6809 CPU Card for RC2014

An RC2014 bus CPU card for the classic 6809 processors

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This board allows you to plug a 6809 processor into an RC2014 system. It should also work for the 6309. Note that the 6809E and 6309E need different clocking arrangements so will not work.

The 6809 uses the same Motorola bus arrangement as other Motorola 8bit processors. It's also not multiplexed on the 6809 so the support logic is quite simple.

A 74HC00 (U3) converts the 6809 E and RW lines into the Z80 \RD and \WR lines as expected by the RC2014 bus. The 74HC688 (U4) matches $FExx addresses as I/O space and generated \IORQ and \MREQ accordingly.

The various 6809 signals that don't fit on the standard bus are exposed via an additional 6 pin connector so that FIR, NMI and the like are usable. As with all my 68xx/65xx cards the E clock and RW lines can be jumpered to user pins 38/39 for use with Motorola style peripherals like the 6522 VIA card.


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View all 6 files

  • 1 × 6809 or 6309 CPU (not E version)
  • 1 × 74HC688 Electronic Components / Misc. Electronic Components
  • 1 × 74HC00 Electronic Components / Misc. Electronic Components
  • 1 × 10K resistor
  • 6 × 4K7 resistor

View all 8 components

  • Sorting out CUBIX

    EtchedPixels02/23/2021 at 19:39 0 comments

    I finally got back to working on this board set up a bit more and fixing  a couple of annoying bugs. I've pushed a revised ROM for the board to fix the following

    - Early boot wrongly accessed and read every I/O port. This happened to be harmless for most cases

    - The correct checksum is now computed for the ROM so the boot properly displays ROM... Passed

    It also adds some initial code to support booting another OS off disk instead of Cubix. I need to test and play with it but it ought to be sufficient to let people port things like Flex and NitrOS9 to the board (and of course Fuzix in my case).

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Ben Scherrey wrote 03/21/2021 at 12:36 point

Have you used it in conjunction with the other standard RC2014 boards like the real time clock or compact flash storage? I've held off on buying an RC2014 kit because my true 8-bit love is the 6809. If your card works as a replacement for the Z80 I'd buy the whole lot.

  Are you sure? yes | no

EtchedPixels wrote 04/19/2021 at 14:44 point

Now I've got cubix running and run a few things the compatbility appears to be good and I've run stuff like the CF adapter with it. In general the problems with other processors are always with the Z80 peripheral chip based cards as these devices expect an exactly Z80 bus cycle pattern. The CF can be touchy even on Z80, but does work nicely in Cubix on the 6809 and the 16x50 uart works fine. Not tried a 6850 for authenticity yet. A 6821/6840 I/O card design is done but waiting until I next send stuff off to JLPCB.

  Are you sure? yes | no

EtchedPixels wrote 06/02/2020 at 01:44 point

Yes but I'm not entirely happy with it. It works with the classic 65C02 processors but the WDC ones have different timings - in particular they have no hold on the data lines when the clock cycle ends, whilst the other devices have 30ns. On a single board there is enough capacitance etc that this issue can usually be armwaved away but on RC2014 you've got two gate delays on \RD and \WR *and* you've got a long bus so the 'ignore it and it'll magically work' approach does not work.

I'm currently playing with using phi0 for RD/WR or skewing the clock to the CPU versus RD/WR as I do with the 65C816 board.

  Are you sure? yes | no

Peabody1929 wrote 05/29/2020 at 00:07 point

Do you have a 6502 RC2014 processor board too?

  Are you sure? yes | no

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