Datasheet

Data Sheet ADT7420
Rev. 0 | Page 17 of 24
SERIAL INTERFACE
ADT7420
SCL
CT
INT
A1
A0
SDA
GND
V
DD
10kΩ
10kΩ
10kΩ
PULL-UP
V
DD
PULL-UP
V
DD
0.1µF
10kΩ
PULL-UP
V
DD
TO INTERRUPT PIN
ON MICROCONTROLLER
V
DD
09013-014
Figure 13. Typical I
2
C Interface Connection
Control of the ADT7420 is carried out via the I
2
C-compatible
serial interface. The ADT7420 is connected to this bus as a slave
and is under the control of a master device.
Figure 13 shows a typical I
2
C interface connection.
SERIAL BUS ADDRESS
Like most I
2
C-compatible devices, the ADT7420 has a 7-bit
serial address. The five MSBs of this address for the ADT7420
are hardwired internally to 10010. Pin A1 and Pin A0 set the
two LSBs. These pins can be configured two ways, low and
high, to give four different address options. Table 20 shows
the different bus address options available. The recommended
pull-up resistor value on the SDA and SCL lines is 10 kΩ.
Table 20. I
2
C Bus Address Options
Binary
Hex
A6 A5 A4 A3 A2 A1 A0
1 0 0 1 0 0 0 0x48
1 0 0 1 0 0 1 0x49
1 0 0 1 0 1 0 0x4A
1 0 0 1 0 1 1 0x4B
The serial bus protocol operates as follows:
1. The master initiates data transfer by establishing a start
condition, defined as a high-to-low transition on the serial
data line, SDA, while the serial clock line, SCL, remains
high. This indicates that an address/data stream is going
to follow. All slave peripherals connected to the serial bus
respond to the start condition and shift in the next eight
bits, consisting of a 7-bit address (MSB first) plus a read/
write (R/
W
) bit. The R/
W
bit determines whether data is
written to, or read from, the slave device.
2. The peripheral with the address corresponding to the
transmitted address responds by pulling the data line low
during the low period before the ninth clock pulse, known
as the acknowledge bit. All other devices on the bus then
remain idle while the selected device waits for data to be
read from or written to it. If the R/
W
bit is a 0, the master
writes to the slave device. If the R/
W
bit is a 1, the master
reads from the slave device.
3. Data is sent over the serial bus in sequences of nine clock
pulses, eight bits of data followed by an acknowledge bit
from the receiver of data. Transitions on the data line must
occur during the low period of the clock signal and remain
stable during the high period as a low-to-high transition when
the clock is high, which can be interpreted as a stop signal.
4. When all data bytes have been read or written, stop condi-
tions are established. In write mode, the master pulls the
data line high during the 10
th
clock pulse to assert a stop
condition. In read mode, the master device pulls the data
line high during the low period before the ninth clock
pulse. This is known as a no acknowledge. The master
takes the data line low during the low period before the
10
th
clock pulse, then high during the 10
th
clock pulse to
assert a stop condition.
It is not possible to mix read and write in one operation because
the type of operation is determined at the beginning and cannot
subsequently be changed without starting a new operation.