SIMULINK error: Unable to determine a fixed step size based on the sample times in the model

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I get the following error while running my SIMULINK model (closed-loop control feedback):
'Unable to determine a fixed step size based on the sample times in the model 'normal_force_input', because the model does not have any discrete sample times. Picking a fixed step size of (12.0) based on simulation start and stop times...
In Solver pane, here are the settings:
Start time: 0.0 Stop time: 10.0
Type: Fixed-step Solver: ODE3 (Boga...)
Fixed step size(fundamental sample time): auto
periodic sample time constraint: unconstrained
Tasking mode ...: auto
Automatically handle ....: unchecked
Higher priority: unchecked
What am I missing? Please help. Thank you.

Accepted Answer

ES on 26 Jun 2014
Immediate fix is to choose a variable step solver(in the solver type) if you really don't need a fixed step solver. If you intend to use a fixed type solver, then all the subsystems should have a sample time that is multiple of other subsystems. That is, an Highest Common Factor (HCF) should exist between the sample times of all subsystems. Else make the subsystem inherit the sample time by putting -1 in the Subsystem Properties. More on Solvers here

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