Check out the "Enabled Subsystem". Put your MATLAB function in there and only enable it on your desired timesteps.
One way to do this is with another MATLAB function, that could look like the one below. The input t would be the output of a "Clock"-Block.
function y = fcn(t)
if t == 1.00 % one second
y = 1;
elseif t == 2.00 % two seconds
y = 1;
else
y = 0; % do not execute subsystem
end
end
If you do it like that please use a "Fixed-step" and an appropriate "fundamental sample time" so every desired timestep gets executed as a Simulation step. To have millisecond precision for example, use:
