HP Kayak XU 04xx HP Kayak XU PC Workstation Technical Reference Manual Hardwar - Page 33
LM75 Chip, Main PLL, voltages in the CPU, main fan CPU fan, disk fan, card area fan speed,
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Page 33 highlights
2 System Board Devices on the SM Bus WARNING: LM75 Chip The LM75 chip is a temperature sensor and alarm located on the motherboard. It is used to measure the temperature in one area of the PC Workstation, and to send an alarm to the processor in case of overheating. This chip includes a security mechanism which prevents the system fan from being disabled using software controls so long as the temperature measured by the sensor is above the maximum operating temperature. LM78 Chip The LM78 chip is a general purpose hardware monitoring chip, connected to the SM bus. It monitors several hardware parameters: several critical voltages in the CPU, main fan (CPU fan, disk fan, card area fan) speed, and several general purpose I/Os. Main PLL The registers of the main PLL are accessed through the SM bus. These registers control the PLL clock signal outputs and are write-only. Writing over the SM bus may be destructive to the PC Workstation, as it allows to access information necessary to the System BIOS, without which the system will not run. 25
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