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ADT7518
The second method is applicable when the part is in round
robin measurement mode. The part measures both the internal
and external temperature sensors as it cycles through all pos-
sible measurement channels. The two temperature channels are
measured each time the part runs a round robin sequence. In
round robin mode, the part continuously measures all channels.
Rev. A | Page 23 of 40
Temperature measurement is also initiated after every read or
write to the part when the part is in either single-channel
measurement mode or round robin measurement mode.
Once serial communication has started, any conversion in
progress is stopped and the ADC is reset. Conversion starts
again immediately after the serial communication has finished.
The temperature measurement proceeds normally as described
in the preceding section.
V
DD
Monitoring
The ADT7518 also has the ability to monitor its own power
supply. The part measures the voltage on its V
DD
pin to a
resolution of 10 bits. The resulting value is stored in two 8-bit
registers; the two LSBs are stored in register address 03h and the
eight MSBs are stored in Register Address 06h. This allows the
option of doing just a 1-byte read if 10-bit resolution is not
important. The measured result is compared with the V
HIGH
and
V
LOW
limits. If the V
DD
interrupt is not masked, any out-of-limit
comparison generates a flag in the Interrupt Status 2 register
and one or more out-of-limit results will cause the INT/INT
output to pull either high or low, depending on the output
polarity setting.
Measuring the voltage on the V
DD
pin is regarded as monitoring
a channel along with the internal, external, and AIN channels.
The user can select the V
DD
channel for single-channel
measurement by setting Bit C4 = 1 and setting Bits C0:C2 to all
0s in the Control Configuration 2 register.
When measuring the V
DD
value, the reference for the ADC is
sourced from the internal reference. Table 8 shows the data
format. As the maximum V
DD
voltage measurable is 7 V, internal
scaling is performed on the V
DD
voltage to match the 2.25 V
internal reference value. Below is an example of how the
transfer function works.
V
DD
= 5 V
ADC Reference
= 2.25 V
1
LSB
=
ADC Reference
/2
10
= 2.25/1024
= 2.226 mV
Scale Factor
=
Full-Scale V
CC
/
ADC Reference
= 7/2.25
= 3.07
Conversion Result
=
V
DD
/(
Scale Factor
×
LSB
size
)
= 5/(3.07 × 2.226 mV)
= 2 DCh
Table 8. V
DD
Data Format (V
REF
= 2.25 V)
V
DD
Value (V)
2.7
3
3.5
4
4.5
5
5.5
6
6.5
7
On-Chip Reference
The ADT7518 has an on-chip 1.2 V band gap reference, which
is gained up by a switched capacitor amplifier to give an output
of 2.25 V. The amplifier is powered up for the duration of the
device monitoring phase and is powered down once monitoring
is disabled. This saves on current consumption. The internal
reference is used as the reference for the ADC. The ADC is used
for measuring V
DD
, internal temperature sensor, external temp-
erature sensor, and AIN inputs. The internal reference is always
used when measuring V
DD
, and the internal and external temp-
erature sensors. The external reference is the default power-up
reference for the DACs.
Round Robin Measurement
On power-up, the ADT7518 goes into round robin mode but
monitoring is disabled. Setting Bit C0 of the Configuration
Register 1 to 1 enables conversions. It sequences through all the
available channels, taking a measurement from each in the
following order: V
DD
, internal temperature sensor, external
temperature sensor/(AIN1 and AIN2), AIN3, and AIN4. Pin 7
and Pin 8 can be configured to be either external temperature
sensor pins or standalone analog input pins. Once conversion is
completed on the AIN4 channel, the device loops around for
another measurement cycle. This method of taking a measure-
ment on all the channels in one cycle is called round robin.
Setting Bit C4 of Control Configuration 2 (Address 19h)
disables the round robin mode and in turn sets up the single-
channel mode. The single-channel mode is where only one
channel, e.g., the internal temperature sensor, is measured in
each conversion cycle.
Digital Output
Binary
01 1000 1011
01 1011 0111
10 0000 0000
10 0100 1001
10 1001 0010
10 1101 1100
11 0010 0101
11 0110 1110
11 1011 0111
11 1111 1111
Hex
18B
1B7
200
249
292
2DC
325
36E
3B7
3FF
interest, since the most recently measured value can be read at
any time. For applications where the round robin time is impor-
tant, typical times at 25°C are given in the specifications.
Single-Channel Measurement
Setting C4 of the Control Configuration 2 register enables the
single-channel mode and allows the ADT7518 to focus on one
channel only. A channel is selected by writing to Bits C0:C2 in
the Control Configuration 2 register. For example, to select the
V
DD
channel for monitoring, write to the Control Configuration