Modern handheld devices from digital cameras cell phones must interfac

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AND8163/D Solving Keyboard Problems with NLSF2500
Modern handheld devices from digital cameras cell phones must interface humans keypad arrangement. Some devices have keys many more. semiconductor industry responded with keyboard debounce chips with little input, full blown encoder with many keys full computer style keyboard interface. What been missing until cost, space efficient interfacing microcontroller unit (MCU). NLSF2500 designed fill void between full blown, do-everything keyboard encoders simple passives plus approach. Keyboard debounce critical component many keyboard encoders. Nearly keys required have some form tactile feedback. most popular form today uses dome switch. This consists membrane that brings traces printed circuit board together under plastic dome bubble. bubble coated with conductive material. When user pushes dome, collapses conductive material brought contact with board material. area where switch made usually gold plated longevity. dome collapses conductive material comes contact with circuit board, dome "bounces" several times until positive contact made. This take several tens milliseconds. order respond closure engineer needs employ some kind circuitry debounce keys. This almost always consists circuit that detects closure waits some period time before acting Many times 10-30 milliseconds considered "correct" response time. This debounce done hardware, software combination both hardware software. There basic approaches this problem. Either designer adds complete keyboard encoder circuit relies manufacturer supply good signal denoting keypad enclosure, designer take advantage capabilities multitask take care keyboard scanning debouncing without complex external devices. second approach almost always less expensive solution keyboard scanning relatively light burden MCU, however this usually requires quite passive components accomplish this task. rest this article will show device encoder, using software capabilities MCU, while eliminating need other circuitry. NLSF2500 CMOS device package with pins designed function between 1.65 There outputs inputs. I/Os have large pullup resistors connected supply pin. I/Os open drain would normally connected inputs through Several outputs hard wired together circuit board form these rows. Columns wired cross point switches form matrix. addition, more keys wired from each inputs ground functionality. This permits keys decoded while only using lines. also possible combine several NLSF2500 devices handle more keys. device itself almost entirely passive consumes nearly zero current. Although functionality device could replicated diodes resistors, cost, small size, lack external passive components ease make NLSF2500 viable addition bill material.
Operation
There five individual identical circuits. Each circuit single input outputs. Both input outputs high impedance with pullup resistors attached (see Figure
Figure
Semiconductor Components Industries, LLC, 2004
July, 2004 Rev.
Publication Order Number: AND8163/D
AND8163/D
entire device consists five above identical sections wired Figure
OUTA
OUT1
OUT2
OUT3
OUT4
OUT5
OUTB
Figure
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Closure Ground
outputs connected standard general purpose ports MCU. Each output drain transistor with high pullup connected VCC. Since open drain draws nearly zero current, VCC. outputs connected row-column keyboard matrix, then detect closure rather easily. Several algorithms possible detecting which closed, debouncing key. Basically Columns would low, while Rows pulled high NLSF2500 output pullup resistor. closed, that will low. rather simple perform scan. example, Row, Read low, then depressed, not, continue scan process. This process should only take hundred microseconds depending performance MCU. designer should repeat this process several times 5-10 millisecond
intervals, until certain true keyboard closure. added keys IN1-IN5 special keys invoke special scan routine. Their keys connected ground pullup cause rows simultaneously (Table causes keyboard scan routine different, from routine outlined above.
Table
AND8163/D
GPIO Interface Lines
SW4,1 SW3,1 SW2,1 SW1,1 NLSF2500 OUTA SW4,2 SW3,2 SW2,2 SW1,2 OUT1 SW4,3 SW4,4 SW3,4 SW2,4 SW1,4 OUT4 OUT5 SWIN5 SWIN4 SW3,3 SW2,3 SW1,3 OUT2 OUT3 SWIN2 SWIN3 SWIN1
OUTB
Figure Complete Schematic
Summary
NLSF2500 simple, cost minimizing hardware associated with keypad. eliminates large numbers diodes passive components otherwise associated with this style keyboard design. permits wide
operating voltage that could supported using diodes because drop associated with diodes. gives system designer freedom design debounce algorithm, while eliminating clutter large number components, cost.
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AND8163/D
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AND8163/D

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