AD7150
Rev. 0 | Page 21 of 28
SERIAL INTERFACE
The AD7150 supports an I
2
C-compatible, 2-wire serial
interface. The two wires on the serial bus (interface) are called SCL
(clock) and SDA (data). These two wires carry all addressing,
control, and data information one bit at a time over the bus to
all connected peripheral devices. The SDA wire carries the data,
while the SCL wire synchronizes the sender and receiver during
the data transfer. The devices on the bus are classified as either
master or slave devices. A device that initiates a data transfer
message is called a master, while a device that responds to this
message is called a slave.
To control the AD7150 device on the bus, the following
protocol must be followed. First, the master initiates a data
transfer by establishing a start condition, defined by a high-to-
low transition on SDA while SCL remains high. This indicates
that the start byte follows. This 8-bit start byte is made up of a
7-bit address plus an R/W bit indicator.
All peripherals connected to the bus respond to the start
condition and shift in the next eight bits (7-bit address + R/W
bit). The bits arrive MSB first. The peripheral that recognizes
the transmitted address responds by pulling the data line low
during the ninth clock pulse. This is known as the acknowledge
bit. All other devices withdraw from the bus at this point and
maintain an idle condition. An exception to this is the general
call address, which is described in the
General Call section. In
the idle condition, the device monitors the SDA and SCL lines
waiting for the start condition and the correct address byte.
The R/W bit determines the direction of the data transfer. A
Logic 0 LSB in the start byte means that the master writes
information to the addressed peripheral. In this case, the
AD7150 becomes a slave receiver. A Logic 1 LSB in the start
byte means that the master reads information from the
addressed peripheral. In this case, the AD7150 becomes a slave
transmitter. In all instances, the AD7150 acts as a standard slave
device on the serial bus.
The start byte address for the AD7150 is 0x90 for a write and
0x91 for a read.
READ OPERATION
When a read is selected in the start byte, the register that is
currently addressed by the address pointer is transmitted to the
SDA line by the AD7150. This is then clocked out by the master
device, and the AD7150 awaits an acknowledge from the
master.
If an acknowledge is received from the master, the address auto-
incrementer automatically increments the address pointer
register and outputs the next addressed register content to the
SDA line for transmission to the master. If no acknowledge is
received, the AD7150 returns to the idle state and the address
pointer is not incremented. The address pointers’ auto-incrementer
allows block data to be written to or read from the starting address
and subsequent incremental addresses.
In continuous conversion mode, the address pointers’ auto-
incrementer should be used for reading a conversion result.
This means that the two data bytes should be read using one
multibyte read transaction rather than two separate single byte
transactions. The single byte data read transaction may result in
the data bytes from two different results being mixed. The same
applies for four data bytes if both capacitive channels are enabled.
The user can also access any unique register (address) on a one-
to-one basis without having to update all the registers. The
address pointer register contents cannot be read.
If an incorrect address pointer location is accessed or if the user
allows the auto-incrementer to exceed the required register
address, the following applies:
• In read mode, the AD7150 continues to output various
internal register contents until the master device issues a
no acknowledge, start, or stop condition. The address
pointers’ auto-incrementer contents are reset to point to
the status register at the 0x00 address when a stop
condition is received at the end of a read operation. This
allows the status register to be read (polled) continually
without having to constantly write to the address pointer.
• In write mode, the data for the invalid address is not
loaded into the AD7150 registers, but an acknowledge is
issued by the AD7150.
WRITE OPERATION
When a write is selected, the byte following the start byte is
always the register address pointer (subaddress) byte, which
points to one of the internal registers on the AD7150. The
address pointer byte is automatically loaded into the address
pointer register and acknowledged by the AD7150. After the
address pointer byte acknowledge, a stop condition, a repeated
start condition, or another data byte can follow from the master.
A stop condition is defined by a low-to-high transition on SDA
while SCL remains high. If a stop condition is encountered by
the AD7150, it returns to its idle condition and the address
pointer is reset to 0x00.
If a data byte is transmitted after the register address pointer
byte, the AD7150 loads this byte into the register that is
currently addressed by the address pointer register and sends an
acknowledge, and the address pointer auto-incrementer auto-
matically increments the address pointer register to the next
internal register address. Thus, subsequent transmitted data
bytes are loaded into sequentially incremented addresses.
