Multichannel Remote/Local
Temperature Sensors
MAX1668/MAX1805/MAX1989
†
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
-25
-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
0000
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
MAX1668/MAX1805/MAX1989
slave address that was latched at
POR (Table 8)
FUNCTION
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 (typ) due to the diode current source, a
fault is detected. Note that the diode fault is not
checked until a conversion is initiated, so immediately
after power-on reset, the status byte indicates no fault
is present, even if the diode path is broken.
If any remote channel is shorted (DXP_ to DXN_ or
DXP_ to GND), the ADC reads 0000 0000 so as not to
trip either the T
HIGH
or T
LOW
alarms at their POR set-
tings. In applications that are never subjected to 0°C in
normal operation, a 0000 0000 result can be checked
to indicate a fault condition in which DXP_ is acciden-
tally short circuited. Similarly, if DXP_ is short circuited
to V
CC
, the ADC reads +127°C for all remote and local
channels, and the device alarms.
Interrupts are generated in response to T
HIGH
and T
LOW
comparisons and when a remote diode is disconnected
(for continuity 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 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.
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).
The alert response can activate several different slave
devices simultaneously, similar to the I
2
C general call. If
more than one slave attempts to respond, bus arbitra-
tion 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
ALERT
Interrupts
The
ALERT
interrupt output signal is latched and can
only be cleared by reading the alert response address.
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11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
