G768B View Datasheet(PDF) - Global Mixed-mode Technology Inc

Part Name

Description

Manufacturer

G768B

Remote/Local Temperature Sensor, 2 Fan Controllers with SMBus Serial Interface and System Reset Circuit

Global Mixed-mode Technology Inc 

Global Mixed-mode Technology Inc.

G768B

Ensuring a Valid Reset Output Down to VCC = 0V

When VCC falls below 1V, the G768B 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 Ps with Bi-directional Reset Pins

Ps with bi-directional reset pins (such as the Motorola

68HC11 series) can connect to the G768B reset out-

put. If, for example, the G768B RESET output is as-

serted high and the µP wants to pull it low, indetermi-

nate logic levels may result. To correct this, connect a

4.7kΩ resistor between the G768B RESET output

and the µP reset I/O (Figure 4). Buffer the G768B

RESET output to other system components.

SMBus Digital Interface

From a software perspective, the G768B 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 G768B 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.

Benefits of Highly Accurate Reset Threshold

Most µP supervisor Ics have reset threshold voltages

between 5% and 10% below the value of nominal sup-

ply voltages. This ensures a reset will not occur within

5% of the nominal supply, but will occur when the sup-

ply is 10% below nominal.

When using Ics rated at only the nominal supply ±5%

this leaves a zone of uncertainty where the supply is

between 5% and 10% low, and where the reset may or

may not be asserted.

The G768B use highly accurate circuitry to ensure that

reset is asserted close to the 5% limit, and long before

the supply has declined to 10% below nominal.

BUFFER

VCC

G768B

4.7k

RESET

VCC

µP

RESET

BUFFERED RESET

TO OTHER SYSTEM

COMPONENTS

GND

GND

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

VCC

G768B

RESET

GND

R1

100k

Fig 3. RESET Valid to VCC = Ground Circuit

Ver 1.3

Oct 28, 2002

TEL: 886-3-5788833

http://www.gmt.com.tw

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