The 4Mhz CoCo3 Project

Closeup of 4mhz parts Years ago, I had an idea that would allow the CoCo 3's CPU to run at 4Mhz. It's essentially a clock doubler circuit for the CPU. Whenever the CPU would not be needing the bus in the following cycle, it would execute that cycle sooner - in the blank time between regular cycles. This allows the CPU to run at 4Mhz, while the bus remains at 2Mhz. This picture shows the latest version of the board made. The cable on the right goes to the 'turbo switch', the CPU plugs on top, and the board itself plugs into the CoCo's CPU socket. There are no modifications to the CoCo itself, other than adding a socket for the CPU! There used to be a 'turbo LED' that lit up every time the circuit overclocked a cycle, but I removed it when testing something out.

Photo of newer 4Mhz board More recently, I drew up a circuit layout for a single sided board (easier to make!). This is the board that I showed at Pennfest 2000. Just plug it into the CoCo's CPU socket, and plug the CPU into the board. This photo also shows a Disto 2 Meg upgrade, which also uses the CPU socket, but the 4Mhz board still fits nicely on top - and still works!

I've put together some information about the accelerated CoCo project so people could build their own. I'd still like to put some more things here, mainly specific information about how it all works, components, board layout...etc. But, this is a start. Here is some general information about it and some logic diagrams of the circuit.

There are also some theoretical ideas about making your CoCo3 run even faster than 4Mhz!

If you're wondering about the actual performance, I've tried this simple Basic program as a benchmark. If you have a good idea for a better benchmark program, please let me know and I'll try it out!

10 A=0:TIMER=0

The averaged values returned are:
100 for 1Mhz 6809 (stock CoCo)
114 for 1Mhz 6309 in native mode
200 for 2Mhz 6809 (POKE65497,0 on stock CoCo)
228 for 2Mhz 6309 in native mode
268 for clock doubled 6809 (The 4Mhz accelerator being used with a 6809 CPU)
275 for clock doubled 6309 in native mode (The 4Mhz accelerator being used with a 6309 CPU)

Using the links below, you can check out how many 'bus cycles' each CPU opcode executes in when using a normal 6809, a 6309 or a 6309 with the 4Mhz clock doubler.
There's also a logic diagram of the circuit, and a text file describing this and a few other ideas on how to accelerate the CoCo3.

4Mhz Project Files to Download

CPU cycle chartThe actual numbers behind the circuit. Here is a chart that shows how many cycles each opcode takes with this circuit.
4mhzcoco.txtThe 4Mhz CoCo3 Project. Theoretical ideas and some that have actually been successfully tried.
4mhzcoco.gif 640x400 logic diagram of my 4Mhz accelerator project for CoCo3. This circuit is essentially a clock doubler for the CPU to boost performance
4mhzc .gif 640x200 smaller logic diagram of the clock doubler for easier viewing on a CoCo3

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