9
FN8240.3
August 31, 2006
placed in the low power standby mode until the device is
selected once again.
Principles of Operation
There are three sections of the ISL95710: the input control,
wiper counter and decode section; the nonvolatile memory;
and the resistor array. The input control section operates as
an up/down counter. The output of this wiper counter is
decoded to turn on a electronic switch connecting a point on
the resistor array to the wiper output. The contents of the
wiper counter can be stored in nonvolatile memory and
retained for future use. The resistor array is comprised of
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. The wiper 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 R
W
change). The
R
TOTAL
value for the device can temporarily be reduced by
a significant amount if the wiper is moved several positions.
When the device is powered-down, the last wiper position
stored will be maintained in the nonvolatile memory. When
power is restored, the contents of the memory are recalled
and the wiper is set to the value last stored.
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 wiper counter. The output of this wiper counter is decoded
to select one of 128 wiper positions along the resistive array.
The value of the wiper counter is stored in nonvolatile
memory whenever CS
transitions HIGH while the INC input
is also HIGH.
The system may select the ISL95710, move the wiper and
deselect the device without having to store the latest wiper
position in nonvolatile memory. After the wiper movement is
performed as described above and once the new position is
reached, the system must keep INC
LOW while taking CS
HIGH. The new wiper position will be maintained until
changed by the system or until a power-up/down cycle
recalls the previously stored data.
This procedure allows the system to always power-up to a
preset value stored in nonvolatile memory; then during
system operation minor adjustments could be made. The
adjustments might be based on user preference, system
parameter changes due to temperature drift, etc.
The state of U/D
may be changed while CS remains LOW.
This allows the host system to enable the device and then
move the wiper up and down until the proper trim is attained.
During initial power-up CS
must go high along with or before
V
CC
to avoid an accidental store generation.
TABLE 1. MODE SELECTION
CS INC U/D MODE
L H Wiper up
L L Wiper down
H X Store wiper position
H X X Standby current
L X No store, return to standby
H H X Standby
L H Wiper up one position
(not recommended)
L L Wiper down one position
(not recommended)
ISL95710
