Datasheet

ADT7411
Rev. B | Page 13 of 36
THEORY OF OPERATION
After the power-up calibration routine, the ADT7411 goes into
idle mode. In this mode, the device is not performing any
measurements and is fully powered up.
To begin monitoring, write to the Control Configuration 1
r
egister (Address 18h) and set Bit C0 = 1. The ADT7411 goes
into its power-up default measurement mode, which is round
robin. The device performs measurements in the following
channel sequence:
1. V
DD
channel
2. I
nternal temperature sensor channel
3. E
xternal temperature sensor channel (or AIN1 and AIN2 if
external diode is not set up)
4. AIN3
5. AIN4
6. AIN5
7. AIN6
8. AIN7
9. AIN8
Once it finishes taking measurements on the AIN8 channel, the
de
vice immediately loops back to start taking measurements on
the V
DD
channel and repeats the same cycle as before. This loop
continues until the monitoring is stopped by resetting Bit C0 of
the Control Configuration 1 register to 0. It is also possible to
continue monitoring as well as switching to single-channel
mode by writing to the Control Configuration 2 register
(Address 19h) and setting Bit C4 = 1. Further explanations of
the single-channel and round robin measurement modes are
given in the
Single-Channel Measurement and Round Robin
M
easurement sections. All measurement channels have
a
veraging enabled on them at power-up. Averaging forces the
device to take an average of 16 readings before giving a final
measured result. To disable averaging and consequently
decrease the conversion time by a factor of 16, set C5 = 1 in
the Control Configuration 2 register.
There are eight single-ended analog input channels on the
ADT7411: AI
N1 to AIN8. AIN1 and AIN2 are multiplexed with
the external temperature sensors D+ and D− terminals. Bits C1
and C2 of the Control Configuration 1 register (Address 18h)
are used to select between AIN1/2 and the external temperature
sensor. The input range on the analog input channels is
dependent on whether the ADC reference used is the internal
V
REF
or V
DD
. To meet linearity specifications, it is recommended
that the maximum V
DD
value is 5 V. Bit C4 of the Control
Configuration 3 register is used to select between the internal
reference and V
DD
as the analog inputs’ ADC reference.
The dual serial interface defaults to the I
2
C protocol on power-
up. To select and lock in the SPI protocol, follow the selection
process as described in the Serial Interface Selection section.
The I
2
C protocol cannot be locked in, while the SPI protocol on
selection is automatically locked in. The interface can only be
switched back to I
2
C when the device is powered off and on.
When using I
2
C, the
CS
pin should be tied to either V
DD
or GND.
There are a number of different operating modes on the
ADT7411 de
vices and all of them can be controlled by the
configuration registers. These features consist of enabling and
disabling interrupts, polarity of the INT/
INT
pin, enabling and
disabling the averaging on the measurement channels, SMBus
timeout, and software reset.
POWER-UP CALIBRATION
It is recommended that no communication to the part is initiated
until approximately 5 ms after V
DD
has settled to within 10% of its
final value. It is generally accepted that most systems take a
maximum of 50 ms to power up. Power-up time is directly
related to the amount of decoupling on the voltage supply line.
During the 5 ms after V
DD
has settled, the part is performing a
calibration routine; any communication to the device interrupts
this routine and can cause erroneous temperature measurements.
If it is not possible to have V
DD
at its nominal value by the time
50 ms elapses or that communication to the device starts prior
to V
DD
settling, then it is recommended that a measurement be
taken on the V
DD
channel before a temperature measurement
is taken. The V
DD
measurement is used to calibrate out any
temperature measurement error due to different supply
voltage values.
CONVERSION SPEED
The internal oscillator circuit used by the ADC has the
capability to output two different clock frequencies. This means
that the ADC is capable of running at two different speeds when
doing a conversion on a measurement channel. Therefore, the
time taken to perform a conversion on a channel can be reduced
by setting C0 of the Control Configuration 3 register
(Address 1Ah). This increases the ADC clock speed from
1.4 Hz to 22 kHz. At the higher clock speed, the analog filters
on the D+ and D− input pins (external temperature sensor) are
switched off. This is why the power-up default setting is to have
the ADC working at the slow speed. The typical times for fast
and slow ADC speeds are given in
Table 1.
The ADT7411 powers up with averaging on. This means every
cha
nnel is measured 16 times and internally averaged to reduce
noise. The conversion time can also be reduced by turning the
averaging off. This is done by setting Bit C5 of the Control
Configuration 2 register (Address 19h) to a 1.