ADT7476A
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20
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:
?SPAN class="pst ADT7476AARQZ-R_2298076_4"> Four Dedicated Supply Voltage Inputs
?SPAN class="pst ADT7476AARQZ-R_2298076_4"> Supply Voltage (V
CC
Pin)
?SPAN class="pst ADT7476AARQZ-R_2298076_4"> Local Temperature
?SPAN class="pst ADT7476AARQZ-R_2298076_4"> 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)
Fan TACH measurements are made in parallel and are not
synchronized with the analog measurements in any way.
Status Registers
The results of limit comparisons are stored in Interrupt
Status Register 1 and Interrupt Status Register 2. The status
register bit for each channel reflects the status of the last
measurement and limit comparison on that channel. If a
measurement is within limits, the corresponding status
register bit is cleared to 0. If the measurement is out-of-limits,
the corresponding status register bit is set to 1.
The state of the various measurement channels can be
polled by reading the status registers over the serial bus. In
Bit 7 (OOL) of Interrupt Status Register 1 (0x41), 1 means
an out-of-limit event has been flagged in Interrupt Status
Register 2. This means the user also needs to read Interrupt
Status Register 2. Alternatively, Pin 10 or Pin 14 can be
configured as an SMBALERT
output. This hard interrupt
automatically notifies the system supervisor of an
out-of-limit condition. Reading the status registers clears the
appropriate status bit as long as the error condition that
caused the interrupt has cleared. Status register bits are
sticky. Whenever a status bit is set, indicating an out-of-limit
condition, it remains set even if the event that caused it has
gone away (until read).
The only way to clear the status bit is to read the status
register after the event has gone away. Interrupt mask
registers (0x74 and 0x75) allow individual interrupt sources
to be masked from causing an SMBALERT
. However, if one
of these masked interrupt sources goes out of limit, its
associated status bit is set in the status registers.
Table 28. INTERRUPT STATUS REGISTER 1 (0x41)
Bit
Mnemonic
Description
[7]
OOL
1 denotes a bit in Interrupt Status
Register 2 is set and Interrupt Status
Register 2 should be read.
[6]
R2T
1 indicates that the Remote 2
Temperature High or Low limit has been
exceeded.
[5]
LT
1 indicates that the Local Temperature
High or Low Limit has been exceeded.
[4]
R1T
1 indicates that the Remote 1
Temperature High or Low Limit has
been exceeded.
[3]
5.0 V
1 indicates that the 5.0 V High or Low
Limit has been exceeded.
[2]
V
CC
1 indicates that the V
CC
High or Low
Limit has been exceeded.
[1]
V
CCP
1 indicates that the V
CCP
High or Low
Limit has been exceeded.
[0]
2.5 V
1 indicates that the 2.5 V High or Low
Limit has been exceeded.
If the 2.5 V input is configured as
THERM
, this bit represents the status of
THERM
.
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