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Dual G4/500 CPU Module Bus/CPU Ratio Settings
(These control CPU Speed)
Published: 12/14/2000


    Disclaimer/Warning: This page is for reference information only. Performing modifications or other work inside your Mac will void your Apple warranty, may cause damage to your computer. Any modifications to your CPU module may result in permanent damage requiring expensive parts replacement, data loss or personal injury. The resistors on these modules are very small surface mount devices, requiring very delicate soldering skills   beyond the capability of most end users. The author and site publishers DO NOT recommend attempting any of the mods listed here.


(The G4 CPU module speed settings info used for this mod are from the Nov. 30th, 1999 article on the single G4/AGP CPU module by Michiro Isobe.)


A reader sent photos of a dual G4 CPU module identifying the bus/ratio control resistor locations.

" Hiya Mike,
busy running tests now, just OC'd my 500 Mystic to 550 by changing jumpers from 5x to 5.5x. Seems stable so far, compressed and opened .smi files, ran Gauge pro memory test for 5 min with no faults. Also had several apps open, leaving 5.3 MB of RAM unused out of 348with no probs so far.

I plan to try higher, but this step was too easy... add 2 solder blobs and I'm at 550...cool.

Attached are 4 jpegs, top and bottom and details. One set of bus mult jumpers are on the back, one set on the front. No labels on my proc board, so I labelled by CFG-PLL address. Same multiplier code from Moto's PDF linked to on M. Isobe's (article).
Zack"


I originally thought Zack had sent photos after he had made the modifications for 550Mhz, but from the photos below, these are just illustrating the areas where the resistors control the Bus/CPU ratio settings for a Dual CPU modukle. (Stock G4/500 settings shown). (The previous article here shows the locations on a single G4/AGP system's CPU module.)

Using Table 14 from the Motorola 7400 (G4) PDF file on PLL settings, I created the following (simplified) table which identifies the resistor areas populated for various Bus/CPU ratio settings. (A zero "0" means a resistor is present at the location specified (at positions 0,1,2,3 respectively).

Since the low end dual G4 system is a 450MHz I didn't bother to show settings below 4.5x ratio, and only show ratio settings as high as 6.5x (650mhz with a 100mhz bus speed). Remember these are just settings, and that it's unlikely any current G4 CPU would run even 600mhz reliably (some have reported running 600Mhz on a single CPU with voltage tweaks and additional cooling). It's important to remember even one step up (50Mhz boost) may not be reliable on your CPU. Mods like this are only for those that are willing to risk damage to the module and failure.-Mike


Bus/CPU ratio settings table

Resistor locations for PLL_CFG 0, 1, 2 and 3 are shown in the photos below.

Back View of Dual G4/500 Module Identifying Bus/CPU Ratio Settings Area
Top Side of Modified Module

(Closer Look at Top Side area)
topside detail



Back View of Dual G4/500 Module Identifying Bus/CPU Ratio Settings Area
Back Side of Module

(Closer Look at Back Side area)
backside detail

Note: Traces (circuit paths) on the surface of the board are only protected by a thin solder mask, so if you scratch the solder mask coating over a trace (with a soldering iron for instance), a solder bridge could then cause an undesired short-circuit. (Which could damage the CPU or other module components.)-Mike



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