The Keychain 6809

When you think of tiny microcontroller boards, you probably think of a modern surface mount processor. Not [Andreas Jakob]. His 5×5 cm keychain computer rocks a 6809 CPU at a blistering 1 MHz or, if you prefer, a 6309 that runs at 5 MHz. The RAM — all 32K — is in a SMD package to make it fit, but the board also sports a 27C256 EPROM which means that chip and the CPU take up most of the PCB.

As you might expect, there’s not much else on the board. It doesn’t hurt, too, that the PCB is a 6-layer board. The board features a USB C port for power and data, but we didn’t see the USB interface chip on the schematic until we opened it in Easy EDA using the button that says “open in editor.” The schematic says it is sheet 1 or 1, but there are actually two additional “tabs” you can only see in the editor with the apparently missing pieces.

The ROM contains [Jeff Tranter’s] “combined ROM” which has a monitor and BASIC onboard. You’ll also find an expansion port.

What can you plug into the port? How about a matching expansion board with a bunch of I/O connected to a 6821 chip. The board is the same size, although clearly, you’d add some thickness when you stack the boards. You’ll notice the 40-pin DIP barely fits on the board diagonally.

If you don’t mind a larger build, grab a breadboard. The 6809 appeared in a few computers, but we always liked the look of the Poly-1.

Thanks to [Stephen Walters] for the tip.

16 thoughts on “The Keychain 6809

  1. Not bad, but I have a small Raspberry Pico, running “PicoMite” BASIC, that has probably 100X the speed, 8X the ram, more IO, I2C, SPI, I2S, and anything else for $8 or so, PCB included. Sure, it’s 3.3V IO, but it can talk to just about anything, works on a USB virtual serial port, and has a build in basic editor.

    I actually carry that on one of my keychains, just in case I need an I2C,SPI, UART etc in a pinch.

    1. I imagine the 6809 is pretty much the raison d’etre of this thing really. If you don’t go to the kind of parties where whipping out your ‘9 impresses the ladies, you’re probably better off with the Pico. 😊

  2. My favorite 6809 application was the Vectrex all-in-one vector display (not raster!) video game by MB Electronics (Milton Bradley). I still have one I got at Children’s Palace for $39 in the late 1980s. The game featured a not too bad knock-off of Asteroids.

    The 6809 was a very nice chip to code. There are A and B accumulators, hardware multiply and simple bus interface.

    General Motors also used a version of the 6809 which they manufactured, called GMP-4, in ECUs in the early 1980s. GM operated a large chip foundry in Kokomo, Indiana. I read that GM finished dismantling their chip making operation around 2016. I’ll bet GM wishes it hadn’t made that decision.

    1. I used an machine with the operating system OS9 made for the 6809 (long before Apple OS9). I was designed after Unix, with a nice shell and Unix-like commands. It was modular so you could add and remove device drivers dynamically. And version two of it had support for different MMU, so it could have up to 512 MB RAM, even though the address space of the chip, and thus processes was 64 kB RAM.
      A really impressive OS of it’s time. They later made a version of the OS for 68k and 80386.

        1. Yup that’s correct. I had 3 of them and used OS9 almost exclusively on them after I found it. Those were very good learning tools and I taught myself quite a lot about hardware and software with them. The cartridge interface was easy to hack and extend and with a little careful soldering it was easy to add RAM although that had to be bank switched. Because the CoCo used the 6809E (clocked at .89Mhz as I recall) it was possible to overclock it, but not much for a bunch of reasons. If, and that’s a big if, I remember correctly I was able to overclock it to just under 1Mhz.

          1. Also wasn’t the 6809 and the 6502 hardware compatible? The story I heard was that the engineering team got into a fight and left. The 6502 could be used in the exact same boards with the same pins, the same peripherals, but a different machine language. In theory you could drop a 6809 into a C64/Apple II/Atari 400/800 and change the rom chip and it should work.

          2. No. The 6502, orI guess 6501, was designed to be pin compatible with the 6800. And it was barely available when a lawsuit from Motorola shut it down. Which is why we have the 6502.

            The 6809 was an improvement of the 6800, but came later.

            You can’t plug a 6809 into a 6502 computer.

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