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03/08/11: Piggyback 6309 Upgrade by admin, | Category: Hacks | 11 comments - (Comments are closed)

Piggyback 6309 Upgrade

Here is a reprint from  the  Color Computer Home page on piggyback a 6309 on a 6809. You do not pull the 6809 with this setup.
6809, 6309
Motorola 6809 and Hitachi 63C09 CPU.

GND |1    +–+   40| /HALT
/NMI |2           39| EXTAL
/IRQ |3           38| XTAL
/FIRQ |4           37| /RST
BS |5           36| MRDY
BA |6           35| Q
VCC |7           34| E
A0 |8           33| /BREQ
A1 |9           32| R//W
A2 |10  MC6809  31| D0
A3 |11  H63C09  30| D1
A4 |12          29| D2
A5 |13          28| D3
A6 |14          27| D4
A7 |15          26| D5
A8 |16          25| D6
A9 |17          24| D7
A10 |18          23| A15
A11 |19          22| A14
A12 |20          21| A13

You can stack a 6309 on top of your computer’s existing 6809. Here’s how…

Cut pin 39 (TSC) of the 6809, leaving a small piece sticking out of the
processor so you can solder to it.

Take a 40 pin IC socket. Cut off pins 5, 6, 33, 36, and 38. Bend out pin 39
so you can solder to it.

Stack the 40-pin IC socket on top of the 6809. Solder all of the
corresponding pins together. DO NOT solder the pins 39 together.

Now you have two options:

1) Solder a wire from 6809 pin 39 to +5V, and another wire from socket pin 39
to ground.

2) Solder a 4.7K resistor between each pin 39 and +5V. Then take a SPDT
switch, connect the common contact to ground, and connect each end to a
separate pin 39.

In either case, finish up by plugging the 6309 into the socket. If you picked
option 1, you now have a 6309 system. If you picked option 2, your system is
now switchable between the 6809 and the 6309 (before power-up – not while

I hope this helps some people install their 6309. Good luck!

Chris Burke

good luck


11 comments to Piggyback 6309 Upgrade

  • pilot352


    After reading the instructions you posted which apparently came from some
    publication from the Color Computer home page, I have a few questions and/or
    technical problems with this…

    How can one CPU not effect the other in this configuration?

    Won’t the fact that the CPU’s are “piggybacked” in this manner cause excessive
    current draw (see explanation below)?

    What about the effects of capacitive interference between CPU bus signals?

    My problem is connecting all the pins together (except pin 39). Basically,
    if I understand this, the author is using the XTAL signal to switch between the
    two CPU’s. I’m somewhat OK with this (although there are better ways) but I
    don’t like the idea of connecting all the pins together in this manner. As
    both CPU’s will be powered up, both CPU buses will be active. Removing the XTAL
    signal will not deactivate the signals from the CPU in static mode. The CPU’s
    will be fighting each other. You may get away with this because you are
    merging CMOS and NMOS components but over time the CPU’s can fail.

    A more correct approach would be to use the HALT signal to disable one of the
    CPU’s This forces the static CPU bus signals into a “tristate” or High impedance
    mode. This would prevent bus contention. You can still piggyback the CPU’s as
    described but you need to place one of them in HALT mode to tristate the bus.

    Comments Welcome!!!

    • CocoWal

      Pin 39 is the TSC (Tri-State Control) Pin. When the pin is asserted by pulling it low, it will tri-state the address/data/control lines within a clock cycle . Since both processors share a common clock and sample this pin in phase it should avoid the bus contention issues mentioned (excessive current draw etc). Using the Halt line could potentially cause damage as I don’t believe Halt tri-states the bus immediately but waits for the current instruction to finish , potentially accessing the external bus simultaneously and causing both processors to drive the bus.

      • pilot352


        Maybe I missed something but isn’t PIN 39 the EXTAL signal? Or is there something I’m missing.

        And as for the HALT signal, you are correct if the pin is pulled low while running, but if the HALT is held LOW when coming out of RESET, the processor immediately enters tristate mode. At least according to the MC6809 docs I have.

        • CocoWal

          Ah , that explains the misunderstanding , you are referencing the 6809 datasheet not the 6809E (used in the coco) datasheet. Indeed pin 39 is EXTAL on the 6809 and TSC on the 6809E.

        • pilot352

          Ah ha!!! It’s great when we are all speaking apples!! 🙂

          I need to get me one of them there 6809″E” datasheets.


  • twixter

    Well one thing is for sure, I won’t perform any such upgrade myself! Cut off these pins? Bend that pin and solder to it? All I would do is toast my CoCo.

    IMO it would make more sense to put a 6309 in a motherboard which can handle 3 MHz, or maybe even 5 MHz which I have read can be done. Not that I could design a computer, of course.

    Anyway, the emulation mode on the 6309 is not perfect. For one thing, the switch to native mode is in software, so if the Hitachi chip ever reads that code, it will go native, which of course the 6809 would not do. Also many instructions execute in fewer clock cycles, so timing loops such as delay loops for joystick or keyboard operation of arcade games could become too fast to play any more.

    I would like to hear from anyone who has a 6309 upgraded CoCo, whether they thought it was a worthwhile upgrade or not.


    • Robert Gault

      I’ve upgraded a Coco3 with a 6309 but did it by removing the 6809, installing a socket, and inserting the desired CPU.

      I have never regretted changing to a 6309 which makes a great difference running OS-9. While it is possible that some program might not run with the 6309 if undocumented opcodes are used, with a socket one can always return to a 6809.

  • atarileaf

    No one has ANY information on this game? I can find nothing on the net. This is weird, even for a coco game.

    • briza

      G’Day Atarileaf.

      What game are you referring to mate?

      Maybe I can help you with it!!!

      And it seems the topic has went to the wrong area, Admin will have to move it to the correct place,



  • atarileaf

    Hi Briza, I’m looking for info, or hopefully a manual scan of Don Pan. Hope this goes to the right place 🙂

    • briza

      G’Day Atarileaf.

      Ok I will search around and ask some ppl if they may have a manual floating about for ya.