switch inputs

Garced Winder · 2007-11-09 06:33

I recently bought a Propeller kit, just "learning the ropes" if you will. Completed a few of the sample programs and am starting to grasp the unit and its capabilities. I can't find is how to "one shot" a pushbutton input. What I'd like is to increment Index by 1 for every button event (off-on-off) reguardless of on time - this increments Index as long as P18 is held high.

repeat
    ! outa          'led on P3
    
    if ina[noparse][[/noparse]18] == 1  'n/o momentary on P18
      Index++

    waitcnt(clkfreq / Index + cnt)

What I'd like is to increment Index by 1 for every button event (off-on-off) reguardless of on time - this increments Index by 1 as long as P18 is held high. Is there a one-shot routine or does that need to be coded? Or status register for i/o pins that can be checked then cleared?

I looked at keyboard.spin, got lost in the assembly. My assembly skills are quite rusty (8088 wayyy back in school)

Mike Green · 2007-11-09 06:38

The trick is to also wait for the "button off" event like:

index~ ' index is initially zero
repeat until ina [noparse][[/noparse] 18 ] == 0 ' make sure button initially off
repeat
   repeat until ina[noparse][[/noparse] 18 ] == 1 ' wait for on
   index++
   repeat until ina[noparse][[/noparse] 18 ] == 0 ' wait for off
   waitcnt(clkfreq/index + cnt)

deSilva · 2007-11-09 06:38

You shall also need some debouncing considerations

Nick Mueller · 2007-11-09 06:51

The keyword is "edge triggering". That means you don't wait for L or H, but you wait for a transition from L to H (or H to L).
Debouncing is an issue!

Nick

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Paul M · 2007-11-09 10:48

Since you are polling the input, to de-bounce the switch, all you need to do is introduce a small delay after the 'wait for on'·and 'wait for off' statements

mS:=clkfreq/1000 
delay:=10*mS                   'delay in milliseconds 
index~                         ' index is initially zero
repeat until ina [noparse][[/noparse] 18 ] == 0   ' make sure button initially off
repeat
   repeat until ina[noparse][[/noparse] 18 ] == 1 ' wait for on
   waitcnt(delay + cnt)
   index++
   repeat until ina[noparse][[/noparse] 18 ] == 0 ' wait for off
   waitcnt(delay + cnt)

·You may need to experiment with the delay depending on the type of switch. The click action switch I tested with required only about 2 mS but you may require up to 20 or 30 mS. Since the push button is likely to be·operated by a·human the delay will not be·noticed.

BTW what was the purpose of the waitcnt(clkfreq/index·+·cnt) line?

Paul

Nick Mueller · 2007-11-09 11:03

Sorry for nitpicking!

milli seconds is "ms".
"mS" would be milli Siemens. And that is the inverse of resistance (G = 1/R) AKA conductance.
<http://en.wikipedia.org/wiki/Siemens_(unit)>

Nick, SI-evangelist

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Never use force, just go for a bigger hammer!

The DIY Digital-Readout for mills, lathes etc.:
YADRO

hippy · 2007-11-09 13:55

@ Nick : Thanks for that, I never realised I'd been making that error. The other common SI mistake is to use KHz when it should be kHz.

Paul M · 2007-11-09 13:55

Nick

Sorry to be pedantic, but 'mS' is the name of a variable·:-)

deSilva · 2007-11-09 19:07

(a) the SI rules state that ALL units should be wriiten in lower case except if they derive from a person's name.

This was quite a clever move, as it COULD lead to become a little bit more interested in history. (Who was Volt? Oh it was Volta, Alessandro! And what did he do? Oh dear! Not only a frog eater but a frog tormentor... Sounds like a role model for Dr. Frankenstein....)

Of course there ar many more famous scientists than units... So with Werner Siemens who later became "von Siemens", who was not only a great scientist but also a great industrialist.
Thanks God they stopped then with inverting the units, but - maybe maybe - there is still some for you! I always wondered what a "Silva" could be...

(b) "Bouncing" happens only when closing a switch, no need to debounce when opening it.

Post Edited (deSilva) : 11/9/2007 7:13:17 PM GMT

Garced Winder · 2007-11-09 22:27

Wow, thanks for the quick replies! Thought about it at work and came up with this.

  dira~~        ' Set pin 3 to output
  Index := 1
  P18Flag := 0     ' Set pin 18 flag open
  
  repeat                   'Endless loop

    if ina[noparse][[/noparse]18] == 0        'P18 open 
      P18Flag := 0         'then clear flag   

    ! outa             ' Change the state of pin 3 

    if (ina[noparse][[/noparse]18] == 1 AND P18Flag == 0)  ' If button pressed and not still down from last cycle
      Index++
      P18Flag :=1                       ' set P18 flag for next cycle
      
    waitcnt(clkfreq / Index + cnt)             ' Wait 1/4 second -> 2 Hz

Can hold switch down as long as you want, only increments by 1.

deSilva · 2007-11-09 23:18

This will work as debouncing, but you have a very bad response time when pressing the key.
You can improve it by putting the WAITCNT into the IF.

Note that it will be expected that the kay can be pressed as fast as possible, which is about 10 per second.

What is the meaning of CLKFREG / Index ???

BTW: always insert a space after a [noparse][[/noparse]

Paul M · 2007-11-12 10:51

deSilva said...
(b) "Bouncing" happens only when closing a switch, no need to debounce when opening it.

Not necessarily true. See:

http://www.ganssle.com/debouncing.pdf
http://www.elexp.com/t_bounc.htm

Graham Stabler · 2007-11-12 11:10

It's hard to see why

Graham

parts-man73 · 2007-11-12 14:22

There's been many threads on input from a switch, and more specifically debouncing.

Here's a link to a recent thread that I think covers it rather well
http://forums.parallax.com/showthread.php?p=677248

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deSilva · 2007-11-12 20:02

"Bouncing" wrt opening switches...
I always liked the thorough invstigation of Jack Ganssle, I generally give this reference myself, but I definitely had forgotten the part about opening switches..

The effect itself - as he well describes -is a little bit unclear. It is NOT the mechanical effect when closing a switch... Closing a switch resembles dropping a ball; opening the switch resembles picking it up...

There is always a small inductance involved and local electrical fields at sharp corners maybe can trick a digital circuit into intermediate triggering. Note the time constant for this effect when opening a switch is very different from closing.

SPIN programmers will not need to consider "unclean" effects when closing switches.

The second reference Paul M gave does not sound very reliable wrt to opening switch effects...

But it will do no harm to consider it. It absolutely depends on how short you start looking again for the switch after you noticed it's closing.

I have never used software debouncing for opening switches...

switch inputs

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