M571 - undocumented clock frequencies & overclocking the PCI bus

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Bad news.

There are no useful undocumented frequencies for the PC Chips M571

motherboard (V70A). At best, one can overclock the PCI bus using jumper

JP5-D.

This motherboard uses a clock synthesizer chip from C-Media Electronics:

PLL chip CMA8865-(27)

http://www.cmedia.com.tw

http://www.cmedia.com.tw/pdf/cma8865-27.pdf (data sheet)

mailto:support@cmedia.com.tw (support)

JP5 selects the CPU External Clock and the PCI bus clock.

Jumper D selects the ratio of external clock to PCI clock.

Jumper D1-2 selects synchronous CLK/2 for PCI clock

Jumper D2-3 selects asynchronous CLK x 2/5 for PCI clock

For example, 83MHz x 2/5 = 33MHz PCI clock

83MHz /2 = 41.5MHZ PCI clock

75MHz x 2/5 = 30MHZ PCI clock

75MHz /2 = 37.5MHZ PCI clock

66MHz /2 = 33MHZ PCI clock

This is the clock frequency table from the C-Media data sheet:

PCICLK PCICLK 48MHz 24MHz

SEL2 SEL1 SEL0 CPU,SDRAM BSEL=1 BSEL=0 FDD USB

-------------------------------------------------------------

0 0 0 50.0 1/2 2/5 48 24

0 0 1 60.0 1/2 2/5 48 24

0 1 0 66.8 1/2 2/5 48 24

0 1 1 REF/2 REF/4 REF 2/5 REF/2 REF/4

1 0 0 55.0 1/2 2/5 48 24

1 0 1 75.0 1/2 2/5 48 24

1 1 0 83.3 1/2 2/5 48 24

1 1 1 Tristate Tristate Tristate Tristate Tristate

REF = 14.31818 MHz

Jumpers JP5 (A,B,C,D) are related to SEL0,1,2 and BSEL as

follows:

BSEL = JP5-D

SEL0 = JP5-C

SEL1 = JP5-B

SEL2 = JP5-A

A logic 0 for SEL in the table equates to JP5 in the 2-3

position. The 1-2 position selects logic 1.

A final cautionary note. The SiS 5597/5598 data sheet states that "the

SiS chip is designed to support Pentium CPU host interface at

75/66.667/60/55/50MHz". This implies that a frequency of 83MHz overclocks

the chipset.

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This page was last modified on 17 July 2002