How to loop exactly 1 second?

william chan

Hi,

I need a loop that updates the second and blinks one LED accurately.

My code

dira[noparse][[/noparse]led_pin]~~ 'Set I/O pin to output direction

repeat
rtcsecs++ ' Increment RTC time
if rtcsecs > 59
rtcsecs := 0
rtcmins++
!outa[noparse][[/noparse]led_pin] ' Toggle I/O Pin
waitcnt(clkfreq + cnt) ' Delay 1 sec cycles

I know this code will run a bit slower than real time.
How do I make it loop exactly 1 second per loop?

Thanks.

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Mike Green

The best way to do this is to use the system clock as a reference point as described in the manual:

time = cnt
repeat
   rtcsecs++
   ' ...
   waitcnt(time += clkfreq)

This way the WAITCNT always waits for exactly one second from the last wait regardless of what has to be done after the wait.

Post Edited (Mike Green) : 3/3/2008 4:06:23 AM GMT

Lawson

The trick for rock solid timing is in how you use Waitcnt. First, grab the value of CNT exactly once, then add a constant offset to it so that the time your code is going to wait for is still in the future once your code gets to the waitcnt. Add "mark := cnt + 2000" before the 'repeat' for example. Next, replace "waitcnt(clkfreq + cnt)" with "waitcnt(mark)". Finally add "mark += clkfreq" right after the waitcnt. With this setup Waitcnt is always waiting for a pre-calculated time to arrive and all code delays are rolled into the next wait.

I personally like separating the pacing section of a loop from the logic sections. This makes it easier to add other functions into the scan/timing loop. (a good reason to count 1/100th of a second [noparse]:)[/noparse] )

Marty

p.s. Mike posted a shorter way of doing the same thing above. He must've been programming is something other than spin recently 'cause ":=" is the assignment operator in spin, "=" is a conditional only

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Lunch cures all problems! have you had lunch?

Post Edited (Lawson) : 3/3/2008 4:05:17 AM GMT

Mike Green

Ah, caught me! I'd been doing some work in PBasic. The conditional though is "==".

Javalin

Or if you don't want to use a cog just to blink an LED....

Paul Baker

Well technically that still uses a cog (just doesn't spawn a new one). Good to see people starting to use the template.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Javalin

>Well technically that still uses a cog
Yes - just you can do other things too.

Perhaps I should of said "Or another option would be..." - my post sounds a bit like Mike bashing....

J

Beau Schwabe

You could get "fancy" and just set one of the Counters to oscillate at 1Hz.

That way you would have the COG free to do whatever else you wanted to do as long as you didn't use that counter for anything else.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

deSilva

It might also be helpful to study this example, alas posted in the wrong context some time ago:

'' Taking two (unused) timers to make a true long term clock (some 100 years)
'  As devised and re-devised "from time to time" by Ariba, deSilva and PhiPi 
CON
  commPin = 0      'unused Pin ?  

PUB main   | oldTimerValue

    setupTimer

    REPEAT
      IF PHSB<>oldTimerValue
         oldTimerValue  := PHSB

         ' do what you like here, or check the commPin for any sync needed

PRI setupTimer
' advantage: an LED at comPin can be used to show the second (even withou a resistor) 
 ctra := %00100<<26  + commPin       ' Set NCO mode i.e. toggle per 25 sec
 frqa := POSX/CLKFREQ*2              ' one second
 ctrb := %01010<<26  + commPin       ' set ctrb to POSEDGE detect
 frqb := 1
 dira[noparse][[/noparse]commPin] := 1

PRI setupAltTimer
' just to exercise some other modes
 ctra := %00110<<26  + commPin       ' Set duty cycle i.e. once per 50 sec
 frqa := POSX/CLKFREQ*2              ' one second
 ctrb := %01000<<26  + commPin       ' set ctrb to POS detect
 frqb := 1
 dira[noparse][[/noparse]commPin] := 1