ADT7461
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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 8 conversions per second means that
beginning at 125 ms intervals; the device performs a
conversion on the internal and external temperature
channels.
This 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, as shown in Table 9.
Table 9. CONVERSION RATE REGISTER CODES
Code
Conversion/
Sec
Average Supply Current
mA Typ at VDD = 5.5 V
0x00 0.0625 121.33
0x01 0.125 128.54
0x02 0.25 131.59
0x03 0.5 146.15
0x04 1 169.14
0x05 2 233.12
0x06 4 347.42
0x07 8 638.07
0x08 16 252.44
0x09 32 417.58
0x0A 64 816.87
0x0B to 0xFF Reserved
Limit Registers
The ADT7461 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 13 for address details
of the limit registers 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 may 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. So, if the temperature measurement uses default
binary, the temperature limits also use the binary scale. If the
temperature measurement scale is switched, however, the
temperature limits do not switch automatically. 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 and the default binary scale is being used, the
limit register value should be 0000 1010b. If the scale is
switched to offset binary, the value in the low temperature
limit register should be reprogrammed to be 0100 1010b.
Status Register
The status register is a read-only register at Address 0x02.
It contains status information for the ADT7461.
Bit 7 of the status register indicates the ADC is busy
converting when it is high. The other bits in this register flag
the out-of-limit temperature measurements (Bits 6 to 3 and
Bits 1 to 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’d 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, Bits 6 to 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 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 reset automatically 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 hysteresis value.
