Sebastien GAUME wrote:
>
> Hello,
>
> I want to check how accurate the functions rt_get_time() of RTAI and
> gethrtime() of RTLinux are, and to draw a comparison between these two
> functions. To do so, I connected two PCs through a parallel wire. On
> each PC, one of the two RT systems is running.
>
> One PC is considered as a sender. A periodic task wakes up every
> millisecond, gets time, writes on the parallel port. The other PC is
> considered as a receiver. It's waiting for an interrupt coming from the
> 'sender' PC. Each time an interrupt occures, the associated handler
> wakes up and gets time.
>
> After a quick glince at the different measured times (those of the
> sender, and those of the receiver), I discovered that :
> - the periodic task (that is to say the sender) wakes up every
> millisecond, as expected ;
How was this verified?
> - the handler (the receiver) does not wake up every millisecond !
> There's a drift. The first measured times are OK, not so far from those
> of the sender, but after a while, it's not good anymore.
>
> The results remain the same, whatever the RT system is used for the
> sender or the receiver. I tried the four possibilities, and each time
> there was a drift. Thus, when the receiver is running under RTAI, it's
> an increasing drift, whereas it's a decreasing one when the receiver is
> running under RTLinux.
>
> I would greatly appreciate your help.
>
> Sebastien
>
Do you mean here that the elapsed time as measured in the receiver is not one
millisecond
and that the sum of the deviation from the expected 1 MS period accumulates to a
positive
sum of errors when RTAI is the receiver and accumulates a negative sum of errors when
rtl
is the receiver (respondent)? Or does the receiver initially have no error and after a
while start having a consistent error?
Wayne
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