AD7156
Rev. 0 | Page 22 of 28
SERIAL INTERFACE
The AD7156 supports an I
2
C-compatible, 2-wire serial inter-
face. 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, whereas a device that responds to
this message is called a slave.
To control the AD7156 device on the bus, the following
protocol must be utilized. 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
AD7156 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 AD7156 becomes
a slave transmitter. In all instances, the AD7156 acts as a
standard slave device on the serial bus.
The start byte address for the AD7156 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 AD7156. This is then clocked out by the master
device, and the AD7156 awaits an acknowledge from the master.
If an acknowledge is received from the master, the address
autoincrementer 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 AD7156 returns to the idle state and the address
pointer is not incremented. The address pointers’ autoincrementer
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 autoincrementer to exceed the required register
address, the following applies:
• In read mode, the AD7156 continues to output various
internal register contents until the master device issues
a no acknowledge, start, or stop condition. The address
pointers’ autoincrementer 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 AD7156 registers, but an acknowledge
is issued by the AD7156.
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 AD7156. The
address pointer byte is automatically loaded into the address
pointer register and acknowledged by the AD7156. 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 AD7156, 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 AD7156 loads this byte into the register that is cur-
rently addressed by the address pointer register and sends
an acknowledge, and the address pointer autoincrementer
automatically increments the address pointer register to the
next internal register address. Thus, subsequent transmitted
data bytes are loaded into sequentially incremented addresses.
