Rainbow Electronics MAX6690 - Manual (Page 11)

MAX6690
2°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
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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 1°C (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 +1/2°C to mini-
mize quantization errors; for example, +99.6°C is
reported as +100°C.
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
extended temperature and read internal extended tem-
perature registers, which extend the resolution to
0.125°C per LSB (Table 3).
If a conversion ends after reading the main register but
before reading the extended register, the extended
register contains the 3LSBs from the new conversion
while the main register contains the 8MSBs from the
previous conversion. The extended data in this case is
meaningless. To avoid this problem, read extended
resolution temperature data using one of the following
approaches:
1) Put the MAX6690 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.
2) If the MAX6690 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.
Note: Extended resolution applies only for conver-
sion speeds of 1Hz and below.
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 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 condition
or if DXP is shorted to DXN, GND, or V
CC . If an open
circuit exists, then the temperature register is loaded
with 1000 0000, and bit 3 of the status register is set to
1 at the end of a conversion. This means that immedi-
ately after POR, the status byte indicates no fault is pre-
sent until the end of the first conversion.
TEMP ( °C) ROUNDED TEMP ( °C)
DIGITAL
OUTPUT
130.00 +127 0 111 1111
127.00 +127 0 111 1111
126.00 +127 0 111 1111
25.25 +25 0 001 1001
0.50 +1 0 000 0001
0.00 0 0 000 0000
<0.00 (Note 1) (Normal mode) 1 000 0000
-1 (Extended temp mode) 1 111 1111
<-64 (Extended temp mode) 1 000 0000
Diode fault
(short or open)
1 000 0000
Table 2. Data Format
FRACTIONAL
TEMPERATURE
DIGITAL OUTPUT
0.000 000X XXXX
0.125 001X XXXX
0.250 010X XXXX
0.375 011X XXXX
0.500 100X XXXX
0.625 101X XXXX
0.750 110X XXXX
0.875 111X XXXX
Table 3. Extended Resolution Register