Additional Adc Functions For Temperature Measurement - Analog Devices ADT7473 Manual

ADT7473
Temperature Measurement Limit Registers
Associated with each temperature measurement channel are
high and low limit registers. Exceeding the programmed high or
low limit causes the appropriate status bit to be set. Exceeding
either limit can also generate SMBALERT interrupts.
Reg. 0x4E Remote 1 Temperature Low Limit = 0x01 default
Reg. 0x4F Remote 1 Temperature High Limit = 0x7F default
Reg. 0x50 Local Temperature Low Limit = 0x01 default
Reg. 0x51 Local Temperature High Limit = 0x7F default
Reg. 0x52 Remote 2 Temperature Low Limit = 0x01 default
Reg. 0x53 Remote 2 Temperature High Limit = 0x7F default
Reading Temperature from the ADT7473
It is important to note that the temperature can be read from
the ADT7473 as an 8-bit value (with 1°C resolution) or as a
10-bit value (with 0.25°C resolution). If only 1°C resolution is
required, the temperature readings can be read back at any time
and in no particular order.
If the 10-bit measurement is required, a 2-register read for each
measurement is used. The extended resolution register
(Reg. 0x77) should be read first. This causes all temperature
reading registers to be frozen until all temperature reading
registers have been read from. This prevents an MSB reading
from being updated while its two LSBs are being read, and
vice versa.
ADDITIONAL ADC FUNCTIONS FOR
TEMPERATURE MEASUREMENT
A number of other functions are available on the ADT7473 to
offer the system designer increased flexibility.
Turn-Off Averaging
For each temperature measurement read from a value register,
16 readings have actually been made internally and the results
averaged before being placed into the value register. Sometimes
it is necessary to take a very fast measurement. Setting Bit 4 of
Configuration Register 2 (Reg. 0x73) turns averaging off.
Table 8. Conversion Time with Averaging Disabled
Channel
Voltage Channel
Remote Temperature 1
Remote Temperature 2
Local Temperature
Measurement Time
0.7 ms
7 ms
7 ms
1.3 ms
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Table 9. Conversion Time with Averaging Enabled
Channel
Voltage Channels
Remote Temperature
Local Temperature
Single-Channel ADC Conversions
Setting Bit 6 of Configuration Register 2 (Reg. 0x73) places the
ADT7473 into single-channel ADC conversion mode. In this
mode, the ADT7473 can be made to read a single temperature
channel only. The appropriate ADC channel is selected by
writing to Bits <7:5> of the TACH1 minimum high byte register
(0x55).
Table 10. Programming Single-Channel ADC Mode for
Temperatures
Bits <7:5> Reg. 0x55 Channel Selected
101 Remote 1 Temperature
110 Local Temperature
111 Remote 2 Temperature
Configuration Register 2 (Reg. 0x73)
<4> = 1, averaging off.
<6> = 1, single-channel convert mode.
TACH1 Minimum High Byte (Reg. 0x55)
<7:5> selects ADC channel for single-channel convert mode.
Overtemperature Events
Overtemperature events on any of the temperature channels can
be detected and dealt with automatically in automatic fan speed
control mode. Reg. 0x6A to Reg. 0x6C are the THERM limits.
When a temperature exceeds its THERM limit, all PWM
outputs run at 100% duty cycle or the maximum PWM duty
cycle (Reg. 0x38, Reg. 0x39, and Reg. 0x3A) if bit 3 of
Configuration Register 4, Reg. 0x7D is set. The fans remain
running at this speed until the temperature drops below
THERM minus hysteresis; this can be disabled by setting the
boost bit in Configuration Register 3, Bit 2, Reg. 0x78. The
hysteresis value for that THERM limit is the value programmed
into the hysteresis registers (Reg. 0x6D and Reg. 0x6E). The
default hysteresis value is 4°C.
THERM LIMIT
TEMPERATURE
100%
FANS
Figure 25. THERM Limit Operation
Measurement Time
11 ms
39 ms
12 ms
HYSTERESIS (°C)