Overclocking the VXL-30
The VXL-30 is a 68030-based accelerator for the A500 or A2000. It was made in several versions - 25MHz, 40MHz and 50MHz, some of which used the "EC" version of the
030. The slower versions were designed to be upgraded to 50MHz by replacing the 030 and oscillator module with 50MHz versions. In addition, there was provision for
a 68882 FPU, which could be clocked in synchronism with the 030 clock, or separately with it's own oscillator module. There was also a separate RAM card called the
RAM-32 which plugged into the VXL-30, providing either 2 or 8 MB of 32-bit RAM. The VXL-30 will work without the RAM-32, however it is very slow - being no faster
than an ordinary 7MHz 68000. In other words, if you have a VXL-30 without RAM, don't expect it to be any faster than your original CPU. My board only has 2MB of RAM. I DO NOT know of a source of the 1x4MB type DIL RAMs
needed to upgrade to 8MB. If anyone knows of a place to get them from (preferably in Australia) let me know.
For more general information on the VXL-30, check out the VXL-30 FAQ. I have tried overclocking two versions of the VXL-30, the 25Mhz version, which used a
25MHz 68EC030 and the same board with a 50MHz 68030 fitted. In both cases the FPU was originally from an old workstation (I don't know what type) and was rated at
16MHz. Motorola don't make 16MHz 68882s, so I suspect it was a 25MHz one which was labelled at 16MHz.
Overclocking the 25MHz version.
When I first got my VXL-30, it came with a 25MHz 68EC030. Overclocking was very easy, as sockets were provided for both the CPU and FPU oscillators, which are
standard 14-pin form factor. As the board itself will work to 50MHz, it is simply a matter of seeing how far the CPU would go. In my case I was able to overclock to
36MHz, although it continued to work unreliably at 40MHz. Don't forget that your mileage may vary. You may get more....or less than me. Of course, the 33 and 40MHz
versions are also overclockable. I suggest fitting a small passive (non fan) heatsink to the 030 to keep it cool. You will notice a distinct lack of room when using a heatsink in an A2000,
so you may need to improvise. I actually made a metal bracket which was attached to the bottom of the drive bay assembly which was pressed against the 030 when the bays were fitted to the
A2000. This conducted heat away into the metalwork of the drive bay assembly. Fortunately there is not a lot of heat to dissipate.
Overclocking the 50MHz version.
Later on I upgraded my VXL-30 to 50MHz, by fitting a genuine 50MHz 68030. I encountered a few problems with this, as it seemed that the RAM on the RAM board was not fast
enough. I had to set the jumpers to 80ns, even though the RAMs supplied were rated at 60nS. So I replaced the the original Hyundai brand RAMs with some I found on an
old PC graphics card. These were also rated at 60nS, except in this case they worked as advertised, so I was able to set the RAM speed jumper to it's correct place for
maximum speed. I then tried overclocking, but was only able to get a maximum of 52MHz. It would not even boot at 54MHz.
Overclocking the FPU.
My VXL-30 was not supplied with an FPU, so I fitted one labelled at 16MHz. I was able to overclock it to 45 MHz, which is quite an improvement.
The FPU was only slightly warm, even at this speed, so I did not fit a heatsink. A good means of checking correct FPU operation is to use the BeachBall test in AIBB. If there are any
FPU errors, there will be corruption in the rendered beach ball, even if the FPU errors are not serious enough to crash the Amiga.
The drawing above indicates the position of the various jumpers mentioned below. Also shown is the position of the two relevant oscillator modules, outlined in red boxes.
Pin 1 on each module is shown with a red dot which corresponds to the black dot and square corner on the actual oscillator module.
The FPU clock select jumper is located on the main board, next to the FPU. It is outlined in red in the drawing above. In the default position shown, FPU timing is taken from
the CPU oscillator. The FPU clock oscillator is not used. In the alternate position, with the top two pins connected, FPU timing is derived from the FPU clock oscillator,
which must be fitted.
The other six jumpers are located on the RAM board, shown in blue on the drawing above. They are shown in their default position with the two terminals not connected. Place
a jumper block over the two terminals to select the alternate function. Jumper 1 is at the top, jumper 6 at the bottom. The following is an outline of their functions:
1: RAM Mapping. Default=High. Alternate=low
2: RAM Size. Default=2MB. Alternate=8MB. Set this to reflect installed RAM.
3: ROM Select. Default=Motherboard. Alternate=RAM-32. Selects which Kickstart ROM to boot from.
4: RAM Access. Default=No Burst. Alternate=Burst. Leave on No Burst and select Burst using the CPU command in startup.
5: RAM Speed. Default=80nS. Alternate=60nS. Set to 60nS for best speed, RAM speed permitting.
6: CPU Speed. Default=25MHz. Alternate=40 or 50MHz. Try this on 25MHz if you are running at 40 or 50 MHz. If your RAM is fast enough, it will work, giving a little
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Introduced November 14th 1998. Updated 20th March 1999. Version 1.1.