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Reader Report: 500MHz Overclock of PF400 ZIF w/Voltage Boost
By Katsuya Uehara 2/10/99

Katsuya Uehara wrote to report he had overclocked his PowerForce 400/266/1MB G3 ZIF upgrade (reviewed here) to 500MHz (higher speeds did not prove reliable). He accomplished this by means of boosting the CPU supply voltage as noted in his mail below. We both warn that this is not something we recommend to others as it is risky and could lead to hardware damage or data loss. The information published here is for reference only and not a recommendation for others to attempt this. This information may not be accurate for other present and future models and may change with future revisions hardware from Apple.

" Dear Mr. Breeden,
Please note the following overclocking technique is risky one. Read this article for reference only.

I have succeeded to overclock PowerForce G3/400MHz ZIF to 500MHz. First, I changed the rotary sw. on PowerForce card. The card worked at 466MHz@L2/310MHz fine. I measured the default Vdd (input voltage to PowerPC core) of the card. It was 1.95V.

For further overclocking I decided to raise the Vdd. At the Gossamer motherboard, 5bits of VID code determines the Vdd. There are two methods to changing the VID code. The one is changing the registers on the PCM (Processor and Cache Module
( see http://www.mot.com/SPS/PowerPC/teksupport/teklibrary/specs/PCMhwspec.pdf ).

And the other is to change the setting of VRM (Voltage Regulation Module) on the motherboard. I selected the former method because it was easier for me. (I wanted to return to the normal setting easily.)

In Apple genuine PCM (same as CPU card), the VID code is determined by the position of zero ohm registers. However, PowerForce G3 ZIF determines the VID code by the presence of 1k ohm registers.

• VID[0] --- R14
• VID[1] --- R15
• VID[2] --- R16
• VID[3] --- R17
• VID[4] --- R19

The relation between VID code and Vdd is written in above PCM specifications. The following table is the extracts from Table 3 of PCM specifications (page 12-13).

PCM/Voltage Settings
VID[0-4]	Vdd
00101	1.80V
00100	1.90V
00011	1.90V
00010	1.95V
00001	2.00V
00000	2.05V
11110	2.10V
11101	2.20V
11100	2.30V
11011	2.40V
11010	2.50V
11001	2.60V
11000	2.70V

I achieved CPU:500MHz/L2:250MHz by changing Vdd to 2.54V (measured). I also added a SANYO Socket 7 fan. I added more SANYO fan (total two fans) and could boot at CPU:525MHz/L2:262.5MHz. But this configuration was unstable. I think this was due to the lack of cooling. CPU seems to have more headroom. I have already confirmed CPU:533MHz/no L2 is working.

It seems that if I can use more effective cooling system, further overclocking will be possible. I think further overclocking needs the cooling of L2 chip and the adjustment of OVdd (I/O voltage of PowerPC).

Apple System Profiler Report

MacBench 5.0 Results at 500MHz

In this experiment, I sometimes experienced the load failure of G3 Cache Control (v1.4.1). It might show the overclocking causes software problems. I emphasize again all these data are only experimental one. Please use this information for reference only. I do not recommend this sort of risky overclocking. I take no responsibility for any damage due to overclocking and Vdd adjustment.

I thank friends of NIFTY-Serve Forum FMACHARD Mes(12) which supported me.

Thank you.
Katsuya Uehara
Toyko, Japan"

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