Counter question: how to set-up a counter to detect going from low to high?

vanmunch · 2011-07-28 13:11

I'm trying to figure out how to measure the time between the moment the code initiates an action (starts to move a servo) until a button is pushed and I was thinking that this would be a great job to use a counter for. I'm thinking that it would be very similar to the code used for seeing when a pin goes from high to low (documented in the PEK), only here I would need to know when the pin goes from low to high. I've looked through the manual, but I can't seem to get anything to work. Please let me know if you have any ideas or if you think there's a better way to do this, but I'm guessing/hoping there's a simple answer. Thanks!

Dave

kwinn · 2011-07-28 14:06

Use a pullup resistor to the pin with the push button grounding it, then the pin will go from high to low and you can use the existing code.

kuroneko · 2011-07-28 16:44

I don't know this particular PEK solution (reference?) but can't you just reverse the condition?

vanmunch · 2011-08-03 19:41

Hey Guys,

I apologize for not responding sooner. Lots going on.

kwin,
Thanks for your suggestion. I would have liked to have gone that route, but the way the button (which is a tactile sensor with the center energized and the outer spring being the ground) requires the pin to go from low to high. Physically, this makes more sense and works great... except now I want to try and use the counter funtion.

kuroneko,
The example is found on pg 121-130 in v 1.1 that you can download here:
http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PELabsFunBook-v1.1.pdf
and I'd like to reverse the condition, but it requires more knowledge than I have about how to set-up counters than what I have.

I'm guessing that it's something really simple.

Anyhow, I'd love to hear any more thought's. Thanks!

Dave

kuroneko · 2011-08-03 20:05

vanmunch wrote: »

The example is found on pg 121-130 in v 1.1 that you can download here:
http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PELabsFunBook-v1.1.pdf
and I'd like to reverse the condition, but it requires more knowledge than I have about how to set-up counters than what I have.

There seem to be examples all over the place. So I just use your OP as a guide and guess that you want something like a NEG detector.

CON
  pin = 16

PUB null | n

  ctra := constant(%0_01100_000 << 23 | pin)            ' monitor pin for low
  frqa := 1                                             ' system clock resolution

' If pin reads low then the counter is already busy counting.
' This doesn't matter as we have a specific starting point.

  phsa := 0                                             ' measurement starts now
  waitpne(0, |< pin, 0)                                 ' wait for pin to leave low state

  n := phsa                                             ' grab timer value (may need overhead
                                                        ' adjustment)

Would this work for you?

vanmunch · 2011-08-03 20:18

Hey Kuroneko,

Thanks for the suggestion/idea. I won't be able to try it out until later next week, but I'll let you know how it goes. Thanks again.

Dave

vanmunch · 2011-08-17 20:11

Hey Kuroneko,

I tried to get your code idea to work, but I think that I'm missing something. It wouldn't return a value. (I'm guessing that I'm missing something obvious.)
The code is:

CON

_clkmode = xtal1 + pll16x ' System clock → 80 MHz
_xinfreq = 5_000_000

CR = 13
pin = 0
OBJ

Debug: "FullDuplexSerialPlus" ' Use with Parallax Serial Terminal to
' display values

PUB null | n

Debug.Start(31, 30, 0, 57600)
waitcnt(clkfreq * 2 + cnt)
Debug.Str(String(13, "going "))

ctra := constant(%0_01100_000 << 23 | pin) ' monitor pin for low
frqa := 1 ' system clock resolution

waitcnt(clkfreq*3 + cnt)
' If pin reads low then the counter is already busy counting.
' This doesn't matter as we have a specific starting point.
Repeat
Debug.Str(String(13, "in program loop "))

phsa := 0 ' measurement starts now
waitpne(0, |< pin, 0) ' wait for pin to leave low state

n := phsa

Debug.Str(String(13, "n = "))
Debug.Dec(n)
waitcnt(clkfreq/2 + cnt)

I also tried to modify the RC decay example by switching it from POS to NEG detector, and I get a value, but it's always "770" so I have something wrong.
The code is:

CON

_clkmode = xtal1 + pll16x ' System clock → 80 MHz
_xinfreq = 5_000_000

CR = 13

OBJ

Debug: "FullDuplexSerialPlus" ' Use with Parallax Serial Terminal to
' display values

PUB Init

'Start serial communication, and wait 2 s for connection to Parallax Serial Terminal.

Debug.Start(31, 30, 0, 57600)
waitcnt(clkfreq * 2 + cnt)

' Configure counter module.

ctra[30..26] := %01100 ' Set mode to "Neg detector"
ctra[5..0] := 0 ' Set APIN to 0 (P0)
frqa := 1 ' Increment phsa by 1 for each clock tick

Debug.Str(String(13, "going "))
main ' Call the Main method

PUB Main | time

repeat

phsa~ ' Clear the phsa register
dira[0]~ ' Pin to input stops charging circuit
Debug.Str(String(13, "in program loop (presse button to stop counter)"))

time := phsa

' Display Result

Debug.Str(String(13, "time = "))
Debug.Dec(time)
waitcnt(clkfreq/2 + cnt)

Just to repeat my goal. I'd like to be able to start a counter and have it monitor a pin for me, watching for it to go from low to high. I'd like it to then give me a "time" value. It would be like having the program say "go" and then pushing a button to close the circuit. The program would then tell you how long it took you to read the message and push the button. This isn't my finial objective (stated in an earlier post), but the same idea.

Any help would be great. I appreciate everyone's time and suggestions.

Dave

kuroneko · 2011-08-17 20:56

vanmunch wrote: »

I tried to get your code idea to work, but I think that I'm missing something. It wouldn't return a value. (I'm guessing that I'm missing something obvious.)

Can you elaborate on wouldn't return a value? Do you mean it never prints out n? This would mean the waitpne never sees a high level on pin. I just grabbed the code you posted and added a fake trigger (second cog which sets pin high after a second). And I get repeated output of about 80M + overhead (80009737).

Ariba · 2011-08-17 21:01

I think something like that would do what you want:

PUB Main | time

 ctra := %01010<<26 + pin   'setup POSEDGE counter mode
 frqa := 1

 repeat

   'initate action here
   time := cnt              'remember start time

   phsa := 0                'wait for pos edge
   repeat until phsa > 0

   time := cnt - time       'calc delta time

   debug.str(string(13, "time = "))  'disp time
   debug.dec(time)

As long as you dont fill in code to initate your action, it will just measure the time between low to high changes on pin.
This is untested and you need to add the CON and OBJ section as in your examples above.

Andy

Counter question: how to set-up a counter to detect going from low to high?

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