DS1780 Datasheet, PDF (3/28 Pages) Dallas Semiconductor – CPU Peripheral Monitor

OVERVIEW
A block diagram of the DS1780 is shown in Figure 1.
DS1780
The DS1780 provides six analog inputs, an analog output, five digital inputs, two fan speed inputs, a
temperature sensor, and interrupt registers on a single chip, which communicates on a 2-wire serial bus.
The DS1780 performs power supply, temperature, and fan monitoring for personal computers.
The analog voltages are divided internally by the DS1780. The inputs are then converted to 8-bit digital
words. The analog inputs are intended to be connected to the several power supplies present in a typical
computer. Temperature can be converted to a 9-bit two’s-complement digital word with a 0.5°C LSb.
The analog output is approximately a 0-1.25V output from an 8-bit D/A converter, which is used to
control fan speeds.
Fan inputs measure the period of tachometer pulses from the fans, providing a higher count for lower fan
speeds. The fan inputs are digital inputs with an acceptable range of 0 to V_DD volts and a transition level
of approximately 1.4 volts. Full-scale fan counts are 255 (8-bit counter) and this represents a stopped or
very slow fan. Nominal speeds, based on a count of 153, are programmable from 1100 to 8800 RPM on
FAN1 and FAN2. Signal conditioning circuitry is included to accommodate slow rise and fall times.
The DS1780 provides a number of internal registers, as detailed in Table 1. These include:
Configuration Register: Provides control and configuration, as well as initialization.
Interrupt ( INT ) Status Registers: Two registers to provide status of each interrupt limit or interrupt
event.
Interrupt ( INT ) Mask Registers: Allows masking of individual Interrupt sources, as well as separate
masking for the hardware interrupt output.
Temperature Configuration Register: The lower 2 bits of this register configure the type of
temperature interrupt mode to be used. Bit 7 reflects the lowest bit of the temperature reading.
VID Register, VID4 Register: Bits 0-3 of the VID register reflect the status of the VID0-VID3 pins, bit
0 of the VID4 register reflect the status of VID4 pin. These are simply input pins - not processed in any
way. In a multiprocessor system, these signals will be multiplexed externally from the various processor
sources, with the source being controlled by software.
Value RAM: The monitoring results and limits for temperature, voltages, and fan counts are all
contained in the Value RAM.
When the DS1780 is started, it cycles through each measurement in sequence, and it continuously loops
through the sequence approximately once every second. Each measured value is compared to values
stored in limit registers. When the measured value violates the programmed limit the DS1780 will set a
corresponding System Management Interrupt (SMI) in the Interrupt Status Registers. One hardware
interrupt line, INT , is available to generate an SMI. INT is fully programmable with masking of each
Interrupt source, and masking of the output. In addition, the configuration register has control bits to
enable or disable the hardware Interrupts.
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