10 0 A3 A2 A1 A0
Device Type
Identifier
Device Address
1
3
FN8163.2
August 30, 2006
PIN NAMES
PRINCIPLES OF OPERATION
The X9221A is a highly integrated microcircuit incor-
porating two resistor arrays, their associated registers
and counters and the serial interface logic providing
direct communication between the host and the XDCP
potentiometers.
Serial Interface
The X9221A supports a bidirectional bus oriented pro-
tocol. The protocol defines any device that sends data
onto the bus as a transmitter and the receiving device
as the receiver. The device controlling the transfer is a
master and the device being controlled is the slave.
The master will always initiate data transfers and pro-
vide the clock for both transmit and receive operations.
Therefore, the X9221A will be considered a slave
device in all applications.
Clock and Data Conventions
Data states on the SDA line can change only during
SCL LOW periods (t
LOW
). SDA state changes during
SCL HIGH are reserved for indicating start and stop
conditions.
Start Condition
All commands to the X9221A are preceded by the
start condition, which is a HIGH to LOW transition of
SDA while SCL is HIGH (t
HIGH
). The X9221A continu-
ously monitors the SDA and SCL lines for the start
condition, and will not respond to any command until
this condition is met.
Stop Condition
All communications must be terminated by a stop con-
dition, which is a LOW to HIGH transition of SDA while
SCL is HIGH.
Acknowledge
Acknowledge is a software convention used to provide
a positive handshake between the master and slave
devices on the bus to indicate the successful receipt of
data. The transmitting device, either the master or the
slave, will release the SDA bus after transmitting eight
bits. The master generates a ninth clock cycle and
during this period the receiver pulls the SDA line LOW
to acknowledge that it successfully received the eight
bits of data. See Figure 7.
The X9221A will respond with an acknowledge after
recognition of a start condition and its slave address
and once again after successful receipt of the com-
mand byte. If the command is followed by a data byte
the X9221A will respond with a final acknowledge.
Array Description
The X9221A is comprised of two resistor arrays. Each
array contains 63 discrete resistive segments that are
connected in series. The physical ends of each array
are equivalent to the fixed terminals of a mechanical
potentiometer (V
H
/R
H
and V
L
/R
L
inputs).
At both ends of each array and between each resistor
segment is a FET switch connected to the wiper
(V
W
/R
W
) output. Within each individual array only one
switch may be turned on at a time. These switches are
controlled by the Wiper Counter Register (WCR). The
six least significant bits of the WCR are decoded to
select, and enable, one of sixty-four switches.
The WCR may be written directly, or it can be changed
by transferring the contents of one of four associated
data registers into the WCR. These data registers and
the WCR can be read and written by the host system.
Device Addressing
Following a start condition the master must output the
address of the slave it is accessing. The most signifi-
cant four bits of the slave address are the device type
identifier (refer to Figure 1 below). For the X9221A this
is fixed as 0101[B].
Figure 1. Slave Address
Symbol Description
SCL Serial Clock
SDA Serial Data
A0–A3 Address
V
H0
/R
H0
-V
H1
/R
H1
,
V
L0
/R
H0
-V
L1
/R
L0
Potentiometers
(terminal equivalent)
V
W0
/R
W0
-V
W1
/R
W1
Potentiometers
(wiper equivalent)
RES Reserved (Do not connect)
X9221A
