G767F datasheet(7/14 Pages) GMT

7 / 14 page

Ver: 2.5

Dec 14, 2004

TEL: 886-3-5788833

http://www.gmt.com.tw

G767

Global Mixed-mode Technology Inc.

Figure 2. Recommended DXP/DXN PC Traces

Twisted Pair and Shielded Cables

For remote-sensor distances longer than 8 in., or in

particularly noisy environments, a twisted pair is rec-

ommended. Its practical length is 6 feet to 12feet (typi-

cal) before noise becomes a problem, as tested in a

noisy electronics laboratory. For longer distances, the

best solution is a shielded twisted pair like that used

for audio microphones. Connect the twisted pair to

DXP and DXN and the shield to GND, and leave the

shield’s remote end unterminated.

Excess capacitance at DX_limits practical remote

sensor distances (see Typical Operating Characteris-

tics), For very long cable runs, the cable’s parasitic

capacitance often provides noise filtering, so the

2200pF capacitor can often be removed or reduced in

value. Cable resistance also affects remote-sensor

accuracy; 1

Ω series resistance introduces about + 1°C

error.

Low-Power Standby Mode

Standby mode disables the ADC and reduces the

supply-current drain to less than 10µA. Enter standby

mode by forcing the STBY pin low or via the

RUN/STOP bit in the configuration byte register.

Hardware and software standby modes behave almost

identically: all data is retained in memory, and the

SMB interface is alive and listening for reads and

writes. The only difference is that in hardware standby

mode, the one-shot command does not initiate a con-

version.

Standby mode is not a shutdown mode. With activity

on the SMBus, extra supply current is drawn (see

Typical

Operating

Characteristics).

software

standby mode, the G767 can be forced to perform A/D

conversions via the one-shot command, despite the

RUN/STOP bit being high.

Activate hardware standby mode by forcing the

STBY pin low. In a notebook computer, this line may

be connected to the system SUSTAT# suspend-state

signal.

The STBY pin low state overrides any software con-

version command. If a hardware or software standby

command is received while a conversion is in progress,

the conversion cycle is truncated, and the data from

that conversion is not latched into either temperature

reading register. The previous data is not changed and

remains available.

Supply-current drain during the 125ms conversion

period is always about 450µA. Slowing down the con-

version rate reduces the average supply current (see

Typical Operating Characteristics). In between con-

versions, the instantaneous supply current is about

25µA due to the current consumed by the conversion

rate timer. In standby mode, supply current drops to

about 3µA. At very low supply voltages (under the

power-on-reset threshold), the supply current is higher

due to the address pin bias currents. It can be as high

as 100µA, depending on ADD0 and ADD1 settings.

SMBus Digital Interface

From a software perspective, the G767 appears as a

set of byte-wide registers that contain temperature

data, alarm threshold values, or control bits, A stan-

dard SMBus 2-wire serial interface is used to read

temperature data and write control bits and alarm

threshold data.

Each A/D channel within the device responds to the

same SMBus slave address for normal reads and

writes.

The G767 employs four standard SMBus protocols:

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

(Figure 3). 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 overwrite 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 (Table 2), 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.

GND

DXP

DXN

GND

10 MILS

MINIMUM

10 MILS

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