ADT7476A
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21
Table 27. FAN LIMIT REGISTERS
Register
Description
Default
0x54
TACH1 Minimum Low Byte
0xFF
0x55
TACH1 Minimum High Byte
0xFF
0x56
TACH2 Minimum Low Byte
0xFF
0x57
TACH2 Minimum High Byte
0xFF
0x58
TACH3 Minimum Low Byte
0xFF
0x59
TACH3 Minimum High Byte
0xFF
0x5A
TACH4 Minimum Low Byte
0xFF
0x5B
TACH4 Minimum High Byte
0xFF
Out-of-Limit Comparisons
Once all limits have been programmed, the ADT7476A
can be enabled for monitoring. The ADT7476A measures
all voltage and temperature measurements in round robin
format and sets the appropriate status bit for out-of-limit
conditions. TACH measurements are not part of this round
robin cycle. Comparisons are done differently depending on
whether the measured value is being compared to a high or
low limit.
High Limit: > Comparison Performed
Low Limit:
≤
Comparison Performed
Voltage and temperature channels use a window
comparator for error detecting and, therefore, have high and
low limits. Fan speed measurements use only a low limit.
This fan limit is needed only in manual fan control mode.
Analog Monitoring Cycle Time
The analog monitoring cycle begins when a 1 is written to
the start bit (Bit 0) of Configuration Register 1 (0x40). The
ADC measures each analog input in turn, and, as each
measurement is completed, the result is automatically stored
in the appropriate value register. This round robin
monitoring cycle continues unless disabled by writing a 0 to
Bit 0 of Configuration Register 1.
As the ADC is normally left to free-run in this manner, the
time taken to monitor all the analog inputs is normally not
of interest, because the most recently measured value of any
input can be read out at any time.
For applications where the monitoring cycle time is
important, it can easily be calculated.
The total number of channels measured is:
•
Four Dedicated Supply Voltage Inputs
•
Supply Voltage (V
CC
Pin)
•
Local Temperature
•
Two Remote Temperatures
As mentioned previously, the ADC performs round robin
conversions and takes 11 ms for each voltage measurement,
12 ms for a local temperature reading, and 39 ms for each
remote temperature reading. The total monitoring cycle time
for averaged voltage and temperature monitoring is,
therefore, nominally:
(5
11)
)
12
)
(2
39)
+
145 ms
(eq. 3)
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