Constant (CON) Declaration question

RobertW

Can anyone explain why I get an error message with the following code:

CON
· ScanTime = (clkfreq / 100) + cnt··············· '10 millisecs scan time

I am looking to set a constant for a delay that I can use in different parts of my program.· This way I would not have to retype 'clkfreq / 100 + cnt' everytime.· Is this possible?· Thanks.

-Rob W.

BradC

RobertW said...
Can anyone explain why I get an error message with the following code:


CON

ScanTime = (clkfreq / 100) + cnt '10 millisecs scan time



I am looking to set a constant for a delay that I can use in different parts of my program. This way I would not have to retype 'clkfreq / 100 + cnt' everytime. Is this possible? Thanks.



-Rob W.

cnt is a register in the chip, not a constant (in that context anyway). clkfreq is a value stored by the compiler in location $0 of the hub ram. The interpreter inserts this value when you use the "clkfreq" command. Although in theory it should be available at compile time, none of the current compilers are geared to make that happen.

Every call you use that value you will need the current cnt value. To speed things up you could define a long in VAR and assign that the value of clkfreq / 100 when your program starts up. Every time you want to use that you'd use WAITCNT(cnt + ScanTime).

Despite almost every example people post to the forum where it's "some calculation" + cnt, you should really put "cnt" as the first value inside the brackets. This just makes your time calculation that little bit more accurate as the cnt value gets pushed onto the stack prior to any of the other calculations taking place.

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Life may be "too short", but it's the longest thing we ever do.

Phil Pilgrim (PhiPi)

Neither clkfreq nor cnt are constants. clkfreq is a hub location that's loaded with the clock frequency when the Propeller resets. cnt is a register that increments at the clock frequency's rate. If you know ahead of time that the clock frequency will be, say 80 MHz, you can always do:

CON

  ScanTime = 80_000_000 / 100

But you still have to do your delays with something like this:

  waitcnt(ScanTime + cnt) [i]...or...[/i]
  waitcnt(time += ScanTime) [i]where [b]time[/b] gets initialized from [b]cnt[/b] at some point.[/i]

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 1/20/2010 3:49:18 AM GMT

Kye

Use functions.

PUB pauseForSeconds(number)
 
  result := cnt 
 
  repeat (number #> 0)
    result += clkfreq
    waitcnt(result)  
 
PUB pauseForMilliseconds(number) 
 
  result := cnt
  
  repeat (number #> 0)
    result += (clkfreq / 1000)
    waitcnt(result)      

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Nyamekye,

JonnyMac

I agree with Kye, but don't perform the math once and before the loop -- like this:

pub pause(ms) | tix, base

  tix := clkfreq / 1_000
  base := cnt
  repeat ms
    waitcnt(base += tix)

In actual fact I tend to create delay constants as Phil suggested above, based on my XIN and PLL settings.

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Jon McPhalen
Hollywood, CA

RobertW

Thanks guys. I will give it a try this evening.
-Rob