DS1780

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OVERVIEW

A block diagram of the DS1780 is shown in Figure 1.

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

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.

event.

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 source, and masking of the output. In addition, the configuration register has control bits to

enable or disable the hardware Interrupts.