Shift-Registers and Push Button Debounce

Switching and Logic LabStandard Laboratory

Exercises

Suggestions and Warnings Read for detail and comprehension Should be able to complete within

normal laboratory period. Make sure you program unused

pins as tri-state inputs or you may burnout EPM7128S device on PLDT-2.

Shift Registers Riding without training wheels Riding with training wheels Project 1 -- Basic Component

Shift Register as Behavioral VHDL Follow standard steps from project creation

to exercise of device Project 2 – Push Button Debounce

Count and display debounced presses Count and display non debounced presses Never let inputs float!

Shift Register Entity

ENTITY ShftRgstr ISPORT (

Dp : IN BIT_VECTOR(7 DOWNTO 0);Ld,Ds, CLK : IN BIT;Q : BUFFER BIT_VECTOR(7 DOWNTO 0));

END ShftRgstr;

Dp[7..0]

Ld

Ds

CLK

Q[7..0]

Shf tRgstr

Tx

Shift Register ArchitectureARCHITECTURE Behavioral OF ShftRgstr ISBEGINPROCESSBEGINWAIT UNTIL Clk'event AND Clk = '1';IF Ld = '1' THEN --parallel loadQ <= Dp;

Shift Register ArchitectureELSE --shift right LSb first

Q(0) <= Q(1); --Serial Data Out

Q(1) <= Q(2); --Continue ShiftQ(2) <= Q(3);Q(3) <= Q(4);Q(4) <= Q(5);Q(5) <= Q(6);Q(6) <= Q(7);Q(7) <= Ds; --Serial Data In

END IF;END PROCESS;END Behavioral;

Vector Waveform Format File

Overwrite

Clock

Arbitrary

Value

Waveform

EditingTool

Add Push Button Jumper Wires

Bounce on Release

Cross-Coupled NAND gates Requires

Two I/O pins Two resistors Double pole

switch Output changes

on first contact closure

Project 2 – Non Debounce Parallel In from DIP Switches and

Debounced Switches Serial Out

5 VDC

pb_in(Active-LOW)

nPb_in(Active-HIGH)

Pb_out(Active-HIGH)

Project 2 -- Debounce

4MHz 1KHz

pb_in(Active-LOW)

nPb_in(Active-HIGH)

Pb_out(Active-HIGH)

Debouncer Shift Register

LoadPB_INCLK

!=

nPb_in = Pb_out Parallel LoadnPb_in != Pb_out Shift

nPb_in

Pb_out

Bounce on Press

LoadPB_INCLK

!=nPb_in

Pb_out

nPB_in

Pb_out

Load

SR4

0…0 0 0 1 0 1 0 1 1 1 1 1 1 1 1…10…0 0 0 0 0 0 0 0 0 0 1 1 1 1 1…11…1 1 1 0 1 0 1 0 0 0 1 1 1 1 1…10…0 0 0 1 0 1 0 1 3 7 F F F F F…F

Bounce on Release

LoadPB_INCLK

!=nPb_in

Pb_out

nPB_in

Pb_out

Load

SR4

1…1 1 1 0 1 0 1 0 0 0 0 0 0 0 0…01…1 1 1 1 1 1 1 1 1 1 0 0 0 0 0…01…1 1 1 0 1 0 1 0 0 0 1 1 1 1 1…1F…F F F E F E F E C 8 0 0 0 0 0…0

8 Steps to Success

1. Create Project2. Capture Logic3. Analysis and Synthesis4. Pin Assignments5. Full Compile6. Timing Simulation7. Programming8. Exercise circuit

Unused Pins as Tri-State Inputs Select

“Assignments” Select “Device” Select “Device & Pin

Options” Select “Unused Pins”

Tab Select “As inputs, tri-

stated” OK