roland Tollenaar wrote:
Hi
Xenomai kernel implements the so-called "FIFO" scheduling policy, that
is, the task which runs at any time is the task with the highest
priority that has been ready for the longest time. Calling
rt_task_wait_period suspends the calling task, and let other tasks
run.
If I understand correctly, what you want is to set some outputs at some
You understand correctly.
precise deadline, I see two ways to do what you want:
* either the time when you compute the output has no importance, and the
only thing that matters is when you set the output, in which case you
can call rt_task_wait_period right before setting the outputs, your
task will do
compute next output
go to sleep
set output
compute next output
go to sleep
set output
etc...
Actually this is exactly what I had in mind only it is important
(discrete state space controller computations) that the input is also
read at exact intervals. So maybe:
compute next output k+1
go to sleep
read input for k+2 computations
set output k+1
compute next output k+2
go to sleep
read input for k+3 computations
set output k+2
etc...
* or the time between reading input and setting output has to be as
fixed as possible, in which case what you will have to do is to
calibrate the WCET (worst case execution time) of the computations on
the machine you are running, setup rt_task_set_periodic so that your
task is woken up at (deadline - WCET), and in each loop, do the
computations, then busy wait until the deadline then set the
output. In order to busy wait, you may use rt_timer_tsc.
Off hand I am not exactly sure why this second technique would give me
a more fixed interval between reading input and writing output than
the modified technique 1 I suggest above?
Obviously one problem i have is the duration of obtaining input and
setting output. Both will be coming in and going out over a CANOpen
connection but from the moment the messages are on the bus I have no
control over what happens. Of course this has nothing to do with
xenomai but only with the deterministic characteristics of the CAN
protocol. And AFAIK that is not good?
What do you mean with a CANopen connection? CANopen is a protocol stack
which must run on your system to communicate with CANopen nodes through
the CAN bus.
Wolfgang.
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