Timer Interval Is No Accurate! Why???

[qazwsx123]

Hi Every One.
As I know 1 Sec. is 1000 milliseconds so in the code blow the final number in label must be 1000:

Dim a As Integer

Private Sub Form_Load()
a = 0
Timer1.Interval = 1
Timer2.Interval = 1000
End Sub

Private Sub Timer1_Timer()
a = a + 1
End Sub

Private Sub Timer2_Timer()
Timer1.Interval = 0
Label1.Caption = a
End Sub

But the label shows 129 !!!!!!
Why???
Can any one tell me?

[dilettante]

I don't think you understand what a Timer control is.

They don't have anything to do with real-time intervals. They are "wake me up no sooner than" intervals and these are also low-priority events that can be discarded if you code isn't sleeping on its messagewait when they arrive.

Values below 64ms actually trigger at varying intervals (by machine) from 16ms to 64 ms. Any value below 10ms is first taken as 10ms (USER_TIMER_MINIMUM).

SetTimer function

[Tech99]

Hi Every One.
As I know 1 Sec. is 1000 milliseconds so in the code blow the final number in label must be 1000:
...
But the label shows 129 !!!!!!
Why???
Can any one tell me?

Timing with timer is not deterministic - so to measure events accurately, you must use other methods than timer.

https://msdn.microsoft.com/en-us/lib...or=-2147217396

Even if timer allows one to set 1 millisecond interval does not mean it can fire within millisecond intervals - timer can fire 1/18 Hz intervals - so the minimum interval is about 55-56 milliseconds and non deterministic. In another words, firing interval could be anything from 1 to 56 milliseconds.

[qazwsx123]

Thanks a lot.
Now how can I use real-time intervals in my program?

[Tech99]

In terms of real-time intervals, do you mean hard deterministic real-time** or 'pseudo' real time? Former is possible only with added real-time kernel, latter is possible by using multimedia timer or using queryperformance counter.

** fex...
https://en.wikipedia.org/wiki/RTX_(operating_system)
https://msdn.microsoft.com/en-US/lib...bedded.5).aspx
https://en.wikipedia.org/wiki/INtime

[incidentals]

what do you want the intervals for?

may seem a stupid question

here to help

[dilettante]

Real-time timing can be a chore even in C on an Arduino.

See the video Julian Investigates: How Slow is Arduino? which covers that topic a little.

I'm not sure whether he quite understands that a C statement is not a machine instruction, but then he is an electrician and no computer scientist.

[Tech99]

Here is some measurement. Event timing - ordinary 'std' Win32 assembler code ie. without realtime kernel additions.

http://s241.photobucket.com/user/205...mance.jpg.html
http://s241.photobucket.com/user/205...ance2.jpg.html

[The trick]

Option Explicit
 
Private Declare Function QueryPerformanceFrequencyAny _
                         Lib "kernel32" _
                         Alias "QueryPerformanceFrequency" ( _
                         ByRef lpFrequency As Any) As Long
Private Declare Function QueryPerformanceCounterAny _
                         Lib "kernel32" _
                         Alias "QueryPerformanceCounter" ( _
                         ByRef lpPerformanceCount As Any) As Long
 
Dim i As Long
Dim f As Boolean
 
Private Sub Form_Load()
    Dim SampleRate As Currency
    Dim Freq As Currency
    Dim Ct1 As Currency, Ct2 As Currency
    Dim Num As Currency, i As Long
    Dim Refr As Long
    
    Me.Show
    
    Freq = 1000                         ' Set frequency 1 kHz
    QueryPerformanceFrequencyAny SampleRate
    SampleRate = SampleRate * 10000     ' Get high-performance counter frequency
    Num = SampleRate / Freq / 10000     ' Calculate number of tick per period
    Refr = Freq / 5                     ' Refresh frequency 5 Гц
    
    QueryPerformanceCounterAny Ct1      ' Get current counter
    
    Do
        QueryPerformanceCounterAny Ct2  ' Get current counter
        If Ct2 - Ct1 >= Num Then        ' If number of tick greater
            i = i + 1
            ' ---------------------------------------
            ' This code is performed with needed frequency
            ' ---------------------------------------
            If i Mod Refr = 0 Then       ' Refresh
                Me.Caption = i
                DoEvents
            End If
            Ct1 = Ct2 + ((Ct2 - Ct1) - Num)
        End If
    Loop Until f
End Sub

Private Sub Form_Unload(Cancel As Integer)
    f = True
End Sub

Anyway it is "dirty impulses", because period isn't same and depends on loading the system. However overall this is stable impulses.

[dilettante]

Yes, there are other counters and timers such as the multimedia timers too. They all have different issues and limitations because they serve different purposes. Without knowing the actual goal specific recommendations are difficult to make.