Digital hardware is best modeled in Simulink using the Fixed-step Discrete solver. This warning is informing you that a) you are using a variable-step solver, and b) that you have not specified any discrete rates anywhere in the system. The solution is to change the solver used by Simulink. The solver setting is found in the configuration parameters dialog. Alternatively, you can run:
>> hdlsetup('modelname')
This will set the solver appropriately, as well as set a number of other global model settings to ensure best compatibility for HDL Coder.
