SW Exp #14: Scanning and Deboucing Multiple Inputs
playinmyblues_nscc
Posts: 38
Hi. I am hoping somebody can explain a section of program in StampWorks Experiment #14, page 94 of the book. I have an edited program in addition to the original program. In both programs I do not understand why the buttons have to be held down for the whole period of the FOR NEXT loop. I understand that that is what happens. In my program, I wrote a preceding loop to let me know when the time was approaching where I have to hold down the button. I also extended the time that the button must be held down so I can experiment with the time it is held down. For example, the first loop lets me know when the hold-down time is approaching so I hold down the button. The FOR NEXT loop counts for the debounce. I hold the button down prior to the loop and when the count is part way through I release the button and press it again before the time is up and I hold the button down until the loop is over.
The effect is I know for sure that the original program functions as it should and my program functions as it should. However, I do not understand why. The reason I do not understand why is that at the end of the count for which the button must be pressed, the boolean expression: btns = btns & ~BtnBus would equal a 1 or 1's at the appropriate place for the last count, or so I would think. Why is this not true?
Original program:
My program:
The effect is I know for sure that the original program functions as it should and my program functions as it should. However, I do not understand why. The reason I do not understand why is that at the end of the count for which the button must be pressed, the boolean expression: btns = btns & ~BtnBus would equal a 1 or 1's at the appropriate place for the last count, or so I would think. Why is this not true?
Original program:
' {$STAMP BS2}
' {$PBASIC 2.5}
' -- Program: SW21-EX14-Debounce.bs2
' -- Purpose: This program demonstrates the simultaneous debouncing of multiple inputs. The input subroutine is easily adjusted to handle
' any number of of inputs.
' -- I/O Definitions --
BtnBus VAR INA ' four inputs, P0 - P3
' -- Variables --
btns VAR Nib ' deboucned inputs
idx VAR Nib ' loop counter
' -- Program Code --
Main:
DO
GOSUB Get_Buttons ' get deboucned inputs
DEBUG HOME, "Inputs = ", IBIN4 btns ' display in binary mode
PAUSE 50
LOOP
' -- Subroutines --
Get_Buttons:
btns = %1111 ' enable all four inputs
FOR idx = 1 TO 5
btns = btns & ~BtnBus ' test inputs
PAUSE 5
NEXT
RETURN
******************************************************************My program:
' {$STAMP BS2}
' {$PBASIC 2.5}
' -- Program: SW21-EX14-Debounce-Edit8Btns.bs2
' -- Purpose: This program demonstrates the simultaneous debouncing of multiple inputs. The input subroutine is easily adjusted to handle
' any number of of inputs.
' -- I/O Definitions --
BtnBus VAR INL ' four inputs, P0 - P7
' -- Variables --
btns VAR Byte ' deboucned inputs
idx VAR Byte ' loop counter
flip VAR Bit ' a bit sized counter to flip from 1 to 0 to 1 to alternate a DEBUG statement
idy VAR Word ' variable used to count the PAUSE
' -- Program Code --
' -- Initialize Variable --
flip = 0
Main:
DO
GOSUB Get_Buttons ' get deboucned inputs
DEBUG HOME, "Inputs = ", IBIN8 btns, CR ' display in binary mode
flip = flip + 1
IF (flip = 1) THEN
DEBUG "Paws1", CR
ELSEIF (flip = 0) THEN
DEBUG "Paws2", CR
ENDIF
' PAUSE 5000
FOR idy = 1 TO 500 ' This FOR NEXT loop is used to let the user know when the next opportunity to press the button is
DEBUG CRSRXY, 0, 3 ' coming up. Otherwise, the user cannot know when to time the button press as the time is too short
DEBUG DEC4 idy ' unless the FOR NEXT loop below is extended to something like 100.
NEXT
idy = 0
LOOP
' -- Subroutines --
Get_Buttons:
btns = %11111111 ' enable all eight inputs
FOR idx = 1 TO 50 ' the FOR loop is the debounce mechanism. It allows for the mechanical bouncing of the contacts to stop
' and then it exits the loop and returns to the DEBUG statement in the Main part of the program.
' Using a higher value for the upper limit of the FOR NEXT loop allows the person to experiment with
' timing of the button press. If the button is released before the end of the loop occurs, the button
' will show as not being pressed.
btns = btns & ~BtnBus ' test inputs
' DEBUG CRSRXY, 0, 1, "idx: ", DEC1 idx
DEBUG CRSRXY, 4, 4, DEC2 idx ' this line gives a number that changes in one position on the DEBUG terminal
PAUSE 5
NEXT
RETURN
Sorry for the poor code posting - recently fixed.
Comments
[SIZE=1][FONT=courier new]btns VAR Nib idx VAR Nib Get_Buttons: btns = % 1111 ' assume all four pressed FOR idx = 1 TO 5 btns = btns & ~inA ' test 4 inputs on inA PAUSE 5 NEXT RETURN[/FONT][/SIZE]When a button is pressed, its bit in inA goes to 0. The ~inA operator inverts that, a pressed button comes out as 1. The % 1111 starts by assuming all buttons are 1=pressed. If a button stays pressed for all 5 scans, then its bit stays at 1&1&1&1&1 = 1. But if a button is let up even once during the scan, then the result is, e.g. 1&1&0&0&1 = 0. Debounced result is, "not pressed". Does that help?
Thanks again!