ADT7461A
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10
Conversion Rate Register
The conversion rate register is Address 0x04 at read and
Address 0x0A at write. The lowest four bits of this register
are used to program the conversion rate by dividing the
internal oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256,
512, or 1024 to give conversion times from 15.5 ms (Code
0x0A) to 16 seconds (Code 0x00). For example, a
conversion rate of eight conversions per second means that
beginning at 125 ms intervals, the device performs a
conversion on the internal and the external temperature
channels.
The conversion rate register can be written to and read
back over the SMBus. The higher four bits of this register are
unused and must be set to 0. The default value of this register
is 0x08, giving a rate of 16 conversions per second. Use of
slower conversion times greatly reduces the device power
consumption.
Table 5. Conversion Rate Register Codes
Code Conversion/Second Time (Seconds)
0x00 0.0625 16
0x01 0.125 8
0x02 0.25 4
0x03 0.5 2
0x04 1 1
0x05 2 500 m
0x06 4 250 m
0x07 8 125 m
0x08 16 62.5 m
0x09 32 31.25 m
0x0A 64 15.5 m
0x0B to 0xFF Reserved
Limit Registers
The ADT7461A has eight limit registers: high, low, and
THERM
temperature limits for both local and remote
temperature measurements. The remote temperature high
and low limits span two registers each, to contain an upper
and lower byte for each limit. There is also a THERM
hysteresis register. All limit registers can be written to, and
read back over, the SMBus. See Table 9 for details of the
limit register addresses and their power−on default values.
When Pin 6 is configured as an ALERT
output, the high
limit registers perform a > comparison, while the low limit
registers perform a ≤ comparison. For example, if the high
limit register is programmed with 80°C, then measuring
81°C results in an out−of−limit condition, setting a flag in
the status register. If the low limit register is programmed
with 0°C, measuring 0°C or lower results in an out−of−limit
condition.
Exceeding either the local or remote THERM
limit asserts
THERM
low. When Pin 6 is configured as THERM2,
exceeding either the local or remote high limit asserts
THERM2
low. A default hysteresis value of 10°C is
provided that applies to both THERM
channels. This
hysteresis value can be reprogrammed to any value after
powerup (Register Address 0x21).
It is important to remember that the temperature limits
data format is the same as the temperature measurement data
format. Therefore, if the temperature measurement uses
default binary, then the temperature limits also use the
binary scale. If the temperature measurement scale is
switched, however, the temperature limits do not
automatically switch. The user must reprogram the limit
registers to the desired value in the correct data format. For
example, if the remote low limit is set at 10°C with the
default binary scale, the limit register value is 0000 1010b.
If the scale is switched to offset binary, the value in the low
temperature limit register needs to be reprogrammed to
0100 1010b.
Status Register
The status register is a read−only register at Address 0x02.
It contains status information for the ADT7461A.
When Bit 7 of the status register is high, it indicates that the
ADC is busy converting. The other bits in this register flag the
out−of−limit temperature measurements (Bit 6 to Bit 3, and
Bit 1 to Bit 0) and the remote sensor open circuit (Bit 2).
If Pin 6 is configured as an ALERT
output, the following
applies: If the local temperature measurement exceeds its
limits, Bit 6 (high limit) or Bit 5 (low limit) of the status
register asserts to flag this condition. If the remote
temperature measurement exceeds its limits, then Bit 4 (high
limit) or Bit 3 (low limit) asserts. Bit 2 asserts to flag an open
circuit condition on the remote sensor. These five flags are
NOR’ed together, so if any of them is high, the ALERT
interrupt latch is set and the ALERT output goes low.
Reading the status register clears the five flags, Bit 6 to
Bit 2, provided the error conditions causing the flags to be
set have gone away. A flag bit can be reset only if the
corresponding value register contains an in−limit
measurement or if the sensor is good.
The ALERT
interrupt latch is not reset by reading the
status register. It resets when the ALERT
output has been
serviced by the master reading the device address, provided
the error condition has gone away and the status register flag
bits are reset.
When Flag 1 and/or Flag 0 are set, the THERM
output
goes low to indicate that the temperature measurements are
outside the programmed limits. The THERM
output does
not need to be reset, unlike the ALERT
output. Once the
measurements are within the limits, the corresponding status
register bits are automatically reset and the THERM
output
goes high. The user may add hysteresis by programming
Register 0x21. The THERM
output is reset only when the
temperature falls to limit value minus the hysteresis value.
When Pin 6 is configured as THERM2
, only the high
temperature limits are relevant. If Flag 6 and/or Flag 4 are
set, the THERM2
output goes low to indicate that the
temperature measurements are outside the programmed
limits. Flag 5 and Flag 3 have no effect on THERM2
. The
behavior of THERM2
is otherwise the same as THERM.
