Ver: 1.0
Oct 02, 2002
TEL: 886-3-5788833
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1
Global Mixed-mode Technology Inc.
G781
1C Remote and Local Temperature Sensor with
SMBus Serial Interface
Features
Two Channels: Measures Both Remote and
Local Temperatures
No Calibration Required
SMBus 2-Wire Serial Interface
Programmable Under/Overtemperature Alarms
Supports SMBus Alert Response
Accuracy:
1C (+60C to +100C, remote)
3C (+60C to + 100C, local)
320A (typ) Average Supply Current During
Conversion
+3V to +5.5V Supply Range
Small 8-Lead SO Package
Applications
Desktop and Notebook
Central Office
Computers Telecom
Equipment
Smart Battery Packs
Test and Measurement
LAN Servers
Multi-Chip Modules
Industrial Controllers
General Description
The G781 is a precise digital thermometer that reports
the temperature of both a remote sensor and its own
package. The remote sensor is a diode-connected
transistor typically a low-cost, easily mounted 2N3904
NPN type that replace conventional thermistors or
thermocouples. Remote accuracy is 1C with no cali-
bration needed. The remote channel can also meas-
ure the die temperature of other ICs, such as micro-
processors, that contain an on-chip, diode-connected
transistor.
The 2-wire serial interface accepts standard System
Management Bus (SMBus) Write Byte, Read Byte,
Send Byte, and Receive Byte commands to program
the alarm thresholds and to read temperature data.
The data format is 11bits plus sign, with each bit cor-
responding to 0.125C, in two's-complement format.
Measurements can be done automatically and
autonomously, with the conversion rate programmed
by the user or programmed to operate in a single-shot
mode. The adjustable rate allows the user to control
the supply current drain.
The G781 is available in a small, 8-pin SOP sur-
face-mount package.
Ordering Information
PART* TEMP.
RANGE PIN-PACKAGE
G781
-20C to +120C
8-SOP
Pin Configuration
Typical Operating Circuit
THERM
0.1F
3V TO 5.5V
VCC
DXP
DXN
SMBCLK
SMBDATA
ALERT
10k
EACH
CLOCK
DATA
INTERRUPT TO C
GND
2200pF
2N3904
DXN
8
6
5
1
2
3
4
VCC
DXP
SMBDATA
SMBCLK
GND
8 Pin SOP
G781
7
ALERT
THERM
THERM
0.1F
3V TO 5.5V
VCC
DXP
DXN
SMBCLK
SMBDATA
ALERT
10k
EACH
CLOCK
DATA
INTERRUPT TO C
GND
2200pF
2N3904
DXN
8
6
5
1
2
3
4
VCC
DXP
SMBDATA
SMBCLK
GND
8 Pin SOP
G781
7
ALERT
THERM
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Global Mixed-mode Technology Inc.
G781
Absolute Maximum Ratings
VCC to GND......................................-0.3V to +6V
DXP to GND..............................-0.3V to VCC + 0.3V
DXN to GND..................................-0.3V to +0.8V
SMBCLK, SMBDATA, ALERT to GND.....-0.3V to +6V
SMBDATA, ALERT Current.............-1mA to +50mA
DXN Current................................................1mA
ESD Protection (SMBCLK, SMBDATA, ALERT , human
body model)...............................................2000V
ESD Protection (other pins, human body model)..2000V
Continuous Power Dissipation (T
A
= +70C) ..SOP
(derate 8.30mW/C above +70C)..................667mW
Operating Temperature Range.........-20C to +120C
Junction Temperature.................................+150C
Storage temperature Range.............-65C to +165C
Lead Temperature (soldering, 10sec).................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the opera-
tional sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may
affect device reliability.
Electrical Characteristics
(VCC = + 3.3V, T
A
= 0C to +85C, unless otherwise noted.)
PARAMETER CONDITIONS
MIN
TYP
MAX
UNITS
T
R
= +60C to +100C, VCC = 3.0V to 3.6V
-1
+1
Temperature Error, Remote Di-
ode (Note 1)
T
R
= 0C to +125C (Note 2)
-3
+3
C
T
A
= +60C to +100C
-3
+3
Temperature Error, Local Diode
T
A
= 0C to +85C (Note 2)
-5
+5
C
Supply-Voltage
Range
3.0 5.5 V
Undervoltage Lockout Threshold VCC input, disables A/D conversion, rising edge
2.8
V
Undervoltage Lockout Hysteresis
50
mV
Power-On Reset Threshold
VCC, falling edge
1.7
V
POR Threshold Hysteresis
50
mV
SMBus static
3
Standby Supply Current
Logic inputs forced to VCC or GND Hardware or software
standby, SMBCLK at 10kHz
4
A
0.5
conv/sec
35
Average Operating Supply
Current
Auto-convert mode. Logic inputs
forced to VCC or GND
8.0 conv/sec
320
A
Conversion Time
From stop bit to conversion complete (both channels)
125
ms
Conversion Rate Timing
Conversion-Rate Control Byte=04h, 1Hz
1
sec
High level
176
Remote-Diode Source Current
DXP forced to 1.5V
Low
level
11
A
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Global Mixed-mode Technology Inc.
G781
Electrical Characteristics
(continued)
(VCC = + 3.3V, T
A
= 0 to +85C, unless otherwise noted.)
PARAMETER CONDITIONS
MIN
TYP
MAX
UNITS
SMBus Interface
Logic Input High Voltage
STBY
, SMBCLK, SMBDATA; Vcc = 3V to 5.5V
2.4 V
Logic Input Low Voltage
STBY
, SMBCLK, SMBDATA; Vcc = 3V to 5.5V
0.8
V
Logic Output Low Sink Current
ALERT , SMBDATA forced to 0.4V
6 mA
ALERT Output High Leakage Current
ALERT forced to 5.5V
1
A
Logic Input Current
Logic inputs forced to VCC or GND
-2
2
A
SMBus Input Capacitance
SMBCLK, SMBDATA
5
pF
SMBus Clock Frequency
100 kHz
SMBus Timeout
SMBCLK low time for interface reset
30
ms
SMBCLK Clock Low Time
t
LOW
, 10% to 10% points
4.7
s
SMBCLK Clock High Time
t
HIGH
, 90% to 90% points
4
s
SMBus Start-Condition Setup Time
4.7
s
SMBus Repeated Start-Condition Setup Time t
SU :
STA ,
90% to 90% points
500
ns
SMBus Start-Condition Hold Time
t
HD: STA ,
10% of SMBDATA to 90% of SMBCLK
4
s
SMBus Stop-Condition Setup Time
t
SD: STO ,
90% of SMBCLK to 10% of SMBDATA
4
s
SMBus Data Valid to SMBCLK Rising-Edge
Time
t
SU: DAT ,
10% or 90% of SMBDATA to 10% of
SMBCLK
800 ns
SMBus Data-Hold Time
t
HD : DAT
300
ns
SMBCLK Falling Edge to SMBus Data-Valid
Time
Master clocking in data
1
s
Note 1: A remote diode is any diode-connected transistor from Table1. T
R
is the junction temperature of the remote
of the remote diode. See Remote Diode Selection for remote diode forward voltage requirements.
Note 2: Guaranteed by design but not 100% tested.
Pin Description
PIN NAME
FUNCTION
1
VCC
Supply Voltage Input, 3V to 5.5V. Bypass to GND with a 0.1F capacitor.
2 DXP
Combined Current Source and A/D Positive Input for remote-diode channel. Do not leave DXP float-
ing; tie DXP to DXN if no remote diode is used. Place a 2200pF capacitor between DXP and DXN for
noise filtering.
3
DXN
Combined Current Sink and A/D Negative Input.
4
THERM
Open-drain output. Requires pull-up to VCC.
5 GND
Ground
6
ALERT
SMBus Alert (interrupt) Output, open drain
7
SMBDATA SMBus Serial-Data Input / Output, open drain
8
SMBCLK SMBus Serial-Clock Input
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Global Mixed-mode Technology Inc.
G781
Detailed Description
The G781 is a temperature sensor designed to work in
conjunction with an external microcontroller (C) or
other intelligence in thermostatic, process-control, or
monitoring applications. The C is typically a power-
management or keyboard controller, generating
SMBus serial commands by "bit-banging" general-
purpose input-output (GPIO) pins or via a dedicated
SMBus interface block.
Essentially an serial analog-to digital converter (ADC)
with a sophisticated front end, the G781 contains a
switched current source, a multiplexer, an ADC, an
SMBus interface, and associated control logic (Figure
1). Temperature data from the ADC is loaded into two
data registers, where it is automatically compared with
data previously stored in several over/under- tem-
perature alarm registers.
ADC and Multiplexer
The ADC is an averaging type that integrates over a
60ms period (each channel, typical), with excellent
noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes, measures their
forward voltages, and computes their temperatures.
Both channels are automatically converted once the
conversion process has started, either in free-running
or single-shot mode. If one of the two channels is not
used, the device still performs both measurements,
and the user can simply ignore the results of the un-
used channel. If the remote diode channel is unused,
tie DXP to DXN rather than leaving the pins open.
The worst-case DXP-DXN differential input voltage
range is 0.25V to 0.95V.
Excess resistance in series with the remote diode
causes about +0.6C error per ohm. Likewise, 240V
of offset voltage forced on DXP-DXN causes about
1C error.
Figure 1. Functional Diagram
V
CC
DXP
DXN
MUX
+
+
+
REMOTE
LOCAL
ADC
CONTROL
LOGIC
SMBUS
7
SMBDATA
SMBCLK
READ WRITE
DIODE
FAULT
2
8
8
COMMAND BYTE
(INDEX) REGISTER
STATUS BYTE
REGISTER
CONFIGURATION
BYTE REGISTER
CONVERSION RATE
REGISTER
ALERT RESPONSE
ADDRESS REGISTER
8
8
LOCAL EMPERATURE
DATA REGISTER
HIGH-TEMPETATURE
THRESHOLD (LOCALT
HIGH
)
LOW-TEMPETATURE
THRESHOLD (LOCAL T
LOW
)
8
DIGITAL COMPARATOR
(LOCAL)
SELECTED VIA
SLAVE ADD = 0001 100
11
11
ALERT
Q
S
R
REMOTE TEMPERATURE
DATA REGISTER
HIGH-TEMPETATURE
THRESHOLD (REMOTE
HIGH
)
LOW-TEMPETATURE
THRESHOLD (REMOTE
LOW
)
DIGITAL COMPARATOR
(REMOTE)
THERM LIMIT AND
HYSTERESIS REGISTER
11
COMPARATOR
THERM
V
CC
DXP
DXN
MUX
+
+
+
REMOTE
LOCAL
ADC
CONTROL
LOGIC
SMBUS
7
SMBDATA
SMBCLK
READ WRITE
DIODE
FAULT
2
8
8
COMMAND BYTE
(INDEX) REGISTER
STATUS BYTE
REGISTER
CONFIGURATION
BYTE REGISTER
CONVERSION RATE
REGISTER
ALERT RESPONSE
ADDRESS REGISTER
8
8
LOCAL EMPERATURE
DATA REGISTER
HIGH-TEMPETATURE
THRESHOLD (LOCALT
HIGH
)
LOW-TEMPETATURE
THRESHOLD (LOCAL T
LOW
)
8
DIGITAL COMPARATOR
(LOCAL)
SELECTED VIA
SLAVE ADD = 0001 100
11
11
ALERT
Q
S
R
REMOTE TEMPERATURE
DATA REGISTER
HIGH-TEMPETATURE
THRESHOLD (REMOTE
HIGH
)
LOW-TEMPETATURE
THRESHOLD (REMOTE
LOW
)
DIGITAL COMPARATOR
(REMOTE)
THERM LIMIT AND
HYSTERESIS REGISTER
11
COMPARATOR
THERM
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Global Mixed-mode Technology Inc.
G781
A/D Conversion Sequence
If a Start command is written (or generated automati-
cally in the free-running auto-convert mode), both
channels are converted, and the results of both meas-
urements are available after the end of conversion. A
BUSY status bit in the status byte shows that the de-
vice is actually performing a new conversion; however,
even if the ADC is busy, the results of the previous
conversion are always available.
Remote Diode Selection
Temperature accuracy depends on having a good-
quality, diode-connected small-signal transistor. The
G781 can also directly measure the die temperature of
CPUs and other integrated circuits having on-board
temperature-sensing diodes.
The transistor must be a small-signal type with a rela-
tively high forward voltage; otherwise, the A/D input
voltage range can be violated. The forward voltage
must be greater than 0.25V at 10A; check to ensure
this is true at the highest expected temperature. The
forward voltage must be less than 0.95V at 300A;
check to ensure this is true at the lowest expected
temperature. Large power transistors don't work at all.
Also, ensure that the base resistance is less than
100
. Tight specifications for forward-current gain
(+50 to +150, for example) indicate that the manufac-
turer has good process controls and that the devices
have consistent V
be
characteristics.
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the G781's ef-
fective accuracy. The thermal time constant of the
SOP- package is about 140 in still air. For the G781
junction temperature to settle to within +1C after a
sudden +100C change requires about five time con-
stants or 12 minutes. The use of smaller packages for
remote sensors, such as SOT23s, improves the situa-
tion. Take care to account for thermal gradients be-
tween the heat source and the sensor, and ensure that
stray air currents across the sensor package do not
interfere with measurement accuracy. Self-heating
does not significantly affect measurement accuracy.
Remote-sensor self-heating due to the diode current
source is negligible. For the local diode, the worst-case
error occurs when auto-converting at the fastest rate
and simultaneously sinking maximum current at the
ALERT
output. For example, at an 8Hz rate and with
ALERT
sinking 1mA, the typical power dissipation is
VCC x 320A plus 0.4V x 1mA. Package theta J-A is
about 120C /W, so with VCC = 3.3V and no copper PC
board heat-sinking, the resulting temperature rise is:
dT = 1.45mW x 120C /W = 0.17C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
Table 1. Remote-Sensor Transistor Manufacturers
MANUFACTURER MODEL
NUMBER
Philips PMBS3904
Motorola(USA) MMBT3904
National Semiconductor (USA)
MMBT3904
Note:Transistors must be diode-connected (base
shorted to collector).
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection. Micropower operation places con-
straints on high-frequency noise rejection; therefore,
careful PC board layout and proper external noise fil-
tering are required for high-accuracy remote meas-
urements in electrically noisy environments.
High-frequency EMI is best filtered at DXP and DXN
with an external 2200pF capacitor. This value can be
increased to about 3300pF(max), including cable ca-
pacitance. Higher capacitance than 3300pF introduces
errors due to the rise time of the switched current
source.
Nearly all noise sources tested cause the ADC meas-
urements to be higher than the actual temperature,
typically by +1C to 10C, depending on the frequency
and amplitude.
PC Board Layout
Place the G781 as close as practical to the remote
diode. In a noisy environment, such as a computer
motherboard, this distance can be 4 in. to 8 in. (typical)
or more as long as the worst noise sources (such as
CRTs, clock generators, memory buses, and ISA/PCI
buses) are avoided.
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Global Mixed-mode Technology Inc.
G781
Do not route the DXP-DXN lines next to the deflection
coils of a CRT. Also, do not route the traces across a
fast memory bus, which can easily introduce +30C
error, even with good filtering, Otherwise, most noise
sources are fairly benign.
Route the DXP and DXN traces in parallel and in close
proximity to each other, away from any high-voltage
traces such as +12V
DC
. Leakage currents from PC
board contamination must be dealt with carefully,
since a 10M
leakage path from DXP to ground
causes about +1C error.
Connect guard traces to GND on either side of the
DXP-DXN traces (Figure 2). With guard traces in place,
routing near high-voltage traces is no longer an issue.
Route through as few vias and crossunders as
possible to minimize copper/solder thermocouple ef-
fects.
When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. In general, PC board-induced ther-
mocouples are not a serious problem, A copper-solder
thermocouple exhibits 3V/C, and it takes about
240V of voltage error at DXP-DXN to cause a +1C
measurement error. So, most parasitic thermocouple
errors are swamped out.
Use wide traces. Narrow ones are more inductive and
tend to pick up radiated noise. The 10 mil widths and
spacing recommended on Figure 2 aren't absolutely
necessary (as they offer only a minor improvement in
leakage and noise), but try to use them where practi-
cal.
Keep in mind that copper can't be used as an EMI
shield, and only ferrous materials such as steel work
will. Placing a copper ground plane between the
DXP-DXN traces and traces carrying high-frequency
noise signals does not help reduce EMI.
PC Board Layout Checklist
Place the G781 close to a remote diode.
Keep traces away from high voltages (+12V bus).
Keep traces away from fast data buses and CRTs.
Use recommended trace widths and spacing.
Place a ground plane under the traces
Use guard traces flanking DXP and DXN and con
necting to GND.
Place the noise filter and the 0.1F VCC bypass
capacitors close to the G781.
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 sen-
sor distances (see Typical Operating Characteristics),
For very long cable runs, the cable's parasitic capaci-
tance often provides noise filtering, so the 2200pF ca-
pacitor can often be removed or reduced in value. Ca-
ble resistance also affects remote-sensor accuracy; 1
series resistance introduces about + 0.6C error.
Low-Power Standby Mode
Standby mode disables the ADC and reduces the
supply-current drain to about 10A. Enter standby
mode by forcing high to the
RUN
/STOP bit in the con-
figuration byte register. Software standby mode be-
haves such that all data is retained in memory, and the
SMB interface is alive and listening for reads and
writes.
Software 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 G781 can be forced to perform A/D
conversions via the one-shot command, despite the
RUN
/STOP bit being high.
GND
DXP
DXN
GND
10 MILS
MINIMUM
10 MILS
10 MILS
10 MILS
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Global Mixed-mode Technology Inc.
G781
If software standby command is received while a con-
version is in progress, the conversion cycle is trun-
cated, 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 320A. 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
25A due to the current consumed by the conversion
rate timer. In standby mode, supply current drops to
about 3A. 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 100A, depending on ADD0 and ADD1 settings.
SMBus Digital Interface
From a software perspective, the G781 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 G781 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 11bits plus sign in
twos-complement form for remote channel, with each
data bit representing 0.125C (Table 2,Table 3),
transmitted MSB first.
Table 2. Temperature Data Format
(Two's-Complement)
DIGITAL OUTPUT
DATA BITS
TEMP.
(C)
SIGN MSB LSB EXT
+127.875 0 111 1111 111
+126.375 0 111 1110 011
+25.5 0 001 1001 100
+1.75 0 000 0001 110
+0.5 0 000 0000 100
+0.125 0 000 0000 001
-0.125 1 111 1111 111
-1.125 1 111 1110 111
-25.5 1 110 0110 100
-55.25 1 100 1000 110
-65.000 1 011 1111 000
Table 3. Extended Temperature Data Format
EXTENDED
RESOLUTION
DATA BITS
0.000C 0000
0000
0.125C 0010
0000
0.250C 0100
0000
0.375C 0110
0000
0.500C 1000
0000
0.625C 1010
0000
0.750C 1100
0000
0.875C 1110
0000
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Global Mixed-mode Technology Inc.
G781
Write Byte Format
S ADDRESS WR ACK
COMMAND
ACK
DATA
ACK
P
7 bits
8 bits
8 bits
1
Slave Address: equivalent to chip- select line of a 3-wire interface
Command Byte: selects which register you are writing to
Data byte: data goes into the register set by the command byte (to set thresholds, configuration masks, and sam
pling rate)
Read Byte Format
S ADDRESS WR ACK
COMMAND ACK
S ADDRESS RD ACK
DATA
/// P
7 bits
8bits
7bits
8 bits
Slave Address: equivalent to chip- select line
Command Byte: selects which register you are reading from
Slave Address: repeated due to change in data-flow direction
Data byte: reads from the register set by the command byte
Send Byte Format
S ADDRESS
WR
ACK
COMMAND
ACK
P
7 bits
8 bits
Command Byte: sends command with no data , usually used for one-shot command
Receive Byte Format
S ADDRESS
RD
ACK
DATA
/// P
7 bits
8 bits
Data Byte: reads data from the register commanded by the last Read Byte or Write Byte transmission; also used
for SMBus Alert Response return address
S = Start condition Shaded = Slave transmission P = Stop condition /// = Not acknowledged
Figure 3. SMBus Protocols
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Global Mixed-mode Technology Inc.
G781
Slave Address
The G781 appears to the SMBus as one device hav-
ing a common address for both ADC channels. The
G781 device address is set to 1001100.
The G781 also responds to the SMBus Alert Re-
sponse slave address (see the Alert Response Ad-
dress section).
One-Shot Register
The One-shot register is to initiate a single conversion
and comparison cycle when the device is in standby
mode and auto conversion mode. The write operation to
this register causes one-shot conversion and the data
written to it is irrelevant and is not stored.
Serial Bus Interface Reinitialization
When SMBCLK are held low for more than 30ms (typical)
during an SMBus communication the G781 will reinitiate
its bus interface and be ready for a new transmission.
Alarm Threshold Registers
Four registers store alarm threshold data, with
high-temperature (T
HIGH
) and low-temperature (T
LOW
)
registers for each A/D channel. If either measured
temperature equals or exceeds the corresponding
alarm threshold value, an
ALERT
interrupt is as-
serted.
The power-on-reset (POR) state of both T
HIGH
registers
is full scale (01010101, or +85
C). The POR state of
both T
LOW
registers is 0
C.
Diode Fault Alarm
There is a fault detector at DXP that detects whether
the remote diode has an open-circuit condition. At the
beginning of each conversion, the diode fault is
checked, and the status byte is updated. This fault de-
tector is a simple voltage detector. If DXP rises above
VCC 1V (typical) due to the diode current source, a
fault is detected and the device alarms through pulling
ALERT
low while the remote temperature reading
doesn't update in this condition. Note that the diode
fault isn't checked until a conversion is initiated, so im-
mediately after power-on reset the status byte indicates
no fault is present, even if the diode path is broken.
If the remote channel is shorted (DXP to DXN or DXP to
GND), the ADC reads 1000 0000(-128
C) so as not to
trip either the T
HIGH
or T
LOW
alarms at their POR settings.
ALERT Interrupts
The
ALERT
interrupt output signal is latched and can
only be cleared by reading the Alert Response ad-
dress. Interrupts are generated in response to T
HIGH
and T
LOW
comparisons and when the remote diode is
disconnected (for fault detection). The interrupt does
not halt automatic conversions; new temperature data
continues to be available over the SMBus interface
after
ALERT
is asserted. The interrupt output pin is
open-drain so that devices can share a common in-
terrupt line. The interrupt rate can never exceed the
conversion rate.
The interface responds to the SMBus Alert Response
address, an interrupt pointer return-address feature
(see Alert Response Address section). Prior to taking
corrective action, always check to ensure that an in-
terrupt is valid by reading the current temperature.
Alert Response Address
The SMBus Alert Response interrupt pointer provides
quick fault identification for simple slave devices that
lack the complex, expensive logic needed to be a bus
master. Upon receiving an
ALERT
interrupt signal,
the host master can broadcast a Receive Byte trans-
mission to the Alert Response slave address (0001
100). Then any slave device that generated an inter-
rupt attempts to identify itself by putting its own ad-
dress on the bus (Table 4).
The Alert Response can activate several different
slave devices simultaneously, similar to the SMBus
General Call. If more than one slave attempts to re-
spond, bus arbitration rules apply, and the device with
the lower address code wins. The losing device does
not generate an acknowledge and continues to hold
the
ALERT
line low until serviced (implies that the
host interrupt input is level-sensitive). Successful
reading of the alert response address clears the inter-
rupt latch.
Table 4. Read Format for Alert Response Address
(0001 100)
BIT NAME
7(MSB) ADD7
6 ADD6
5 ADD5
4 ADD4
3 ADD3
2 ADD2
1 ADD1
0(LSB) 1
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Global Mixed-mode Technology Inc.
G781
Command Byte Functions
The 8-bit command byte register (Table 5) is the mas-
ter index that points to the various other registers
within the G781. The register's POR state is 0000
0000, so that a Receive Byte transmission (a protocol
that lacks the command byte) that occurs immediately
after POR returns the current local temperature data.
The one-shot command immediately forces a new
conversion cycle to begin. In software standby mode
(
RUN
/STOP bit = high), a new conversion is begun,
after which the device returns to standby mode. If a
conversion is in progress when a one-shot command
is received in auto-convert mode (
RUN
/STOP bit =
low) between conversions, a new conversion begins,
the conversion rate timer is reset, and the next auto-
matic conversion takes place after a full delay elapses.
Configuration Byte Functions
The configuration byte register (Table 6) is used to
mask interrupts and to put the device in software
standby mode. The other bits are empty.
Status Byte Functions
The status byte register (Table 7) indicates which (if
any) temperature thresholds have been exceeded.
This byte also indicates whether or not the ADC is
converting and whether there is an open circuit in the
remote diode DXP-DXN path. After POR, the normal
state of all the flag bits is zero, assuming none of the
alarm conditions are present. The status byte is
cleared by any successful read of the status, unless
the fault persists. Note that the
ALERT
interrupt latch
is not automatically cleared when the status flag bit is
cleared.
When reading the status byte, you must check for in-
ternal bus collisions caused by asynchronous ADC
timing, or else disable the ADC prior to reading the
status byte (via the
RUN
/STOP bit in the configura-
tion byte). In one-shot mode, read the status byte only
after the conversion is complete, which is approxi-
mately 125ms max after the one-shot conversion is
commanded.
Table 5. Command-Byte Bit Assignments
REGISTER COMMAND POR
STATE
FUNCTINON
RLTS
00h
0000 0000*
Read local temperature. It returns latest temperature
RRTE
01h
0000 0000*
Read remote temperature. It returns latest temperature
RSL
02h
N/A
Read status byte (flags, busy signal)
RCL
03h
0000 0000
Read configuration byte
RCRA
04h
0000 1000
Read conversion rate byte
RLHN
05h
0101 0101 (85) Read local T
HIGH
limit
RLLI
06h
0000 0000
Read local T
LOW
limit
RRHI
07h
0101 0101 (85) Read remote T
HIGH
limit
RRLS
08h
0000 0000
Read remote T
LOW
limit
WCA
09h
N/A
Write configuration byte
WCRW
0Ah
N/A
Write conversion rate byte
WLHO
0Bh
N/A
Write local T
HIGH
limit
WLLM
0Ch
N/A
Write local T
LOW
limit
WRHA
0Dh
N/A
Write remote T
HIGH
limit
WRLN
0Eh
N/A
Write remote T
LOW
limit
OSHT
0Fh
N/A
One-shot command (use send-byte format)
RTEXT
10h
0
Remote temperature extended byte
RTOFS
11h
0
Remote temperature offset high byte
RTOFSEXT
12h
0
Remote temperature offset extended byte
RLEXT 13h
0
Remote
T
HIGH
limit extended byte
RHEXT 14h
0
Remote
T
LOW
limit extended byte
RTTHERM
19h
0101 0101 (85) Remote temperature THERM limit
LTTHERM
20h
0101 0101 (85) Local temperature THERM limit
THERMHYST
21h
0000 1010 (10)
THERM hysteresis
ALERTFQ 22h
0 ALERT fault queue code
MFGIO
FEh
0100 0111
Manufacturer ID
DEVID
FFh
0000 0001
Device ID
*If the device is in standby mode at POR, both temperature registers read 0
C.
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Global Mixed-mode Technology Inc.
G781
Table 6. Configuration-Byte Bit Assignments
BIT NAME
POR
STATE
FUNCTION
7 (MSB)
MASK
0
Masks all
ALERT
interrupts when high.
6
RUN
/
STOP
0
Standby mode control bit. If high, the device immediately stops converting and en-
ters standby mode. If low, the device converts in either one-shot or timer mode.
5-0
RFU
0
Reserved for future use
Table 7. Status-Byte Bit Assignments
BIT NAME
FUNCTION
7 (MSB)
BUSY
A high indicates that the ADC is busy converting.
6
LHIGH*
A high indicates that the local high-temperature alarm has activated.
5
LLOW*
A high indicates that the local low-temperature alarm has activated.
4
RHIGH*
A high indicates that the remote high-temperature alarm has activated.
3
RLOW*
A high indicates that the remote low-temperature alarm has activated.
2
OPEN*
A high indicates a remote-diode continuity (open-circuit) fault.
1 RTHRM
A high indicates a remote temperature
THERM
alarm has activated.
0 (LSB)
LTHRM
A high indicates a local temperature
THERM
alarm has activated.
*These flags stay high until cleared by POR, or until the status byte register is read.
Table 8. Conversion-Rate Control Byte
DATA
CONVERSION RATE (Hz)
00h 0.0625
01h 0.125
02h 0.25
03h 0.5
04h 1
05h 2
06h 4
07h 8
08h 16
09h to FFh
RFU
To check for internal bus collisions, read the status
byte. If the least significant seven bits are ones, dis-
card the data and read the status byte again. The
status bits LHIGH, LLOW, RHIGH, and RLOW are
refreshed on the SMBus clock edge immediately fol-
lowing the stop condition, so there is no danger of los-
ing temperature-related status data as a result of an
internal bus collision. The OPEN status bit (diode con-
tinuity fault) is only refreshed at the beginning of a
conversion, so OPEN data is lost. The
ALERT
inter-
rupt latch is independent of the status byte register, so
no false alerts are generated by an internal bus colli-
sion.
When auto-converting, if the THIGH and TLOW limits
are close together, it's possible for both high-temp and
low-temp status bits to be set, depending on the
amount of time between status read operations (espe-
cially when converting at the fastest rate). In these
circumstances, it's best not to rely on the status bits to
indicate reversals in long-term temperature changes
and instead use a current temperature reading to es-
tablish the trend direction.
For bit 1 and bit 0, a high indicates a temperature
alarm happened for remote and local diode respec-
tively.
THERM
pin also asserts. These two bits
wouldn't be cleared when reading status byte.
Conversion Rate Byte
The conversion rate register (Table 8) programs the
time interval between conversions in free-running
auto-convert mode. This variable rate control reduces
the supply current in portable-equipment applications.
The conversion rate byte's POR state is 08h (16Hz).
The G781 looks only at the 4 LSB bits of this register,
so the upper 4 bits are "don't care" bits, which should
be set to zero. The conversion rate tolerance is
25%
at any rate setting.
Valid A/D conversion results for both channels are
available one total conversion time (125ms,typical)
after initiating a conversion, whether conversion is
initiated via the
RUN
/STOP bit, one-shot command,
or initial power-up.
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Global Mixed-mode Technology Inc.
G781
POR AND UVLO
The G781 has a volatile memory. To prevent ambiguous
power-supply conditions from corrupting the data in
memory and causing erratic behavior, a POR voltage
detector monitors VCC and clears the memory if VCC
falls below 1.7V (typical, see Electrical Characteristics
table). When power is first applied and VCC rises above
1.7V (typical), the logic blocks begin operating, although
reads and writes at V
CC
levels below 3V are not recom-
mended. A second VCC comparator, the ADC UVLO
comparator, prevents the ADC from converting until there
is sufficient headroom (VCC= 2.8V typical).
ALERT Fault Queue
To suppress unwanted
ALERT
triggering the G781 em-
bedded a fault queue function. The
ALERT
won't as-
sert until consecutive out of limit measurements have
reached the queue number. The mapping of fault
queue register (ALERTFQ, 22h) value to fault queue
number is shown in the Table 9.
Table 9. Alert Fault Queue
ALERTFQ
VALUE
FAULT QUEUE NUMBER
XXXX000X 1
XXXX001X 2
XXXX010X 3
XXXX011X 3
XXXX100X 4
XXXX101X 4
XXXX110X 4
XXXX111X 4
Operation of The THERM Function
A local and remote
THERM
limit can be programmed
into the G781 to set the temperature limit above which
the
THERM
pin asserts low and the bit 1, of status
byte will be set to 1 corresponding to remote and local
over temperature. These two bits won't be cleared to 0
by reading status byte it the over temperature condi-
tion remain. A hysteresis value is provided by writing
the register 21h to set the temperature threshold to
release the
THERM
pin alarm state, The releasing
temperature is the value of register 19h, 20h minus the
value in register 21h. The format of register 21h is 2's
complement. The
THERM
signal is open drain and
requires a pull-up resistor to power supply.
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Global Mixed-mode Technology Inc.
G781
Figure 4. SMBus Write Timing Diagram
A = start condition
H = LSB of data clocked into slave
B = MSB of address clocked into slave
I = slave pulls SMBDATA line low
C = LSB of address clocked into slave
J = acknowledge clocked into master
D = R/W bit clocked into slave
K = acknowledge clocked pulse
E = slave pulls SMBDATA line low
L = stop condition data executed by slave
F = acknowledge bit clocked into master
M = new start condition
G = MSB of data clocked into slave
Figure 5. SMBus Read Timing Diagram
A = start condition
G = MSB of data clocked into master
B = MSB of address clocked into slave
H = LSB of data clocked into master
C = LSB of address clocked into slave
I = acknowledge clocked pulse
D = R/ W bit clocked into slave
J = stop condition
E = slave pulls SMBDATA line low
K= new start condition
F =acknowledge bit clocked into master
SMBCLK
SMBDATA
A
B
C
D
E F
G
H
I J
K
L
M
t
SU:STA
t
HD:STA
t
SU:DAT
t
HD:DAT
t
SU:STO
t
BUF
t
LOW
t
HIGH
SMBCLK
SMBDATA
A
B
C
D
E F
G
H
I
J
K
t
SU:STA
t
HD:STA
t
SU:DAT
t
SU:STO
t
BUF
t
LOW
t
HIGH
SMBCLK
SMBDATA
A
B
C
D
E F
G
H
I
J
K
t
SU:STA
t
HD:STA
t
SU:DAT
t
SU:STO
t
BUF
t
LOW
t
HIGH
Ver: 1.0
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TEL: 886-3-5788833
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Global Mixed-mode Technology Inc.
G781
Package Information
8 Pin SOP Package
Note:
1. Package body sizes exclude mold flash and gate burrs
2. Dimension L is measured in gage plane
3. Tolerance 0.10mm unless otherwise specified
4. Controlling dimension is millimeter converted inch dimensions are not necessarily exact.
DIMENSION IN MM
DIMENSION IN INCH
SYMBOL
MIN. NOM. MAX. MIN. NOM. MAX.
A 1.35 1.60 1.75 0.053 0.063 0.069
A1
0.10 ----- 0.25 0.004
----- 0.010
A2 ----- 1.45 ----- ----- 0.057
-----
B 0.33 ----- 0.51 0.013
----- 0.020
C 0.19 ----- 0.25 0.007
----- 0.010
D 4.80 ----- 5.00 0.189
----- 0.197
E 3.80 ----- 4.00 0.150
----- 0.157
e ----- 1.27 ----- ----- 0.050
-----
H 5.80 ----- 6.20 0.228
----- 0.244
L 0.40 ----- 1.27 0.016
----- 0.050
y ----- ----- 0.10 ----- ----- 0.004
0 ----- 8 0 ----- 8
Taping Specification
GMT Inc. does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and GMT Inc. reserves the right at any time without notice to change said circuitry and specifications.
D
E
H
7
(4X)
A1
A2
A
e
B
y
C
L
Feed Direction
Typical SOP Package Orientation
Feed Direction
Typical SOP Package Orientation
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