4
FN8161.5
April 9, 2014
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Resistor Array Description
The X9118 is comprised of a resistor array. The array
contains 1023, in effect, discrete resistive segments that are
connected in series (see Figure 1). The physical ends of
each array are equivalent to the fixed terminals of a
mechanical potentiometer (R
H
and R
L
inputs).
At both ends of each array and between each resistor
segment is a CMOS switch (transmission gate) connected to
the wiper (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 10 bits
of the WCR (WCR[9:0]) are decoded to select, and enable,
one of 1024 switches.
The WCR may be written directly. The Data Registers and
the WCR can be read and written by the host system.
Serial Interface Description
SERIAL INTERFACE – 2-WIRE
The X9118 supports a bi-directional bus oriented protocol.
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 provide the clock for both transmit
and receive operations. Therefore, the X9118 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. The SDA state changes during SCL HIGH are
reserved for indicating start and stop conditions,
see Figure 3.
START CONDITION
All commands to the X9118 are preceded by the start
condition, which is a HIGH-to-LOW transition of SDA while
SCL is HIGH. The X9118 continuously monitors the SDA
and SCL lines for the start condition and will not respond to
any command until this condition is met, see Figure 3.
STOP CONDITION
All communications must be terminated by a stop condition,
which is a LOW-to-HIGH transition of SDA while SCL is
HIGH, see Figure 3.
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 8 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.
The X9118 will respond with an acknowledge after
recognition of a start condition and its slave address and
once again after successful receipt of the command byte. If
the command is followed by a data byte, the X9118 will
respond with a final acknowledge, see Figure 2.
SERIAL DATA PATH
FROM INTERFACE
REGISTER 0
SERIAL
BUS
INPUT
PARALLEL
BUS
INPUT
COUNTER
REGISTER
R
L
10 10
C
O
U
N
T
E
R
D
E
C
O
D
E
If WCR = 000[HEX] then R
W
= R
L
If WCR = 3FF[HEX] then R
W
= R
H
WIPER
(WCR)
(DR0)
CIRCUITRY
REGISTER 1
(DR1)
REGISTER 2
(DR2)
REGISTER 3
(DR3)
FIGURE 1. DETAILED POTENTIOMETER BLOCK DIAGRAM
R
W
R
H
X9118
