User guide

GPIO Function Register (15h)
The GPIO function register (15h) sets the GPIO_ states.
Write a zero to set a GPIO as an output. Write a one to
set a GPIO as an input.
GPIO Value Register (16h)
The GPIO value register (16h) contains the state of
each GPIO input when a GPIO is configured as an
input. When configured as an output, write a one or
zero to set the value of the GPIO output.
Applications Information
Remote-Diode Considerations
Temperature accuracy depends upon having a good-
quality, diode-connected, small-signal transistor.
Accuracy has been experimentally verified for all the
devices listed in Table 7. The MAX6678 can also direct-
ly measure the die temperature of CPUs and other ICs
with on-board temperature-sensing diodes.
The transistor must be a small-signal type with a rela-
tively high forward voltage. This ensures that the input
voltage is within the A/D input voltage range. The for-
ward voltage must be greater than 0.25V at 10µA at the
highest expected temperature. The forward voltage
must be less than 0.95V at 100µA at the lowest expect-
ed temperature. The base resistance has to be less
than 100Ω. Tight specification of forward-current gain
(+50 to +150, for example) indicates that the manufac-
turer has good process control and that the devices
have consistent characteristics.
Effect of Ideality Factor
The accuracy of the remote-temperature measurements
depends on the ideality factor (n) of the remote “diode”
(actually a transistor). The MAX6678 is optimized for n =
1.008, which is the typical value for the Intel Pentium® III
and the AMD Athlon™ MP model 6. If a sense transistor
with a different ideality factor is used, the output data is
different. Fortunately, the difference is predictable.
Assume a remote-diode sensor designed for a nominal
ideality factor n
NOMINAL
is used to measure the tem-
perature of a diode with a different ideality factor, n
1
.
The measured temperature T
M
can be corrected using:
where temperature is measured in Kelvin.
As mentioned above, the nominal ideality factor of the
MAX6678 is 1.008.
As an example, assume the MAX6678 is configured
with a CPU that has an ideality factor of 1.002. If the
diode has no series resistance, the measured data is
related to the real temperature as follows:
For a real temperature of +85°C (358.15K), the mea-
sured temperature is +82.87°C (356.02K), which is an
error of -2.13°C.
Effect of Series Resistance
Series resistance in a sense diode contributes addition-
al errors. For nominal diode currents of 10µA and
100µA, change in the measured voltage is:
Since 1°C corresponds to 198.6µV, series resistance
contributes a temperature offset of:
Assume that the diode being measured has a series
resistance of 3Ω. The series resistance contributes an
offset of:
The effects of the ideality factor and series resistance
are additive. If the diode has an ideality factor of 1.002
and series resistance of 3Ω, the total offset can be cal-
culated by adding error due to series resistance with
error due to ideality factor:
1.36°C - 2.13°C = -0.77°C
for a diode temperature of +85°C.
In this example, the effect of the series resistance and
the ideality factor partially cancel each other.
For best accuracy, the discrete transistor should be a
small-signal device with its collector connected to GND
and base connected to DXN. Table 7 lists examples of
discrete transistors that are appropriate for use with the
MAX6678.
30453 1 36Ω
Ω
×
°
=°..
C
C
90
198 6
0 453
µ
µ
°
=
°
V
V
C
C
Ω
Ω
.
.
∆VR A A AR
MS S
=µµ=µ×−()100 10 90
TT
n
n
TT
ACTUAL M
NOMINAL
MM
=






=






=
.
.
(. )
1
1 008
1 002
1 00599
TT
n
n
M ACTUAL
NOMINAL
=






1
MAX6678
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
______________________________________________________________________________________ 15
Pentium is a registered trademark of Intel Corp.
Athlon is a trademark of AMD.