Slide 1'&$%ECE/CS 3720: Embedded System Design(ECE 6960/2 and CS 6968)Chris J. MyersLecture 6: Debouncing and Matrix KeypadsSlide 2'&$%Interfacing a Switch to a Computer1Slide 3'&$%Switch BounceSlide 4'&$%Hardware Debouncing Using a Capacitor2Slide 5'&$%Software DebouncingSlide 6'&$%Software Debouncing with Gadfly SynchronizationWaitPress: ldaa PORTA ;PA0=0 if pressedanda #$01bne WaitPress ;loop till pressedldd TCNTaddd #20000 ;10ms delaystd TOC5ldaa #$08staa TFLG1 ;clear OC5FloopP: ldaa TFLG1 ;10ms for bouncinganda #$08 ;OC5F set?beq loopPrts3Slide 7'&$%Applications of Output Compare• Can create a fixed time delay.1. Read the current 16-bit TCNT2. Calculate TCNT+fixed3. Set 16-bit output compare register to TCNT+fixed4. Clear the output compare flag5. Wait for the output compare flag to be set• Delay of steps 1 to 4 sets the minimum delay.• Maximum delay is 65,536 cycles.Slide 8'&$%Output Compare Interface on 68HC114Slide 9'&$%Control Bits and FlagsOMn OLn Effect of when TOCn=TCNT0 0 Does not affect OCn0 1 Toggle OCn1 0 Clear OCn=01 1 Set OCn=1Slide 10'&$%Software Debouncing with Gadfly SynchronizationWaitRel: ldaa PORTA ;PA0=1 if releasedanda #$01beq WaitRel ;loop until releasedldd TCNTaddd #20000 ;10ms delaystd TOC5ldaa #$08staa TFLG1 ;clear OC5FloopR: ldaa TFLG1 ;10ms for bouncinganda #$08 ;OC5F set?beq loopRrts5Slide 11'&$%Software DebouncingSlide 12'&$%Software Debouncing with Gadfly Synchronization* Reg B is the return valueReadPA0: ldd TCNTaddd #20000 ;10ms delaystd TOC5ldaa #$08staa TFLG1 ;clear OC5Fldab PORTA ;0 if pressedandb #$01 ;B=old valueSame: ldaa TFLG1 ;10ms bouncinganda #$08 ;OC5F set?bne Doneldaa PORTA ;0 if pressedanda #$01 ;A=new valuecba ;same as beforebeq Samebra ReadPA0 ;different, start overDone: rts6Slide 13'&$%Basic Approaches to Interfacing Multiple KeysRow Out3 Out2 Out1 Out03 0 HiZ HiZ HiZ2 HiZ 0 HiZ HiZ1 HiZ HiZ 0 HiZ0 HiZ HiZ HiZ 0Row Out3 Out2 Out1 Out0 15 14 . . . 015 1 1 1 1 0 HiZ . . . HiZ14 1 1 1 0 HiZ 0 . . . HiZ. . . . . . . . . . . . . . . . . . . . . . . . . . .0 0 0 0 0 HiZ HiZ . . . 0Slide 14'&$%4 by 4 Scanned KeyboardRow 3 Row 2 Row 1 Row 0 Col 3 Col 2 Col 1 Col 00 1 1 1 a b c d1 0 1 1 e f g h1 1 0 1 i j k l1 1 1 1 m n o p7Slide 15'&$%4 by 4 Scanned Keyboard• There are two steps to scan a particular row:1. Select that row by driving low while other rows arenot driven.2. Read the columns to see if any keys are pressed inthat row (0 means key pressed, 1 means not pressed).• The scanned keyboard operates properly if:1. No key is pressed.2. Exactly one key is pressed.3. Exactly two keys are pressed.Slide 16'&$%Software for Matrix Scanned KeyboardRitual: ldaa #$20staa PIOC ;CWOM=1ldaa #$F0 ;PC7-PC4 outputsstaa DDRC ;PC3-PC0 inputsrtsScanTab: fcb %01110000 ;PC7 rowfcc "abcd" ;charactersfcb %10110000 ;PC6 rowfcc "efgh" ;charactersfcb %11010000 ;PC5 rowfcc "ijkl" ;charactersfcb %11100000 ;PC4 rowfcc "mnop" ;charactersfcb 08Slide 17'&$%Software for Matrix Scanned Keyboard; Returns RegA ASCII key pressed,; RegY number of keys pressed; Y=0 if no key pressedScan: ldy #0 ;Number pressedldx #ScanTabLoop: ldab 0,xbeq Donestab PORTC ;select rowldab PORTC ;read columnslsrb ;PC0 into carrybcs NotPC0ldaa 4,xinySlide 18'&$%Software for Matrix Scanned KeyboardNotPC0: lsrb ;PC1 into carrybcs NotPC1ldaa 3,xinyNotPC1: lsrb ;PC2 into carrybcs NotPC2ldaa 2,xinyNotPC2: lsrb ;PC3 into carrybcs NotPC3ldaa 1,xinyNotPC3: ldab #5 ;Size of entryabxbra LoopDone:
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