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Message-ID: <3A4066E3.6811305B@esg-gmbh.de>
Date: Wed, 20 Dec 2000 08:59:31 +0100
From: Martin Gangkofer <mgangkof@esg-gmbh.de>
Organization: ESG - Elektroniksystem- und Logistik- GmbH, Muenchen
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Subject: Re: Using "delay until" in real-time
References: <915jl7$jt5$1@nnrp1.deja.com> <3A3F133E.4C13077C@esg-gmbh.de>
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Ted Dennison schrieb:

> In article <3A3F133E.4C13077C@esg-gmbh.de>,
>   Martin Gangkofer <mgangkof@esg-gmbh.de> wrote:
>
> > translate into English like this: "Between the blind, the one-eyed is
> > the king". Well closing one eye I see a solution for your problem ..
>
> I've heard it in English as "In the land of the blind, the one-eyed man
> is king."
>
> (
> > Use integer arithmetics, which is 100% accurate to calculate next
> > time:
> >     NextTime  =  StartTime + ( 1/f ) * n  =  StartTime + ( n * dx ) /
> >     dy;
>
> Something pretty much like that has been suggested. My problem with it
> is that "n" is a counter that has to be monotonically increasing. That
> means that it will eventually hit an archetectural limit. Of course on
> my 32-bit machine at 240Hz that limit won't happen until the machine has
> been running for about 1/3 of a year, but the limit is there.

The solution is to increment the start time:

task body executive is
    function "+"( L : Ada.Real_Time.Time; R : Ada.Real_Time.Time_Span )
        return Ada.Real_Time.Time renames Ada.Real_Time."+";

    Period_Enumerator : constant Some_Integer_Type := 1;
    Period_Denominator : constant Some_Integer_Type := 60;
    Limit : constant Some_Integer_Type := Some_Integer_Type'Last;

    Start_Time : Ada.Real_Time.Time;
    Next_Time : Ada.Real_Time.Time;
    Cycles_Counter : Some_Integer_Type := 0;


    function Increment( Step : in Some_Integer_Type ) return
Ada.Real_Time.Time_Span is
        use Ada.Real_Time;

        -- use float type to avoid constraint error due to "*"
        -- single precision float should be sufficient, error is O( 1/10 )
usec
        -- if you need higher precision scheduling (???) take long float
        Val : constant Float := Float( Period_Enumerator * Step )
                / Float( Period_Denominator);
    begin
        return To_Time_Span( Duration( Val ) );
    end Increment;

begin

    -- startup initialisation, sync, etc.

    Start_Time := Ada.Real_Time.Clock;
    loop
        if ( Cycles_Counter < Limit ) then
            Cycles_Counter := Cycles_Counter + 1;
        else
            Cycles_Counter := 0;
            Start_Time := Start_Time + Increment( Limit );
        end if;


        -- do_your_job;


        Next_Time := Start_Time + Increment( Cycles_Counter );
        delay until Next_Time;
    end loop;

end Executive;

I did'nt compile it, but it should work.

Martin Gangkofer