PowerBook 5300 FAQ - A.1.2) Overclocking: Oscillator

The system bus of the 5300 connects the CPU to the main memory and other subsystems such as the hard drive and serial ports. Whenver the CPU requests information, it must travel across the system bus. Thus it should be pretty obvious that increasing the system bus speed would also increase the overall speed of the 5300. Additionally, the bus-to-cpu ratio (or multiplier) is used by many PowerPC based Macintoshes to determine how fast the CPU should run. The CPU takes a sample of the computer's bus speed, then uses the multiplier to set its own speed. Thus, increasing the system bus speed also directly increases the CPU speed.

The 5300/100 (5300, 5300cs, and 5300c) has a bus-to-cpu ratio of 1:3 (or a multiplier of 3x). That means that a 33.3MHz bus (which is standard on all 5300s) yields a 100MHz (ok, 99.9MHz) CPU.

The 5300/117 (5300ce) has a bus-to-cpu ratio of 1:3.5 (or a multiplier of 3.5x). Thus a 33MHz bus yields a 117MHz CPU.

The chip that determines the bus speed on the 5300 is called an oscillator. It generates a 33MHz signal that the computer uses to set it's bus speed. Replacing the crystal with another similar part of a different speed will change the bus speed. On the 5300, the standard crystal is a 4-pin IC. My 5300cs's oscillator is labled ECE 5350A. The part number may change from 5300 to 5300, as another user's oscillator is labeled ECE 6050A. Each part also has 33.3333 labeled on it. You can locate the oscillator at position G1 on the motherboard between the CPU and the PDS connector.

The following table shows some standard oscillator speeds and how they would change the 5300's CPU speed.

Oscillator Speed	PowerBook 5300/100 (3x bus speed)	PowerBook 5300/117 (3.5x bus speed)
33MHz	100MHz	117MHz
36MHz	108MHz	126MHz
38MHz	114MHz	133MHz
40MHz	120MHz	140MHz
(A 40MHz bus is probably unusable on anything but a 5300ce with a 3x bus ratio) I haven't experimented with this yet, but I would bet even money that the highest practical bus speed is 38MHz. Some older Macs exhibit component faillure when their bus speed gets too high. Some symptoms include non-working serial ports, overworked video chips developing visible artifacts on the screen, overheating, or crashes caused by RAM operating above its specified speed tolerance.

Thus, if you have a 5300/100, you can overclock a 5300 fairly significantly by just changing the clock generator.

This page describes how one would overclock the PowerBook 5300 by changing bus speed.