Remote/Local Temperature Sensor
with SMBus Serial Interface
MAX1617A
Table 2. Data Format (Two’s Complement)
TEMP.
(°C)
+130.00
+127.00
+126.50
+126.00
+25.25
+0.50
+0.25
0.00
-0.25
-0.50
-0.75
-1.00
-25.00
-25.50
-54.75
-55.00
-65.00
-70.00
ROUNDED
TEMP.
(°C)
+127
+127
+127
+126
+25
+1
0
0
0
0
-1
-1
-25
-26
-55
-55
-65
-65
DIGITAL OUTPUT
DATA BITS
SIGN
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
MSB
111
111
111
111
001
000
000
000
000
000
111
111
110
110
100
100
011
011
LSB
1111
1111
1111
1110
1001
0001
0000
0000
0000
0000
1111
1111
0111
0110
1001
1001
1111
1111
Table 3. Read Format for Alert Response
Address (0001100)
BIT
7
(MSB)
6
5
4
3
2
1
0
(LSB)
NAME
ADD7
ADD6
ADD5
ADD4
ADD3
ADD2
ADD1
1
Logic 1
Provide the current MAX1617A
slave address that was latched at
POR (Table 8)
FUNCTION
the T
HIGH
or T
LOW
alarms at their POR settings. In
applications that are never subjected to 0°C in normal
operation, a 0000 0000 result can be checked to indi-
cate a fault condition in which DXP is accidentally short
circuited. Similarly, if DXP is short circuited to V
CC
, the
ADC reads +127°C for both remote and local channels,
and the device alarms.
ALERT
Interrupts
The
ALERT
interrupt output signal is latched and can
only be cleared by reading the Alert Response address.
Interrupts are generated in response to T
HIGH
and T
LOW
comparisons and when the remote diode is disconnect-
ed (for continuity fault detection). The interrupt does not
halt automatic conversions; new temperature data con-
tinues 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 interrupt 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 inter-
rupt is valid by reading the current temperature.
Alarm Threshold Registers
Four registers store alarm threshold data, with high-
temperature (T
HIGH
) and low-temperature (T
LOW
) reg-
isters for each A/D channel. If either measured
temperature equals or exceeds the corresponding
alarm threshold value, an
ALERT
interrupt is asserted.
The power-on-reset (POR) state of both T
HIGH
registers
is full scale (0111 1111, or +127°C). The POR state of
both T
LOW
registers is 1100 1001 or -55°C.
Diode Fault Alarm
There is a continuity fault detector at DXP that detects
whether the remote diode has an open-circuit condi-
tion. At the beginning of each conversion, the diode
fault is checked, and the status byte is updated. This
fault detector is a simple voltage detector; if DXP rises
above V
CC
- 1V (typical) due to the diode current
source, a fault is detected. Note that the diode fault
isn’t checked until a conversion is initiated, so immedi-
ately 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 0000 0000 so as not to trip either
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 transmission
to the Alert Response slave address (0001 100). Then
any slave device that generated an interrupt attempts
to identify itself by putting its own address on the bus
(Table 3).
11
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