Trigger function with precission

Hi all,

I want to execute a function every 200 ms in my script. The precission of the trigger is very important in this case since I'm acquiring samples from a Bluetooth device. I'm currently using the following timer to trigger the function:

    my_timer = timer('ExecutionMode','fixedSpacing','Period',0.2,'BusyMode','queue','TimerFcn',{@my_function_callback});

the issue here is, when I execute my_timer.AveragePeriod, it yields 0.2312 s and not 0.200 s (what I was expecting). I'd like to know why the timer is not triggering every 200 ms and if it depends on the tasks my script is performing during the timers period.

Thanks in advance,

Edu

Answer 1

[Guillaume]

Mathworks doesn't document how their timer is implemented, therefore it's impossible to tell what the actual resolution and potential jitter is. On Windows, the standard resolution for timers is 15.6 ms. If matlab uses these timers, then you're certainly not going to get better than that and I wouldn't be suprised if your 30 ms error came from that.

The fact that the timers period is specified in seconds is a strong hint that they can't achieve ms accuracy. If you do need that sort of accuracy you may have to use something else than matlab.

[Lask]

Hey Guillaume, thanks for answering! The fact is, the following text is taken from Matlab documentation on timers:

'Period': Number greater than 0.001 that specifies the delay, in seconds, between executions of TimerFcn

so I expect Matlab precission on triggering the timer to be 1 ms.

[Guillaume]

Achieving 1 ms accuracy for timers on Windows is very hard and I doubt that mathworks have done the effort to get that with their timer.

Windows is not a real-time OS, therefore you never have any guarantee that a timer will execute on time. You can be pretty sure that every now and then, it'll miss a tick or two. As I said, the standard tick resolution is ~15 ms, so it's not surprise that you get an error of ~30 ms.

If you want an accuracy of 1 ms, you may have to switch to a real time OS. Failing that, you'll have to live with the jitter, measure the actual period and correct for it.

[Stephen23]

https://docs.microsoft.com/en-us/windows-hardware/drivers/kernel/high-resolution-timers