5
FN8229.3
October 14, 2005
Pin Descriptions
RH and RL
The ISL90461 contains a digital potentiometer configured as
a variable resistor. The wiper of the potentiometer is tied to
one end of the potentiometer at terminal RH, and the RL pin
is the other terminal of the potentiometer. The resistance
from the RH pin to the RL pin will vary with the potentiometer
setting. At the highest setting the resistance will be
maximum (Rtot) and at the lowest setting it will be minimum.
As the wiper position is incremented, the wiper will move
from the Low terminal to the High terminal.
Up/Down (U/D)
The U/D input controls the direction of the wiper movement
and whether the counter is incremented or decremented.
Chip Select (CS)
The device is selected when the CS input is LOW. The
current counter value is stored in volatile memory when CS
is returned HIGH. When CS
is high, the device is placed in
low power standby mode.
Principles of Operation
There are two sections of the ISL90461: 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 31
individual resistors connected in series. At either end of the
array and between each resistor is an electronic switch that
transfers the connection at that point to the wiper. The wiper
is connected to the RH terminal, forming a variable resistor
from RH to RL.
The direction of the wiper movement is defined when the
device is selected. If during CS
transition from High to Low
the U/D
input is LOW, the wiper will move down on each
rising edge of U/D
toggling. Similarly, the wiper will move up
on each rising edge of U/D
toggling if, during CS transition
from High to Low, the U/D
input is High.
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.
If the wiper is moved several positions, multiple taps are
connected to the wiper for t
SETTLE
(U/D to RH change). The
2-terminal resistance value for the device can temporarily
change by a significant amount if the wiper is moved several
positions.
CS
U/D
RH - RL
t
CU
t
CI
t
IL
t
IH
t
IC
t
SETTLE
FIGURE 2. SERIAL INTERFACE TIMING DIAGRAM, DECREMENT
ISL90461
