6
FN6126.0
August 22, 2005
AC Timing
Power Up and Down Requirements
In order to prevent unwanted tap position changes, bring the
CS
and INC high before or concurrently with the V
CC
pin on
power-up. The potentiometer voltages must be applied after
this sequence is completed. During power-up, the data sheet
parameters for the DCP do not fully apply until 1ms after
V
CC
reaches its final value. The wiper will be set to its initial
value (64d) once V
CC
exceeds V
POR
.
Pin Descriptions
R
H
and R
L
The high (R
H
) and low (R
L
) terminals of the ISL23710 are
equivalent to the fixed terminals of a mechanical
potentiometer. The terminology of R
L
and R
H
references the
relative position of the terminal in relation to wiper movement
direction selected by the U/D
input and not the voltage
potential on the terminal.
R
W
R
w
is the wiper terminal and is equivalent to the movable
terminal of a mechanical potentiometer. The position of the
wiper within the array is determined by the control inputs.
Up/Down (U/D)
The U/D input controls the direction of the wiper movement
and whether the counter is incriminated or decremented.
Increment (INC)
The INC input is negative-edge triggered. Toggling INC will
move the wiper and either increment or decrement the
counter in the direction indicated by the logic level on the
U/D
input.
Chip Select (CS)
The device is selected when the CS input is LOW.
Principles of Operation
There are three sections of the ISL23710: the input control,
counter and decode section, and the resistor array. The input
control section operates just like an up/down counter. The
output of this counter is decoded to turn on a single
electronic switch connecting a point on the resistor array to
the wiper output. The resistor array is comprised of 127
individual resistors connected in series. At either end of the
array and between each resistor is an electronic switch that
transfers the potential at that point to the wiper.
The wiper, when at either fixed terminal, acts like its
mechanical equivalent and does not move beyond the last
position. That is, the counter does not wrap around when
clocked to either extreme.
The electronic switches on the device operate in a “make
before break” mode when the wiper changes tap positions. If
the wiper is moved several positions, multiple taps are
connected to the wiper for t
IW
(INC to V
W
change). The
R
TOTAL
value for the device can temporarily be reduced by
a significant amount if the wiper is moved several positions.
Instructions and Programming
The INC, U/D and CS inputs control the movement of the
wiper along the resistor array. With CS
set LOW the device
is selected and enabled to respond to the U/D
and INC
inputs. HIGH to LOW transitions on INC
will increment or
decrement (depending on the state of the U/D
input) a seven
bit counter. The output of this counter is decoded to select
CS
INC
U/D
R
W
t
CI
t
IL
t
IH
t
CYC
t
ID
t
DI
t
IW
MI
(3)
t
ICL
t
F
t
R
10%
90% 90%
ISL23710
