It is best to let Simulink keep track of time for you, and try to interface with your camera when Simulink needs a new frame of data. Often, interfacing with real hardware requires that you run your simulation in pseudo-real-time or real-time depending on the application. This can be done only if the simulation currently runs faster than real time.
You can use an S-function to accomplish both goals of pacing the simulation and interfacing to your camera hardware. At the beginning of the simulation when tsim=0, capture the real time from your system treal0. With each step of the S-function you can pause just long enough to let treal catch up with tsim. For each step of the simulation you will need to pause until treal-(tsim+treal0) is greater than or equal to 0.
The benefit of using the S-function for your camera interface too is that it will ensure you synchronize captures at the right times during the simulation.
If all you need to do is slow the simulation down to real-time, the Simulation Pace block from the Aeropsace Blockset provides this capability.
You might also want to search on the MATLAB Central File Exchange for real-time pace. There are at least two submissions that show promise in this area.
