Ver 1.0
Dec 11, 2001
TEL: 886-3-5788833
http://www.gmt.com.tw
7
Global Mixed-mode Technology Inc.
G766
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).
In
software
standby mode, the G766 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 G766 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 G766 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
10 MILS
10 MILS
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