Full Speed Ahead … Finally

I have been building my 68030 computer around a 25MHz-rated part, so that has always been my target. My original wire-wrap prototype initially ran at half that, but as I continued to expand the project, the best I could manage was 6MHz. Eventually, the whole thing got so unstable I had to abandon plans to continue adding FPU, DRAM, IDE, etc.

I do still want to add those parts to the project, to learn more about working with them. That’s why I ordered the custom PCBs — I was hoping a PCB would be a more stable platform for future expansion and allow me to finally run the machine at the 25MHz target.

Once I got the new PCB prototype working I tried again with a 25MHz oscillator (my new glue logic no longer divides the clock like my original).

It didn’t work.

In fact, at 25MHz it failed in the same way it always had on the old wire-wrap prototype. It seemed 12 MHz was my limit.

Or was it? Maybe I just had a bad oscillator. Perhaps a new one would work better?

A ridiculous line of thought, given that the 25MHz oscillator I had did indeed run the computer at one time. But, I did want to see how fast I could get it to run, and there are some respectable speeds between 12MHz and 25MHz. So I placed an order for a few oscillators, 16MHz, 20MHz, 24MHz … I also stocked up on some common resistors and capacitors to make the most of the shipping fee.

Got the new parts in, threw on the 24MHz oscillator and … nothing. It didn’t work.

There is a problem I noticed when I did my original troubleshooting on this PCB — some of the wired-or signals had very slow rise times. I don’t have a proper oscilloscope, so it’s hard to tell sometimes if analog problems like that are measurement error or induced by stray capacitance of the measurement leads. But, the 8kΩ resistor networks I had gotten from surplus to use as pull-ups on this project were a bit high. Perhaps a lower-value pull-up resistor might help here.

Among the resistors I ordered were some 4.7kΩ and 1kΩ resistor networks. 4k7 is a fairly standard pull-up value, and some rough math had shown 1k might be a good value for this project. So I swapped out the 8k resistor networks for the new 1k networks and gave it a shot. It still ran without issue at 12MHz…

And at 16MHz.

And at 20MHz.

And at 24MHz.

And at 25MHz.

And at 32MHz.

And at 40MHz.

My MC68030 rated for 25MHz was running BASIC stable at a 60% overclock. Even the RAM was overclocked at this point, with cycles reduced to 50ns for SRAM rated for 55ns. It would seem that all this time my choice of pull-up resistor value had as much or more to do with my speed limits as the method of prototyping.

It wasn’t perfect though, and as soon as I added my SE-VGA card back into the mix it would no longer successfully load BASIC at 40MHz. It did however run just fine at 32MHz, even with the SE-VGA card.

Ludicrous Speed

In the years since I started this project I acquired another 68030 CPU — a 40MHz-rated 68EC030 (the EC units lacking the on-board MMU). I’m sure you can see where this is going.

First, I needed to modify my glue logic. RAM access cycles were going to need another wait state, ROM another two. While I was at it, I created a new cycle specifically for my SE-VGA card, with three wait states. Since my UART (68B50) is actually rated for 2MHz, and my timing was originally factored for 1MHz with a 25MHz base clock, I left the UART timing alone to push it closer to its rated speed. All-around, the new timing should support up to 50MHz base clock.

New logic, everything ran fine at 32MHz. Swapped in the EC030 and no problems. Time to see how fast it will go.

40MHz, no sweat

50MHz, still running cool

56MHz, no problems

I’m out of oscillators. My 68030 homebrew, with a 40MHz-rated EC030 is running reliably at 56MHz, a 40% overclock. Even the SE-VGA card is working, and much happier with its custom timing added to the glue logic.

I’ve been using a simple Mandelbrot rendering BASIC program as a benchmark. On my original 6MHz 68000 build, this program takes around 9 minutes to complete. When I first ran it on the 68030, running at 12MHz, with cache disabled, and BASIC in 8-bit ROM, it took just under 5 minutes. Now, with BASIC running from RAM on the 32-bit bus, L1 cache enabled, and CPU at 56MHz, the Mandelbrot program completes in 14 seconds. That is an incredible performance increase for a simple homebrew computer.

It does generate some heat now, so I added a small heatsink to the CPU to be safe. Current consumption for the system is up 300mA just from raising the clock speed.

Motorola originally sold 68030 CPUs rated as high as 50MHz. I wonder if my later production units just benefit from what they learned pushing the architecture that high, or if it’s reflective of what the CPU can handle in general. Could I push a 50MHz part up 40%? A 70MHz 68030 homebrew certainly would be interesting.

Early Hacks

Here we see a hack I did more than fifteen years ago, when I lacked much of the knowledge, experience, and materials I have at my disposal now. I overclocked my PowerMac G4 Sawtooth. Stock it ran at 450MHz; I was able to get it to run stable at 500MHz.

The CPU card for these old G4s used a few resistors to set the bus multiplier. For this old Sawtooth model, it was R7, R9, R11, R13.

I removed the resistors that were there originally and soldered some enameled magnet wire in their place. On the other end, I soldered a set of DIP switches to allow me to experiment with the multiplier without a lot of effort. I only had this bank of six switches, so two are unused. And then, lacking anything better, I wrapped it in tons of electrical tape.

I tried setting the multiplier up to 5.5x to get it up to 550MHz, but it wasn’t stable enough to boot. At 600MHz it wouldn’t even power on. I believe it could be set down to 3.5 or 3.0, should I ever have any reason to underclock the machine.

I used this computer, overclocked to 500MHz and running MacOS 10.5 for years before passing it on to a family member who used it for several more years. Never once had an issue with the overclock or my crude hack.

In the 15 years since, the tape has obviously collected a lot of dust, but the computer still runs fine. I’m just a extra cautious when it comes to working around those fine wires.