Fixed step call to mex file and continous block

Dear all,

I have a mex function which MUST be called exactly every simulated millisecond.

This mex block is instantiated in a simulink model, whcih has continues blocks (like 1/s, PT1) ...

If I select fixed step solver, then the the whole simulink model is fix step and I get a error, that continues functions are used.

If I select euler1 solver or other one, how do I guaranty that mex function is call exactly every milisecond. (exactly means, for example 1ms +/- certain N nanosec)

What is the best approach to have a fix type mex inside of continues model.

Thank you!