AD5228

Rev. 0 | Page 11 of 20

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

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.

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

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.

M40.0ms

A CH1

2.38V

1

Figure 31. Typical Tactile Switch Characteristics

M100

µs

A CH1

2.38V

1

Figure 32. Close-Up of Initial Bounces

M10.0

µs

A CH1

2.38V

1

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

Repeatedly pressing the PU button for fast adjustment without

missing steps is allowed, provided that each press is not shorter

an advanced video game player can press a pushbutton switch

in 40 ms.