Datenblatt-Suchmaschine für elektronische Bauteile |
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AD5228 Datenblatt(PDF) 11 Page - Analog Devices |
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AD5228 Datenblatt(HTML) 11 Page - Analog Devices |
11 / 20 page AD5228 Rev. A | 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 and to zero-scale when PRE is tied to VDD. 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 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 VDD 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. CH1 1.00V M40.0ms A CH1 2.38V 1 T 20.40% Figure 31. Typical Tactile Switch Characteristics CH1 1.00V M100 μs A CH1 2.38V 1 T 20.20% Figure 32. Close-Up of Initial Bounces CH1 1.00V M10.0 μs 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 RWB by one step. Repeatedly pressing the PU button for fast adjustment without missing steps is allowed, provided that each press is not shorter than tPU, which is 12 ms (see ). As a point of reference, an advanced video game player can press a pushbutton switch in 40 ms. Figure 2 |
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