Re: [Xenomai-help] timing within a task and assessing the timing of CAN messages

[Wolfgang Grandegger <wg@domain.hid>]

Roland Tollenaar wrote:
> Hello,
> 
> I have three questions I would like to pose here.
> 
> 1-Is it possible to subdivide a task to control at what time a certain 
> command is executed within the task? Example: I have a 1ms task. I send 
> a CAN sync message at the beginning of the task but I would like to wait 
> about 300us before performing a read to ensure that all the messages 
> emitted as a result of this sync have come in. Now I will have an odd 
> 700us to use the data that has come in to calculate outputs. However I 
> want the output to be put out as close as possible to the end of the 
> task time. Is there an elegant manner to delay the read and the write to 
> give me this kind of control within a taks. If not any suggestions as to 
> how I should tackle this.

The task timing services allow to suspend and then resume the task at a 
defined time, either by specifying a releative delay or an absolute 
time, e.g. rt_task_sleep() and rt_task_sleep_until() for the native skin.

> 
> 2-For development I am still using the PEAK CAN dongle. We have 
> establisched that the ISR responding to an incoming message interrupt 
> takes 200us to complete. Is anything known about how long it takes 
> before a message that has been sent is put on the bus? How does this 
> happen? Reason I ask is to know what kind of variance I can expect in 
> time between the moment I send the output values and the moment it is on 
> the bus. If there are routines between those to points that allow 
> themselves to be pre-empted or for some other reason have varying 
> execution times my control performance will be badly degraded.

Reading, but also writing a byte to the port register task time. I 
think, that setting up a message for transmit takes as much time as 
reading it. Writing is also done with interrupts disabled.

> 3-How do I interpret the relative time stamps that can be made visible 
> with rtcanrev ? IS the time indicated the time between each message and 
> the last time the message was posted? Or between the message and its 
> direct predecessor?

The latter one if the option "-R" was used:

   $ rtcanrecv -h
   ...
	-T, --timestamp       with absolute timestamp
	-R, --timestamp-rel   with relative timestamp

Note that the timestamp is added in the ISR.

Wolfgang