Analog Devices dBCool ADT7473 Manual page 18

ADT7473
To factor this i n, the user can w
offset register. Then, the ADT7473 automatically a
or su btracts it from the tempe
•
Some CPU m anufacturers spe
levels of the subst rate transisto
the ADT7473, I , is 96 µA a
HIGH
I , is 6 µA. If the ADT7473 cur
LOW
the current levels specified by the
might be necessary to remove an offset. The CPU's dat
sheet advises whether this offset needs to be removed and
how to calculate it. This offset can be programmed to the
offset register. It is important to note that, if more than
offset must be considered, the algebraic sum of these
offs ets must be programmed to the offset register.
If a discrete transistor is used with the ADT7473, the best
accuracy is obta ined by choosing devices according to the
following criteria:
•
Base-emitter voltage gr eater than 0.25 V at 6 µA
highest operating temp erature
•
Base-emitter voltage les
lowest operating temperature
•
Base resistance less than 100 Ω
•
Small variation in h (such as 50 to
FE
tight control of V characteristics
BE
Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23
packages, are suitable devices to use.
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, some temperature errors can still be attributable to
noise coupled onto the D+/D– lines. Constant high frequency
noise usually attenuates or increases temperature measurements
by a linear, constant value.
The ADT7473 has temperature offset registers at Register 0x70
and Register 0x72 for the Remote 1 and Remote 2 temperature
channels. By performing a one-time calibration of the system,
the user 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 LSBs add +0.5°C offset to the
temperature reading so the 8-bit register effectively allows
temperature offsets of up to ±64°C with a resolution of +0.5°C.
This ensures that the readings in the temperature measurement
registers are as accurate as possible.
Temperature Offset Registers
Register 0x70, Remote 1 Temperature Offset = 0x00 (0°C default)
Register 0x71, Local Temperature Offset = 0x00 (0°C default)
Register 0x72, Remote 2 Temperature Offset = 0x00 (0°C default)
rite the ∆T value to the
dds it to
rature measurement.
cify the hig h and low current
rs. The hi gh current level of
nd the low level current,
rent levels do not match
CPU manufacturer, it
, at the
s than 0.95 V at 100 µA, at the
150) that indicates
ADT7460/ADT7473 Backw
By setting Bit 1 of Configuration Register 5 (0x7C), all tempera-
ture measurements are stored in the zone temperature value
registers (Register 0x25, Register 0x26, and Register 0x27) in
twos complement, in the range −63°C to +127°C. (The
ADT7473 still makes calculations based on the Offset 64
extended range and clamps the results, if necessary.) The
temperature limits must be reprogrammed in twos comple-
a
ment. If a twos complement temperature below −63°C is
entered, the temperature is clamped to −63°C. In this mode, th
diode fault condition remains −128°C = 1000 0000, while in th
one
extended temperature range (−64°C to +191°C), the fault
condition is represented by −64°C = 0000 0000.
Temperature Measurement Reg
Register 0x25, Remote 1 Temperature
Register 0x26, Lo
Register 0x27, Remote 2 Temperature
Register 0x77, Extended Resolution 2 = 0x00 default
Bits [7:6] TDM2, Remote 2 temperature LSBs
Bits [5:4] LTMP, local temperature LSBs
Bits [3:2] TDM1, Remote 1 temperature LSBs
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
Register 0x4E, Remote 1 Temperature Low Limit = 0x01 default
Register 0x4F, Remote 1 Temperature High Limit = 0x7F default
Register 0x50, Local Temperature Low Limit = 0x01 default
Register 0x51, Local Temperature High Limit = 0x7F default
Register 0x52, Remote 2 Temperature Low Limit = 0x01 default
Register 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
(Register 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.
Rev. A | Page 18 of 76
ards-Compatible Mode
isters
cal Temperature
interrupts.
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