I want my code conditions checked every 1/2000th of a second, how can I do that?

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Aaqib Ahmad 2024-8-3

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编辑： dpb 2024-8-4

I have multiple if conditions that is being checked to provide pulse and gate signals to switches I want that these conditions be checked every 1/2000th of a second. I tried for loop and tic toc methods but the condition is being checked at a frequency of 200kHz. I am not able to solve this issue kindly help

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dpb 2024-8-3

编辑：dpb 2024-8-3

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Is based on internal CPU clock and isn't interrput driven so will be variable; user events and OS task-switching, etc., will still take place and on occasion, cause quite a bit of system latency.

If you really need anything more than an approximation, you'll probably have to use the Instrumentation TB and an external interrupt-driven board; I've never tried to do real time stuff w/ MATLAB, in lieu of alternate instrumentation solutions.

But, a MATLAB-only try would look something like

WAIT_MSEC=0.5;                      % the desired time delay, msed
PAUSE_SEC=WAIT_MSEC/1000;           % convert to sec for pause()
continueFlag=true;                  % make it go...
while(continueFlag)
  values=getData(someInputSource);  % whatever returns your data
  caseValue=values(1);              % whichever is the decision one, or other variable
  switch caseValue
    case 0                          % an out; read something or however to stop      
      continueFlag=false;           % call any cleanup routine, set exit flag
    case 1
    case 2
    ...
  end
  pause(PAUSE_SEC)                  % wait for the desired time; may have to adjust...
end

You can add in a toc call at the bottom of the loop and keep a running total of elapsed times and instrument to tell you min, max, average, etc., to see if is at all feasible. I wouldn't have too high of hopes, however, at these rates; slow "real" time apps can be reasonably consistent with such techniques; the variability and relative accuracy degrades quickly with shorter time intervals. If the machine can be left totally alone while it's running, that can help by eliminatng user interaction responses, of course.

Aaqib Ahmad 2024-8-3

@dpb dear sir/ma'am the code you provided is for case statements, will it work in my code where I have only if conditions that I am checking?

dpb 2024-8-4

Sure, just replace the guts with yours; it is phantom code to illustrate the structure of the loop...but, depending upon the tests, you might consider replacing a bunch of "if...elseif...elseif...else...end" with a switch construct. Just depends on details you've not provided...

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Answer 1

Walter Roberson 2024-8-3

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You will not be able to pause() more accurately than 1/100 of a second on Windows. (On Mac it is about 1/1000 of a second.)

Therefore you will need to "busy-wait"

start_time = tic();
while toc(start_time) < 1/2000
  %idle
end

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dpb 2024-8-3

编辑：dpb 2024-8-4

在 MATLAB Online 中打开

untitled.jpg

N=1000;
i=0; 
t=zeros(N,1);
WAIT_MSEC=0.5;                      % the desired time delay, msec
PAUSE_SEC=WAIT_MSEC/1000;           % convert to sec for pause()
continueFlag=true;                  % make it go...
while(continueFlag)
  t0=tic;
  pause(PAUSE_SEC)                  % wait for the desired time; may have to adjust...
  i=i+1; t(i)=toc(t0); 
  continueFlag=(i<N);               % make it stop...
end
t=t*1000;                           % back to msec for display...
[min(t) max(t) mean(t)]
ans =
    0.5019    0.8487    0.5072
histogram(t)
hAx=gca; hAx.YScale='log'; hAx.YLim=[0.1 N]; grid on

does reasonably well although as is known, it isn't reliable...nor, of course, will be the polled waiting solution...

I was thinking I read somewhere (Yair, maybe???) quite some time ago that TMW changed over to using the Windows high resolution timer...seems to be the case.

It appears can go to about 50 usec resolution ...for it I got

t=t*1000;[min(t) max(t) mean(t)]
ans =
    0.0518    0.7194    0.0551

for N=10000. The most frequent however, was in the third histogram bin and the two most prevalent were the 2nd and 3rd, about 36% and 52%, respectively. And, of course, the loop has no real work in it...

ADDENDUM:

The above is a delay after the code runs; if the intent is to have the code execute on the 0.005 cycle, then the delay time must be shortened by the average run time of the functional code; that, of course, then means the difference in execution time between branches also adds jitter.

All in all, unless "on or about" is good enough, it needs a hardware interrupt scheme...

dpb 2024-8-3

在 MATLAB Online 中打开

With my results above, I thought "just maybe!" and tried

PAUSE_SEC=2/1000;
hT=timer('period',PAUSE_SEC,'executionmode','fixedRate','timerfcn',@timercallback)
hT = 
   Timer Object: timer-1

   Timer Settings
      ExecutionMode: fixedRate
             Period: 0.002
           BusyMode: drop
            Running: off

   Callbacks
           TimerFcn: @timercallback
           ErrorFcn: ''
           StartFcn: ''
            StopFcn: ''

Unfortunately, whether MATLAB could physically do it or not, it's been neutered... the above (per documentation, just hoping maybe it would/could do more)..

That's interesting; on Windows (R2021b) the above results in

Warning: Period property is limited to 1 millisecond precision. Sub-millisecond precision will be ignored. 
> In timer/set.Period (line 365)
In timer (line 159) 

So, can we believe the non-Windows platform??? That won't help @Aaqib Ahmad, probably, even if so, but would be interesting to know.

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I want my code conditions checked every 1/2000th of a second, how can I do that?

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