Data Sheet AD5228
Rev. B | Page 11 of 18
THEORY OF OPERATION
The AD5228 is a 32-position manual up/down digitally con-
trolled potentiometer with selectable power-on preset. The
AD5228 presets to midscale when the PRE pin is tied to ground
and to zero-scale when PRE is tied to V
DD
. Floating the PRE pin
is not allowed. The step-up and step-down operations require
the activation of the
PU
(push-up) and
PD
(push-down) pins.
These pins have 100 kΩ internal pull-up resistors that the
PU
and
PD
activate at logic low. The common practice is to apply
external pushbuttons (tactile switches) as shown in Figure 30.
04422-0-031
UP/DOWN
CONTROL
LOGIC
DISCRETE
STEP/AUTO
SCAN DETECT
ADAPTIVE
DEBOUNCER
ZERO- OR MID-
SCALE PRESET
AD5228
PUSH-UP
BUTTON
PUSH-DOWN
BUTTON
R1 R2
D
E
C
O
D
E
A
W
B
V
DD
PRE GND
PU
PD
Figure 30. Typical Pushbutton Interface
Because of the bounce mechanism commonly found in the
switches during contact closures, a single pushbutton press
usually generates numerous bounces during contact closure.
Note that the term pushbutton refers specifically to a
pushbutton tactile switch or a similar switch that has 10 ms or
less bounce time during contact closure. Figure 31 shows the
characteristics of one such switch, the KRS-3550 tactile switch.
Figure 32 and Figure 33 show close ups of the initial bounces
and end bounces, respectively.
04422-0-032
CH1 1.00V M40.0ms A CH1 2.38V
1
T 20.40%
Figure 31. Typical Tactile Switch Characteristics
04422-0-033
CH1 1.00V M100s A CH1 2.38V
1
T 20.20%
Figure 32. Close-Up of Initial Bounces
04422-0-034
CH1 1.00V M10.0
s A CH1 2.38V
1
T 20.20%
Figure 33. Close-Up of Final Bounces
The following paragraphs describes the
PU
incrementing
operation. Similar characteristics apply to the
PD
decrementing
operation.
The AD5228 features an adaptive debouncer that monitors the
duration of the logic-low level of
PU
signal between bounces. If
the
PU
logic-low level signal duration is shorter than 7 ms, the
debouncer ignores it as an invalid incrementing command.
Whenever the logic-low level of
PU
signal lasts longer than
11 ms, the debouncer assumes that the last bounce is met and
therefore increments R
WB
by one step.
Repeatedly pressing the
PU
button for fast adjustment without
missing steps is allowed, provided that each press is not shorter
than t
PU
, which is 12 ms (see Figure 2). As a point of reference,
an advanced video game player can press a pushbutton switch
in 40 ms.
