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Building the Address Decoder

A project log for Yet Another m68k Homebrew

A Motorola 68k homebrew on breadboard, this time with a 68010 running at 8MHz

Ross BamfordRoss Bamford 05/08/2019 at 15:360 Comments

So I finally gathered all the parts I needed, and since I have some time off work this week I managed to build the address decoder. I'm happy to say it was fairly painless although time-consuming - routing all the wires has taken the best part of a day. To be fair, a couple of hours of that were spent debugging a couple of mis-wired connections though.

It looks like this:

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This handles all the address decoding I've talked about previously, and has the /BOOT line generator as well (the slightly out-of-place looking IC in the center top is a DM54174 - a military-grade variant of the 74174 Hex-D Flip-flop I found in my parts bin, and which pulls /BOOT low for 4 memory cycles after reset).

I did realise while building this that I had the odd/even RAM/ROM selects mixed up - the m68k is big-endian of course, so I've fixed that in the build and also pushed new schematics and simulation to Github. I'll also update the previous log in which I showed the address decoder to reflect the fix.

Here's another pic showing the build in-progress, when I had all the inputs wired through jumpers so I could make sure things worked as I built it:

And here is a full shot of the board in its current state. You'll notice that I've added a bare board in the center which I'm using to break out the control, address and data buses (not everything is wired up yet):

Finally, if you want to see it in action, I made a short video where I manually drive the control lines and have some LEDs displaying the output of the decoder:

That's all for now, tomorrow I'm planning to finish hooking up the memory chips, and then burn something simple to the ROMs and get this thing running some real code!

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