1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
extended temperature and read internal extended tem-
Table 2. Data Format
perature registers, which extend the resolution to
DIGITAL
OUTPUT
0.1251C per LSB (Table 3).
TEMP (°C)
ROUNDED TEMP (°C)
If a conversion ends after reading the main register but
before reading the extended register, the extended
register will contain the 3LSBs from the new conversion
while the main register will contain the 8MSBs from the
previous conversion. The extended data in this case
will be meaningless. To avoid this problem, read
extended resolution temperature data using one of the
following approaches:
130.00
127.00
126.00
25.25
+127
0 111 1111
0 111 1111
0 111 1110
0 001 1001
0 000 0001
0 000 0000
1 000 0000
1 111 1111
1 000 0000
+127
+126
+25
0.50
+1
0.00
0
<0.00 (Note 1)
-1
(Normal mode)
(Extended temp mode)
(Extended temp mode)
1) Put the MAX6654 into standby mode by setting bit 6
of the configuration register to 1. Initiate a one-shot
conversion using Command Byte 0Fh. When this
conversion is complete, read the contents of the
temperature data registers.
<-64
Diode fault
(short or open)
1 000 0000
2) If the MAX6654 is in run mode and the conversion
rate is not set to either 1Hz or 8Hz, read the Status
Byte. If the BUSY bit indicates that a conversion is
in progress, wait until the conversion is complete as
indicated by the BUSY bit. Then immediately read
the contents of the temperature data registers. If no
conversion is in progress, the data can be read
within a few µs, which is a sufficiently short period
to ensure that a new conversion can’t be completed
until after the data has been read.
Table 3. Extended Resolution Register
FRACTIONAL
DIGITAL OUTPUT
TEMPERATURE
0.000
0.125
0.250
0.375
0.500
0.625
0.750
0.875
000X XXXX
001X XXXX
010X XXXX
011X XXXX
100X XXXX
101X XXXX
110X XXXX
111X XXXX
Note: Extended resolution applies only for conversion
speeds of 1Hz and below.
Alarm Threshold Registers
Four registers store alarm threshold data, with high-
temperature (T
) and low-temperature (T
) reg-
LOW
HIGH
isters for each A/D channel. If either measured
temperature equals or exceeds the corresponding
alarm threshold value, an ALERT interrupt is asserted.
When the conversion rate is greater than 1Hz, tempera-
ture data can be read from the read internal tempera-
ture (00h) and read external temperature (01h)
registers. The temperature data format is 7 bits plus
sign in two’s-complement form for each channel, with
the LSB representing 11C (Table 2), transmitted MSB
first. When the conversion rate is less than 1Hz, the
extended data can be read from the read external
extended temperature register (10h) and the read inter-
nal extended temperature register (11h), and the first 3
bits of the register represent 1/2, 1/4, and 1/8 of a
degree. Measurements are offset by +1/21C to mini-
mize quantization errors; for example, +99.61C is
reported as +1001C.
The POR state of both T
registers is full scale
HIGH
(0111 1111, or +1271C). The POR state of both T
LOW
registers is 1100 1001 or -551C.
Diode Fault Alarm
There is a continuity fault detector at DXP that detects
whether the remote diode has an open-circuit condition
or if DXP is shorted to DXN, GND, or V . If an open
CC
circuit exists, then the temperature register will be
loaded with 1000 0000, and bit 3 of the status register
will be set to 1 at the end of a conversion. This means
that immediately after POR the status byte will indicate
no fault is present until the end of the first conversion.
When the conversion rate is 1Hz or less, the first 8 bits
of temperature data can be read from the read internal
temperature (00h) and read external temperature (01h)
registers, the same as for faster conversion rates. An
additional 3 bits can be read from the read external
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
