Add warning about Control task timer source in Blinky LED example

[pierre-haessig]

Hello OwnTech,

In the Blinky LED example, it is suggested that a critical task can be launched by uncommenting the following two lines, see
https://github.com/owntech-foundation/examples/blob/main/SPIN/LED/blinky/main.cpp#L65

task.createCritical(loop_critical_task, 500);
task.startCritical();

However, this is in contradiction with the way the critical task works (thanks @jalinei for the link): https://docs.owntech.org/latest/core/docs/task_introduction/#the-control-task-sources. This only works if the PWM timer is started, which is not the case in the Blinky LED example.

I suggest to:

1. add a link that doc in the source ode
2. make the timer source explicit in this example

In the longer run, it may be good to make the timer source explicit in most examples.

[jalinei]

Hello Pierre, indeed this make complete sense.

I think this problem derive from the fact that earlier the default value for control task was timer6, independant from the PWMs. We later changed that default behavior, because in most case you want your control task to be PWM synchronous.

owntech-foundation/Core#32

We also want to change the naming of the parameter to something more hardware agnostic, PWM and PWM_independant.

[pierre-haessig]

Thanks for the pointer to Core issue. I've replied there.

Perhaps in the end, it's not a good idea to have a default value?