
SiS5595 PCI System I/O Chipset
Preliminary V2.0 Nov. 2, 1998 39 Silicon Integrated Systems Corporation
The Data Acquisition Module internally contains an Analog-to-Digital Converter (ADC) to
continuously transform analog signals from power supply voltages or temperature sensor
into 8-bit digital binary numbers. The rotational speed of fans can be calculated by counting
the cycle time of digital pulses from tachometer output based on an internally generated
22.5KHz clock. For every input sources to be monitored, the actual readings which have
been calculated or transformed will be compared with those upper and lower limits as
programmed in their associated registers. Upon a limit being exceeded, a SMI# or IRQ can
be optionally generated to inform the operating software to take proper steps to protect the
system from critical failure.
8-Bit
Analog
To
Digital
Converter
(ADC)
Limit
Registers
and
Comparators
Interrupt
and
SMI#
Mask
Registers
Temperature
Sensor
VIN1
VIN2
VIN3
Index Address Reg.
Data Register
SMI#
IRQ
AD BUS
BTI#
VIN0
Fan Speed
Counter
FAN1
FAN2
DXP/VIN4
Figure 3.5-1 Data Acquisition Module Block Diagram
3.5.2 POWER SUPPLY VOLTAGE MONITORING
The SiS5595 Data Acquisition Module supports up to five positive voltage inputs monitoring.
Analog inputs from power supply voltages, in term, will be converted into 8-bit unsigned
binary numbers by the internal Analog-to-Digital Converter (ADC) with a resolution of 16mV.
The resulting measurable voltage range is therefore from 0V to 4.096V. For voltages higher
than 4.096V, an external voltage attenuator circuit consisting of two properly selected
resistors can be used to scale down the voltage into the measurable range. Typically, the
four voltages to be monitored in a PC system are +12V, +5V, +3.3V and +2.5V respectively.
The +12V and +5V require external attenuators. The SiS5595 application notes contain the
recommended values of the resistors for the attenuators.
3.5.3 FAN SPEED MONITORING
The fan inputs are from fans equipped with tachometer pin, which carries continuous digital
pulses, indicating its current rotational speed. The longer the pulse’s cycle time, the slower
the fan rotates. The tachometer output requires a pull-up resistor connected to +5V. The
SiS5595 Data Acquisition Module supports two fan inputs monitoring. The typical application
will be connecting one to CPU fan and the other one to the case fan.