software timed loops using wait (ms) and wait until next ms multiple - developer zone - national instruments.mht

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<H1>Software Timed Loops using Wait (ms) and Wait Until Next ms =
Multiple</H1>
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<DIV class=3Dratings>14 =E8=AF=84=E7=BA=A7 | <STRONG>4.50</STRONG> out =
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<P>This document will discuss using two VIs, <B>Wait (ms)</B> and =
<B>Wait Until=20
Next ms Multiple</B>, to give software timing to loops. This document =
assumes=20
that the LabVIEW RT Engine is the execution platform. In our discussion =
we will=20
assume that the VI is running in a time critical thread and that it is =
in the=20
only time critical loop in the system. For a complete discussion of =
priorities=20
and threads in LabVIEW RT, follow the link at the bottom of the page.=20
<BR><BR></P>
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<H3>=E7=9B=AE=E5=BD=95</H3>
<OL>
  <LI><A href=3D"http://zone.ni.com/devzone/cda/tut/p/id/4120#toc0">Wait =
Until=20
  Next ms Multiple</A>
  <LI><A =
href=3D"http://zone.ni.com/devzone/cda/tut/p/id/4120#toc1">Using Wait=20
  Until Next ms Multiple to Time a Loop in Time Critical Priority</A>
  <LI><A href=3D"http://zone.ni.com/devzone/cda/tut/p/id/4120#toc2">Wait =

  (ms)</A></LI></OL></DIV><A name=3Dtoc0></A>
<H2>Wait Until Next ms Multiple</A><BR></H2>
<DIV align=3Dcenter>
<P><IMG height=3D67 alt=3D""=20
src=3D"http://zone.ni.com/cms/images/devzone/tut/a/1b136ce2506.gif"=20
width=3D329></P></DIV><BR>When Wait Until Next ms Multiple is called, it =
will=20
return, or finish, when the millisecond timer value of the operating =
system is a=20
multiple of the <B>millisecond multiple</B> input. For instance, if the =
VI is=20
called with a <B>millisecond multiple</B> input of 10 ms, and the =
millisecond=20
timer value is 112 ms, then the VI will only wait 8 more millisecond, at =
which=20
time the millisecond timer value will be 120 ms, which is a multiple of =
10 ms.=20
Using this VI to time a loop means that the loop will be synchronized =
with the=20
operating system millisecond timer value multiples. If called in =
parallel with=20
other code in the same loop, the loop will have the period of =
<B>millisecond=20
multiple</B>. This will not be the case if the other code takes longer =
than the=20
<B>millisecond multiple</B>. <A name=3Dtoc1></A>
<H2>Using Wait Until Next ms Multiple to Time a Loop in Time Critical=20
Priority</A><BR></H2><BR>You must use this VI carefully in the time =
critical=20
priority of LabVIEW Real-Time. Remember that if any VI sleeps in a time =
critical=20
thread, then the entire thread sleeps. For this reason, you should =
control when=20
the <B>Wait Until Next ms Multiple</B> executes, rather than placing the =
VI in=20
parallel with other code. To understand why, let's look at an example =
control=20
loop. In this example, we will read an analog input and then write an =
analog=20
output. The desired timing of the system is shown in Figure 1. The =
vertical=20
arrows represent 100 millisecond multiples. <BR>
<DIV align=3Dcenter><IMG height=3D96 alt=3D""=20
src=3D"http://zone.ni.com/cms/images/devzone/tut/a/1b136ce2507.gif"=20
width=3D423><BR>Figure 1: Desired timing of control system</DIV>
<DIV align=3Dcenter>Our first attempt to solve this problem is a simple =
loop with=20
<B>Wait Until Next ms Multiple</B> (see Figure 2).<BR><IMG height=3D113 =
alt=3D""=20
src=3D"http://zone.ni.com/cms/images/devzone/tut/a/1b136ce2508.gif"=20
width=3D265><BR>Figure 2: First attempt at timed loop</DIV><BR>In this =
first=20
attempt, we do not dictate when in the sequence the <B>Wait Until Next =
ms=20
Multiple</B> executes. The behavior of this loop will differ depending =
on when=20
the <B>Wait Until Next ms Multiple</B> gets executed. If the <B>Wait =
Until Next=20
ms Multiple</B> executes first, then the analog input will be followed=20
immediately by the analog output. This behavior is usually desired, but =
is not=20
guaranteed in the example above. If the <B>Wait Until Next ms =
Multiple</B>=20
executes immediately after the analog input, then the analog output will =
not be=20
able to execute until the <B>Wait Until Next ms Multiple</B> is =
finished. Again,=20
this is because when any portion of a time critical priority VI sleeps, =
the=20
entire thread sleeps. The resulting timing would be as shown below, in =
Figure 3.=20
This behavior is usually not desired.=20
<DIV align=3Dcenter><IMG height=3D96 alt=3D""=20
src=3D"http://zone.ni.com/cms/images/devzone/tut/a/1b136ce2509.gif"=20
width=3D446><BR>Figure 3: Wait statement executes after =
AI<BR></DIV><BR>Instead of=20
placing the <B>Wait Until Next ms Multiple</B> in parallel with other =
code, we=20
can force it to execute in a specific sequence by using data flow as=20
demonstrated below. With the example below, the analog input will occur=20
immediately after the millisecond multiple, and the output will occur=20
immediately after that. <BR>
<DIV align=3Dcenter><IMG height=3D167 alt=3D""=20
src=3D"http://zone.ni.com/cms/images/devzone/tut/a/1b136ce2510.gif"=20
width=3D287><BR>Figure 4: Controlling the sequencing of events <A =
name=3Dtoc2></A>
<H2>Wait (ms)</A><BR></H2></DIV>
<DIV align=3Dcenter><IMG height=3D69 alt=3D""=20
src=3D"http://zone.ni.com/cms/images/devzone/tut/a/1b136ce2511.gif"=20
width=3D330><BR></DIV><BR>When <B>Wait (ms)</B> is called, it will =
return or=20
finish, after the time in milliseconds specified in the <B>milliseconds =
to=20
wait</B> input. For instance, if the millisecond timer value is 112 ms =
when the=20
VI is called, and <B>milliseconds to wait</B> equals 10, then the VI =
will finish=20
when the millisecond timer value equals 122 ms. If called in parallel =
with other=20
code, in priorities other than time critical, this means that the loop =
will have=20
the period of <B>milliseconds to wait</B> unless the other code takes =
longer=20
than the <B>milliseconds to wait</B>.<BR><BR>In LabVIEW Real-Time, it is =
not=20
recommended to use this VI in parallel with anything in time critical =
priority.=20
If the wait executes first, the whole thread will sleep until the =
<B>Wait=20
(ms)</B> is finished, and the code in parallel will not execute until =
the=20
<B>Wait (ms)</B> is finished. The resulting loop period will be the code =

execution time plus the <B>milliseconds to wait</B> time. <BR><BR>This =
VI does=20
not provide reliably consistent timing, even in normal priority VIs, =
since=20
jitter is accumulative. <BR><B>Related Links:</B><BR><A=20
href=3D"http://zone.ni.com/devzone/cda/tut/p/id/4324">Timing =
deterministic control=20
loops</A><BR><A=20
href=3D"http://zone.ni.com/devzone/cda/tut/p/id/4954">Understanding =
priorities in=20
LabVIEW Real-Time applications</A><BR>
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