Overclocking Apple Mac mini

Remember the good old 1980's, When things were so uncomplicated?..               ELO, "Time", 1981

Short Intro

I will stick to the very essentials - Mac mini is terrific! Read about it everywhere. There is also some mystery to it.

Overclocking

Here is the bottom of the board. Or whatever they call it nowadays ( many years ago there were component side and solder side.)

Speed of MC7447 processor is defined by the logic levels of its five PLL_CFG pins. They set bus-to-core multiplier and together with bus frequency define the value of CPU core clock. These pins have internal pull-up resistors so the processor speed is set by leaving each pin either disconnected or grounded with zero Ohm resistors (call them jumpers if you wish).

Space for four of the resistors is located in the area directly underneath the CPU. Having removed them you don't have to put them back. Since their value is zero Ohm, replacing them with a tiny blob or bridge of solder connecting necessary pads together is just as good, but much quicker. It is also easier to remove if needed. Look here to get an idea. You will find magnifying glass to be very helpful.

But before you rush into it, take a look at the size of the resistors.

I have not located the fifth resistor (MSB PLL_CFG) but it only needs to be removed for speeds higher than 1.58GHz which I did not try.
If you name PLL_CFG bits in the 7447's datasheet (p.47) as ABCDE, resistors in the picture above are lined up as BECD.

Results

I have settled on 1.42GHz speed for my original 1.25GHz Mac mini. This gives me about 14% extra CPU power, totally reliable operation of Mac mini and no associated heat issues.

1.5GHz setting caused some glitches in Xbench Altivec routines. At 1.58GHz the system profiler reported the speed as 750MHz (Open Firmware does not recognise unexpected clock frequency) but apart from mentioned Altivec problems system was usable. As the result I have 1.42GHz Mac mini and it works just fine.

So, enjoy this new knowledge! It's all great fun in a true spirit of Woz!

P.S. Oh, yes, did I say that it comes with no guarantees and voids your warranty?

© Leo Bodnar, originally published 2nd February 2005. Home