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G771 Datasheet(PDF) 8 Page - Global Mixed-mode Technology Inc

Part No. G771
Description  Two Remote Temperature Sensors with SMBus Serial Interface and System Reset Circuit
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Maker  GMT [Global Mixed-mode Technology Inc]
Homepage  http://www.gmt.com.tw
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G771 Datasheet(HTML) 8 Page - Global Mixed-mode Technology Inc

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Ver 0.2 Preliminary
Dec 11, 2001
TEL: 886-3-5788833
http://www.gmt.com.tw
8
G771
Global Mixed-mode Technology Inc.
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 G771 reset
output. If, for example, the G771 RESET output is
asserted high and the µP wants to pull it low,
indeterminate logic levels may result. To correct this,
connect a 4.7k
Ω resistor between the G771 RESET
output and the µP reset I/O (Figure 4). Buffer the
G771 RESET output to other system components.
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 G771 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.
Fig 3. RESET Valid to VCC = Ground Circuit
SMBus Digital Interface
From a software perspective, the G771 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 G771 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.
Fig 4. Interfacing to µPs with Bi-directional Reset I/O
V
CC
G771
RESET
GND
R1
100k
V
CC
G771
RESET
GND
R1
100k
V
CC
G771
RESET
GND
BUFFER
RESET
GND
V
CC
µP
4.7k
BUFFERED RESET
TO OTHER SYSTEM
COMPONENTS
V
CC
G771
RESET
GND
BUFFER
RESET
GND
V
CC
µP
4.7k
BUFFERED RESET
TO OTHER SYSTEM
COMPONENTS


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