Additional Adc Functions For Temperature Measurement - Analog Devices ADT7460 Manual

Nulling Out Temperature Errors
As CPUs run faster, it becomes more difficult to avoid high
frequency clocks when routing the D+, D− traces around a
system board. Even when recommended layout guidelines are
followed, there may still be temperature errors attributed to
noise being coupled onto the D+/D− lines. High frequency
noise generally has the effect of giving temperature measure-
ments that are too high by a constant amount. The ADT7460
has temperature offset registers at Addresses 0x70, 0x72 for the
Remote 1 and Remote 2 temperature channels. By doing a one-
time calibration of the system, one can determine the offset
caused by system board noise and null it out using the offset
registers. The offset registers automatically add a twos
complement 8-bit reading to every temperature measurement.
The LSB adds 0.25°C offset to the temperature reading so the
8-bit register effectively allows temperature offsets of up to
±32°C with a resolution of 0.25°C. This ensures that the
readings in the temperature measurement registers are as
accurate as possible.
Table 15. Temperature Offset Registers
Register Description
0x70 Remote 1 temperature offset
0x71 Local temperature offset
0x72 Remote 2 temperature offset
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.
Table 16. Temperature Measurement Limit Registers
Register Description
0x4E Remote 1 temperature low limit
0x4F Remote 1 temperature high limit
0x50 Local temperature low limit
0x51 Local temperature high limit
0x52 Remote 2 temperature low limit
0x53 Remote 2 temperature high limit
Overtemperature Events
Overtemperature events on any of the temperature channels can
be detected and dealt with automatically in automatic fan speed
control mode. Registers 0x6A to 0x6C are the THERM limits.
When a temperature exceeds its THERM limit, all fans run at
100% duty cycle. The fans continue running at 100% until the
temperature drops below THERM – Hysteresis. (This can be
disabled by setting the BOOST bit in Configuration Register 3,
Bit 2, Register 0x78). The hysteresis value for that THERM limit
is the value programmed into Registers 0x6D and 0x6E
(hysteresis registers). The default hysteresis value is 4°C.
THERM LIMIT
FANS
ADDITIONAL ADC FUNCTIONS FOR
TEMPERATURE MEASUREMENT
A number of other functions are available on the ADT7460 to
offer the systems 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 may be necessary to take a very fast measurement, for
Default
example, of CPU temperature. Setting Bit 4 of Configurat
0x00 (0°C)
Register 2 (Re
0x00 (0°C)
every 15.5 ms. Each remote temperature measurem
0x00 (0°C)
4 ms and the local temperature measurement take
Singl
Settin
A DT7460 into single-channel ADC conversion mode. In this
m ode, the ADT7460 can be made to read a single temperature
channel only. The appropriate ADC channel is selected b
to B its <
(Reg. 0 55).
Default
Table
0x81
Bit
0x7F
<4>
0x81
<6>
0x7F
0x81
Table 18. TACH1 Minimum High Byte (Reg. 0x55)
0x7F
B it
<7:5>
Rev. C | Page 19 of 52
TEMPERATURE
100%
Figure 28. THERM Limit Operation
g. 0x73) turns averaging off. This takes a reading
e-Ch annel ADC Con versions
g Bit 6 of Configuration Register 2 (R
7:5> of the TA CH1 minimum high byte register
x
17 . Conf iguration Register 2 (
Description
1: Averaging off
1: single-channel convert mode
Description
Selects ADC channel for single-channel convert mode
Value
101
110
111
ADT7460
HYSTERESIS = (°C)
ion
ent takes
s 1.4 ms.
eg. 0x73) places the
y writing
Reg. 0x73)
Channel Selected
Remote 1 temp
Local temp
Remote 2 temp