (how dare I right? )   Lets face it, its never any fun to be told that you have a terrible non-starter of an idea.  But to be honest I rather appreciated it ... though it did do a number on my mental space for a few hours, I'll live this is how we learn.

    ... Read more »

  • Here's the plan...

    Dave Collins07/19/2022 at 05:33 3 comments

    Here we go.....

    So I needed a pallet cleanse of my CMOS CPU project that's kind of hit a snag.  I bought this 6809 in a vintage electronics suppler that is about 40 minutes from my house called Chester electric. They are a small electronics suppler and they have everything! Seriously from HAM parts to boards you can buy to pull micro's off of; it is my happy place.  I never manage to take pictures when I'm there but  here is a video Rinoa's Auspicious Travels took of the inside of the place: 

    if you are ever in southeast Wisconsin its a very interesting place to visit, I highly recommend it!  

    The CPU: 

    Its important to talk about the different types of 6809's.  As far as I can tell there are 3 speed grades B, A and standard (68B09, 68A09, and 6809) these versions have a "On chip Clock Circuit".  Additionally, there is a E variety that brings the divided clock outside of the chip.   This computer uses the on chip clock variety.  By way of comparison, the Tandy Color Computer (CoCo) uses the E variety All of these chips end with E[Package Type] and so they would be: 6809E, 68A09E, or 68B09E.  The package type labeling varies but simply put P = plastic or C = Ceramic, and vice versa.  Also, as a word of caution, it is easy to confuse the chips as they are all 40 pin packages but the pin outs are greatly different. There is an adapter available although I've not tried it and have no idea if it works well. Here's a cross reference of the CPU's I shamelessly ripped from google image search from this source:  

    The plan:

    I'd like to build a basic computer (at first, though I'm not limiting myself).   I'll base the initial build off of Grant Searle's 6809 basic computer.  If you've never been over to his website you really should its an invaluable resource.  A while back i built a terrible first attempt PCB of his keyboard and TV interface.... i'll not link it here as it's not  a very good example of the design but if you look hard enough I'm sure you can find it.  

    Grant has spent a lot of time and energy on his website, so if you want schematics for his original build, he's asked everybody to not re-post them so head on over there to take a look.

    To start I would like to improve the clock circuit. Since the chip has an internal crystal oscillator amplifier pin (XTAL), grant simply used it with a crystal and two capacitors. This arrangement is actually specified in the datasheet, however, since there are 3 speed grades of the 6809 (non-E Version) it is nice to have a little divider circuit so that we can use the lowest and highest speed grade with a jumper and 1 crystal.  Since were using an ACIA as a the initial display and user input interface, it is fortunate grant selected a 7.3728 Mhz crystal for his design.  Simply, with that clock frequency we can get a baud rate of 115200 out of the AICA. However, by building a clock that can both supply a perfect clock for the serial interface and also optionally divide itself down to provide something closer to 4 Mhz ( ~3.68 Mhz), we can use the lower speed grade chip without harm.   Additionally the CPU divides down the clock further, by a factor of 4.  So in the end we get approximately 1.8 MHz. for a B speed grade chip and .9 Mhz for a standard 6809 (this sounds slow but compared to chips of the day it's actually right in there).

    Since grants design, does not have (or really need for that matter) a power on reset, I had to add one for the clock divider in my clock. And so I went with the standard, substandard, C64 Reset Circuit:  

    For the simple reason I've used it before, and it works.  I may experiment with one of those reset chips, that provides watchdog / brownout protection, but for now this is a very common known design.  Since I had to put in the inverter chip any way...

    Read more »

View all 2 project logs

  • 1
    Very Simple mod to the LM2596 switching regulator module

    I discovered you can simply add a DC jack directly to the input of the module by bending the tab back and running a small 22 gauge ground wire.  While not useful for the build it makes it far easier to use this module with a breadboard 

View all instructions

Enjoy this project?



Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates