Experimental Physics and
Industrial Control System

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<== Date ==>		<== Thread ==>

Subject:	Re: timer delay compensation
From:	Eric Norum <[email protected]>
To:	Andrew Johnson <[email protected]>
Cc:	EPICS core-talk <[email protected]>
Date:	Thu, 14 Jun 2012 16:21:02 -0700

On Jun 14, 2012, at 3:52 PM, Andrew Johnson wrote:

……...

Lets go with 0.999999999, chosen since we only represent time down to the
nanosecond anyway.

The units of the addition are 'ticks' so 9 9's is actually a fraction of a nanosecond (likely 1/50, 1/60 or 1/100).

But even so the '1 extra tick' problem wouldn't show up until around 8 million ticks (10^9 ticks ~= 30 bits, 53 bit double mantissa) so 9 9's it is.

Here's the RTEMS implementation:

void

epicsThreadSleep (double seconds)

{

rtems_status_code sc;

rtems_interval delay;

extern double rtemsTicksPerSecond_double;

if (seconds <= 0.0) {

delay = 0;

}

else {

* Low-overhead approximation of ceil()

delay = seconds * rtemsTicksPerSecond_double + 0.999999999;

}

sc = rtems_task_wake_after (delay);

if(sc != RTEMS_SUCCESSFUL)

errlogPrintf("epicsThreadSleep: %s\n", rtems_status_text (sc));

}

References:: timer delay compensation Hill, Jeff; Re: timer delay compensation Andrew Johnson

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ANJ, 26 Nov 2012