ADG2108 Data Sheet
Rev. B | Page 22 of 28
WRITE OPERATION
When writing to the ADG2108, the user must begin with an
address byte and R/
W
bit, after which the switch acknowledges
that it is prepared to receive data by pulling SDA low. This
address byte is followed by the two 8-bit words. The write
operations for the switch array are shown in Figure 34. Note
that it is only the condition of the switch corresponding to the
bits in the data bytes that changes state. All other switches retain
their previous condition.
READ OPERATION
Readback on the ADG2108 is designed to work as a tool for
debug and can be used to output the status of any of the
80 switches of the device. The readback function is a two-step
sequence that works as follows:
1. Select the relevant X line to be read back from. Note that
there are eight switches connecting that X line to the eight Y
lines. The next step involves writing to the ADG2108 to tell
the part to reveal the status of those eight switches.
a. Enter the I
2
C address of the ADG2108, and set the
R/
W
to 0 to indicate a write to the device.
b. Enter the readback address for the X line of interest,
the addresses of which are shown in Tabl e 8. Note that
the ADG2108 is expecting a 2-byte write; therefore, be
sure to enter another byte of don’t cares (see Figure 35).
c. The ADG2108 then places the status of those eight
switches in a register than can be read back.
2. The second step involves reading back from the register
that holds the status of the eight switches associated with
the X line of choice.
a. As before, enter the I
2
C address of the ADG2108. This
time, set the R/
W
to 1 to indicate a readback from the
device.
b. As with a write to the device, the ADG2108 outputs
a 2-byte sequence during readback. Therefore, the
first eight bits of data out that are read back are all 0s.
The next eight bits of data that come back are the status
of the eight Y lines attached to that particular X line.
If the bit is a 1, the switch is closed (on); similarly, if the
bit is a 0, the switch is open (off).
The entire read sequence is shown in Figure 35.
DATA
AX3
AX2 AX1
AX0 AY2 AY1
AY0
A0
R/W
A1A2
x x x
x x
x x
SCL
SDA
DATA BYTEDATA BYTE
ACK
BY
SWITCH
STOP
COND
BY
MASTER
START
COND
BY
MASTER
ADDRESS BYTE
ACK
BY
SWITCH
ACK
BY
SWITCH
LDSW
05898-005
Figure 34. Write Operation
Table 8. Readback Addresses for Each X Line
X Line RB7 RB6 RB5 RB4 RB3 RB2 RB1 RB0
X0 0 1 1 1 0 1 0 0
X2 0 0 1 1 0 1 0 1
X3 0 0 1 1 1 1 0 1
X4 0 1 1 1 0 1 0 1
X5 0 1 1 1 1 1 0 1
X6 0 0 1 1 0 1 1 0
X7 0 0 1 1 1 1 1 0
X8 0 1 1 1 0 1 1 0
X9 0 1 1 1 0 1 1 0
