Mauricio � wrote: > On Sat, Apr 12, 2008 at 01:08:37PM -0700, mgross wrote: >> On Fri, Apr 04, 2008 at 03:57:53AM -0400, Mauricio A. Araya L�pez wrote: >>> Hi all, >>> >>> I've been searching on the web (and in the rt-wiki) for the current >>> status of the real-time support for devices. For instance >>> the serial port, CAN Bus, etc. >>> > [...] > >> FWIW I was able to get +/- 30usec jitter from a serial port experiment >> where I programmed a PIC to push 5 bytes over the serial line every >> 20ms. > My results are similar. A 30 usec jitter it seems to be the limit, > but indeed the jitter is less, because the latency is near the > 10 usec. But, that is "on average", because I've run several > test during long time periods, and my worst jitter is like 5 msec!, > which is too much. Maybe it is a hardware problem, but I am using > no swap, no usb and no DMA, and I dont know which other thing > i should disable to not have this huge random jitter. > (Obviosly I am locking the memory to have no paging) > >> I had a blocking read from a FIFO pthread that would wake up >> and call gettimeofday, which I used to measure the jitter. > > I have the same idea, but using clock_gettime which use timespec > (nanoseconds) like nanosleep. > >> It was >> very impressive. I'm pretty confident that *if* you use a hardware >> serial port (no-USB) you will be able to implement effective PID >> controls with a real time pthread program running on a RT-preempt >> kernel. > > In summary, i am reading from other computer the data sent by a serial > port, and almost always I have a 30-40 usec jitter, but sometimes > I get a single point where the jitter is like 1-5 msec. The strange > thing is that the sender shows perfect timing (less that 30 usec), > but the reader get these strange jumps. I am downloading the last > kernel to check the serial port driver code: the patch -rt do something > with the serial port? or generally some hardware related pacth? Just to be sure, the interrupt thread of your serial port is running at a higher priority than the others. Wolfgang. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
