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G768D

Two Remote Temperature Sensors and One Fan Controller with SMBus Serial Interface and System Reset Circuit

Global Mixed-mode Technology Inc 

Global Mixed-mode Technology Inc.

G768D

Ensuring a Valid Reset Output Down to VCC = 0V

When VCC falls below 1V, the G768D RESET output

no longer sinks current-it becomes an open circuit.

Therefore, high-impedance CMOS logic inputs con-

nected to RESET can drift to undetermined voltages.

This presents no problem in most applications, since

most µP and other circuitry is inoperative with VCC be-

low 1V. However, in applications where RESET must

be valid down to 0V, adding a pull-down resistor to

RESET causes any stray leakage currents to flow to

ground, holding RESET low (Figure 3). R1's value is

not critical; 100kΩ is large enough not to load

RESET and small enough to pull RESET to ground.

Interfacing to µPs with Bi-directional Reset Pins

A µP with bi-directional reset pins (such as the Mo-

torola 68HC11 series) can connect to the G768D reset

output. If, for example, the G768D RESET output is

asserted high and the µP wants to pull it low, indeter-

minate logic levels may result. To correct this, connect

a 4.7kΩ resistor between the G768D RESET output

and the µP reset I/O (Figure 4). Buffer the G768D

RESET output to other system components.

SMBus Digital Interface

From a software perspective, the G768D appears as a

set of byte-wide registers that contain temperature

data, alarm threshold values, fan speed data, or con-

trol bits, A standard SMBus 2-wire serial interface is

used to read temperature data and write control bits

and alarm threshold data. Each A/D and fan control

channel within the device responds to the same

SMBus slave address for normal reads and writes.

The G768D employs four standard SMBus protocols:

Write Byte, Read Byte, Send Byte, and Receive Byte

(Figure 5). The shorter Receive Byte protocol allows

quicker transfers, provided that the correct data regis-

ter was previously selected by a Read Byte instruction.

Use caution with the shorter protocols in multi-master

systems, since a second master could over-write the

command byte without informing the first master.

The temperature data format is 7bits plus sign in

twos-complement form for each channel, with each

data bit representing 1°C (Table3), transmitted MSB

first. Measurements are offset by +1/2°C to minimize

internal rounding errors; for example, +99.6°C is re-

ported as +100°C.

VCC

G768D

RESET

GND

R1

100k

BUFFER

VCC

G768D

4.7k

RESET

VCC

µP

RESET

GND

GND

BUFFERED RESET

TO OTHER SYSTEM

COMPONENTS

Fig 3 RESET Valid to VCC = Ground Circuit

Fig 4. Interfacing to µPs with Bi-directional Reset I/O

Ver: 1.2

Apr 03, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

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