A
PPLICATION
N
OTES
A V A I L A B L E
AN99 • AN115 • AN120 • AN124 • AN133 • AN134 • AN135
Low Noise/Low Power/2-Wire Bus
X9418
Dual Digitally Controlled Potentiometers (XDCP
™
)
FEATURES
• Two potentiometers in one package
• 2-wire serial interface
• Register oriented format
—Direct Read/Write/Transfer Wiper Position
—Store as many as Four Positions per
Potentiometer
• Power supplies
—V
CC
= 2.7V to 5.5V
—V+ = 2.7V to 5.5V
—V– = –2.7V to –5.5V
• Low power CMOS
—Standby current < 1µA
—Ideal for Battery Operated Applications
• High reliability
—Endurance–100,000 Data Changes per Bit per
Register
—Register Data Retention–100 years
• 8-bytes of nonvolatile memory
• 2.5K
Ω
, 10K
Ω
resistor array
• Resolution: 64 taps each potentiometer
• 24-pin plastic DIP, 24-lead TSSOP and 24-lead
SOIC packages
DESCRIPTION
The X9418 integrates two digitally controlled
potentiometers (XDCP) on a monolithic CMOS
integrated microcircuit.
The digitally controlled potentiometer is implemented
using 63 resistive elements in a series array. Between
each element are tap points connected to the wiper
terminal through switches. The position of the wiper on
the array is controlled by the user through the 2-wire
bus interface. Each potentiometer has associated with
it a volatile Wiper Counter Register (WCR) and 4
nonvolatile Data Registers (DR0:DR3) that can be
directly written to and read by the user. The contents
of the WCR controls the position of the wiper on the
resistor array through the switches. Power up recalls
the contents of DR0 to the WCR.
The XDCP can be used as a three-terminal
potentiometer or as a two-terminal variable resistor in
a wide variety of applications including control,
parameter adjustments, and signal processing.
BLOCK DIAGRAM
V
CC
V
SS
V+
V-
R0 R1
Wiper
Counter
Register
(WCR)
V
H0
/R
H0
WP
SCL
SDA
A0
A1
A2
A3
R2 R3
V
L0
/R
L0
V
W0
/R
W0
Interface
and
Control
Circuitry
8
Data
R0 R1
Wiper
Counter
Register
(WCR)
V
W1
/R
W1
V
H1
/R
H1
R2 R3
Resistor
Array
XDCP1
V
L1
/R
L1
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Characteristics subject to change without notice.
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X9418
PIN DESCRIPTIONS
Host Interface Pins
Serial Clock (SCL)
The SCL input is used to clock data into and out of the
X9418.
Serial Data (SDA)
SDA is a bidirectional pin used to transfer data into
and out of the device. It is an open drain output and
may be wire-ORed with any number of open drain or
open collector outputs. An open drain output requires
the use of a pull-up resistor. For selecting typical
values, refer to the guidelines for calculating typical
values on the bus pull-up resistors graph.
Device Address (A
0
–
A
3
)
The Address inputs are used to set the least
significant 4 bits of the 8-bit slave address. A match in
the slave address serial data stream must be made
with the Address input in order to initiate
communication with the X9418. A maximum of 16
devices may occupy the 2-wire serial bus.
Potentiometer Pins
V
H
/R
H
(V
H0
/R
H0
—V
H1
/R
H1
), V
L
/R
L
(V
L0
/R
L0
—V
L1
/R
L1
)
The V
H
/R
H
and V
L
/R
L
inputs are equivalent to the
terminal connections on either end of a mechanical
potentiometer.
V
W
/R
W
(V
W0
/R
W0
—V
W1
/R
W1
)
The wiper outputs are equivalent to the wiper output of
a mechanical potentiometer.
Hardware Write Protect Input (WP)
The WP pin when low prevents nonvolatile writes to
the Data Registers.
Analog Supplies V+, V-
The Analog Supplies V+, V- are the supply voltages
for the XDCP analog section.
V
CC
R
L0
/V
L0
R
H0
/V
H0
R
W0
/V
W0
A2
WP
SDA
A1
R
L1
/V
L1
R
H1
/V
H1
R
W1
/V
W1
V
SS
PIN CONFIGURATION
DIP/SOIC
1
24
2
23
3
22
4
21
5
20
6
19
X9418
7
18
8
17
9
16
10
5
14
11
12
13
V+
NC
NC
NC
A0
NC
A3
SCL
NC
NC
NC
V-
TSSOP
SDA
A1
R
L1
/V
L1
R
H1
/V
H1
R
W1
/V
W1
V
SS
NC
NC
NC
V-
SCL
A3
1
24
2
23
3
22
4
21
5
20
6
19
X9418
7
18
8
17
9
16
10
15
14
11
13
12
WP
A2
V
W0
/R
W0
V
H0
/R
H0
V
L0
/R
L0
V
CC
NC
NC
NC
V+
A0
NC
PIN NAMES
Symbol
SCL
SDA
A0-A3
V
H0
/R
H0
–V
H1
/R
H1
,
V
L0
/R
L0
–V
L1
/R
L1
V
W0
/R
W0
–V
W1
/R
W1
WP
V+,V-
V
CC
V
SS
NC
Description
Serial Clock
Serial Data
Device Address
Potentiometer Pins
(terminal equivalent)
Potentiometer Pins
(wiper equivalent)
Hardware Write Protection
Analog Supplies
System Supply Voltage
System Ground
No Connection
REV 1.1.5 7/23/02
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Characteristics subject to change without notice.
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X9418
PRINCIPLES OF OPERATION
The X9418 is a highly integrated microcircuit
incorporating two resistor arrays and their associated
registers and counters and the serial interface logic
providing direct communication between the host and
the XDCP potentiometers.
Serial Interface
The X9418 supports a bidirectional 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 X9418 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 X9418 are preceded by the start
condition, which is a HIGH to LOW transition of SDA
while SCL is HIGH (t
HIGH
). The X9418 continuously
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
condition, 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.
The X9418 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 X9418 will respond with a final acknowledge.
Array Description
The X9418 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 CMOS 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 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
significant four bits of the slave address are the device
type identifier (refer to Figure 1 below). For the X9418
this is fixed as 0101[B].
Figure 1. Slave Address
Device Type
Identifier
0
1
0
1
A3
A2
A1
A0
Device Address
The next four bits of the slave address are the device
address. The physical device address is defined by the
state of the A
0
–A
3
inputs. The X9418 compares the
serial data stream with the address input state; a
successful compare of all four address bits is required
for the X9418 to respond with an acknowledge. The
A
0
–A
3
inputs can be actively driven by CMOS input
signals or tied to V
CC
or V
SS
.
Characteristics subject to change without notice.
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X9418
Acknowledge Polling
The disabling of the inputs, during the internal
nonvolatile write operation, can be used to take
advantage of the typical 5ms EEPROM write cycle
time. Once the stop condition is issued to indicate the
end of the nonvolatile write command the X9418
initiates the internal write cycle. ACK polling can be
initiated immediately. This involves issuing the start
condition followed by the device slave address. If the
X9418 is still busy with the write operation no ACK will
be returned. If the X9418 has completed the write
operation an ACK will be returned, and the master can
then proceed with the next operation.
Flow 1. ACK Polling Sequence
Nonvolatile Write
Command Completed
Enter ACK Polling
point to one of the two pots and when applicable they
point to one of four associated registers. The format is
shown below in Figure 2.
Figure 2. Instruction Byte Format
Register
Select
I3
I2
I1
I0
R1
R0
0
P0
Instructions
Wiper Counter
Register Select
The four high order bits define the instruction. The next
two bits (R1 and R0) select one of the four registers
that is to be acted upon when a register oriented
instruction is issued. The last bits (P0) select which
one of the two potentiometers is to be affected by the
instruction. Bit 1 is defined to be 0.
Four of the nine instructions end with the transmission
of the instruction byte. The basic sequence is
illustrated in Figure 3. These two-byte instructions
exchange data between the wiper counter register and
one of the data registers. A transfer from a Data
Register to a Wiper Counter Register is essentially a
write to a static RAM. The response of the wiper to this
action will be delayed t
WRL
. A transfer from the wiper
counter register (current wiper position), to a Data
Register is a write to nonvolatile memory and takes a
minimum of t
WR
to complete. The transfer can occur
between one of the two potentiometers and one of its
associated registers; or it may occur globally, wherein
the transfer occurs between both of the potentiometers
and one of their associated registers.
Four instructions require a three-byte sequence to
complete. These instructions transfer data between the
host and the X9418; either between the host and one
of the Data Registers or directly between the host and
the wiper counter register. These instructions are:
Read Wiper Counter Register (read the current wiper
position of the selected pot), write Wiper Counter
Register (change current wiper position of the selected
pot), read Data Register (read the contents of the
selected nonvolatile register) and write Data Register
(write a new value to the selected Data Register). The
sequence of operations is shown in Figure 4.
Issue
START
Issue Slave
Address
Issue STOP
ACK
Returned?
YES
NO
Further
Operation?
YES
Issue
Instruction
NO
Issue STOP
Proceed
Proceed
Instruction Structure
The next byte sent to the X9418 contains the instruction
and register pointer information. The four most
significant bits are the instruction. The next four bits
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X9418
Figure 3. Two-Byte Instruction Sequence
SCL
SDA
S
T
A
R
T
0
1
0
1
A3
A2
A1
A0
A
C
K
I3
I2
I1
I0
R1 R0 0
P0
A
C
K
S
T
O
P
The Increment/Decrement command is different from
the other commands. Once the command is issued and
the X9418 has responded with an acknowledge, the
master can clock the selected wiper up and/or down in
one segment steps; thereby, providing a fine tuning
capability to the host. For each SCL clock pulse (t
HIGH
)
while SDA is HIGH, the selected wiper will move one
Table 1. Instruction Set
Instruction
Read Wiper Counter
Register
Write Wiper Counter
Register
Read Data Register
Write Data Register
XFR Data Register to
Wiper Counter Register
XFR Wiper Counter
Register to Data Register
Global XFR Data
Registers to Wiper
Counter Registers
Global XFR Wiper
Counter Registers to
Data Register
Increment/Decrement
Wiper Counter Register
Note:
resistor segment towards the V
H
/R
H
terminal. Similarly,
for each SCL clock pulse while SDA is LOW, the
selected wiper will move one resistor segment towards
the V
L
/R
L
terminal. A detailed illustration of the
sequence and timing for this operation are shown in
Figures 5 and 6 respectively.
I
3
1
1
1
1
1
I
2
0
0
0
1
1
Instruction Set
I
1
I
0
R
1
R
0
0
1
1
0
0
1
0
1
0
1
0
0
0
0
P
1
0
0
0
0
0
P
0
Operation
1/0 1/0
1/0 1/0
1/0 1/0
1
1
1
0
1/0 1/0
0
0
0
0
1
1/0 1/0
0
1
0
0
0
1/0 1/0
0
0
0
1
0
0
0
0
1/0 Read the contents of the Wiper Counter Register
pointed to by P
0
1/0 Write new value to the Wiper Counter Register
pointed to by P
0
1/0 Read the contents of the Data Register pointed to
by P
0
and R
1
–R
0
1/0 Write new value to the Data Register pointed to by
P
0
and R
1
–R
0
1/0 Transfer the contents of the Data Register pointed
to by P
0
and R
1
–R
0
to its associated Wiper Counter
Register
1/0 Transfer the contents of the Wiper Counter Register
pointed to by P
0
to the Data Register pointed to by
R
1
–R
0
0 Transfer the contents of the Data Registers pointed
to by R
1
–R
0
of both pots to their respective Wiper
Counter Registers
0 Transfer the contents of both Wiper Counter
Registers to their respective data Registers pointed
to by R
1
–R
0
of both pots
1/0 Enable Increment/decrement of the Wiper Counter
Register pointed to by P
0
(7) 1/0 = data is one or zero
REV 1.1.5 7/23/02
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Characteristics subject to change without notice.
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