We have a motherboard with the Intel 6700PXH 64-bit PCI Hub chip. As I understand the chip automatically selects between 33/66/100/133MHz bus rates based on the slowest PCI card installed. Is there a way to monitor/read-out what the selected bus speed is?
According to the datasheet (6700pxh-64-bit-pci-hub-datasheet.pdf, july 2004, page 99) see http://www.intel.com/content/www/us/en/chipsets/6700pxh-64-bit-pci-hub-datasheet.html, the register at bus=0, device=0, function 2, offset 40-41, bits 9:10 have the encoding:
bits 10:9 RW 0 PCI Frequency (PFREQ): Determines the frequency at which the PCI bus operates. After software determines the bus’ capabilities, it sets this value and the PMODE (bits 14 and 15 of this register) to the desired frequency and resets the PCI bus. The values are encoded as follows:
00 = 33 MHz
01 = 66 MHz
10 = 100 MHz
11 = 133 MHz
Invalid combinations should not be written by software. Results will be indeterminate.
Bits 14:15 show the PCI mode. I think that the frequency and mode are tied.