Rainbow Electronics MAX1669 - Manual (Page 9)

MAX1669
Fan Controller and Remote Temperature Sensor
with SMBus Serial Interface
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mode, the ALERT and OVERT outputs respond correct-
ly according to the last valid A/D result.
Note that the ALERT output does not respond to T CRIT
(OVERT) comparisons.
The OVERT latch can implement an override control to
the FAN output, which forces the fan to V
CC whenever
the T
CRIT
threshold is crossed. This override switch is
the backup fan control loop, and is enabled through the
FAN ON bit in the configuration register (bit 2). Note
that changing the duty to 100% in this way doesn’t
affect the contents of the DUTY register, and the FAN
output reverts to the preprogrammed duty factor (or
DAC voltage) when the OVERT latch is reset.
Diode Fault Alarm
A continuity fault detector at DXP detects whether the
remote diode has an open-circuit condition, short-cir-
cuit to GND, or short-circuit DXP-to-DXN condition. At
the beginning of each conversion, the diode fault is
checked and the status byte updated. This fault detec-
tor is a simple voltage detector; DXP rising above V
CC -
1V or falling below DXN + 40mV constitutes a fault con-
dition. Also, if the ADC has an extremely low differential
input voltage, the diode is assumed to be shorted and
a fault occurs. Note that the diode fault isn’t checked
until a conversion is initiated, so immediately after
power-on reset the status byte indicates no fault is pre-
sent even if the diode path is broken. Any diode fault
will return a +127°C fault reading and cause ALERT to
go low.
Write Byte Format
Read Byte Format
7-bit slave address:
equivalent to chip-select line 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
sampling rate)
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
S = Start condition Shaded = Slave transmission
P = Stop condition A = Not acknowledged
Figure 3. SMBus Protocols
S
1
WR
1
ACK
1
COMMAND
8 bits
ACK
1
S
1
ADDRESS
7 bits
RD
1
ACK
1
DATA
8 bits
A
1
P
1
1
S ADDRESS
7 bits 1
WR ACK
1 8 bits
COMMAND ACK
1 8 bits
DATA ACK
1 1
P
TEMP (°C)
ROUNDED
TEMP (°C)
+0.25 +0
+0.50 +1
+25.25 +25
+126.00 +126
+0.00 +0
-0.25 +0
-0.50 +0
-0.75 -1
+127.00
+130.00 +127
+127
+126.50 +127
-1.00 -1
-25.00 -25
-25.50 -26
-54.75 -55
-55.00 -55
-65.00 -65
-70.00 -65
DIGITAL OUTPUT
DATA BITS
SIGN MSBs LSBs
1 111 1111
0 000 0000
1 110 0111
0 000 0001
0 001 1001
0 111 1110
0 000 0000
0 000 0000
1 110 0110
1 100 1001
0 000 0000
1 111 1111
1 100 1001
1 011 1111
1 011 1111
0 111 1111
0 111 1111
0 111 1111
Table 1. Data Format (Two’s Complement)
ADDRESS
7 bits