Simulink : How to implement a dynamic frequency clock?

Hi everybody! I have a problem with simulink, I have to control a stepper motor with an output signal of my simulink block. The stepper motor will move of one step for each pulse of my signal. In order to control the speed of the motor I would like to dynamicaly control the frequency of these pulses by implementing a "dynamic frequency clock". It seems really simple but I don't manage to do it... I tried to use the Clock block with a variable as parameter, but I don't manage to change the value of this variable dynamicaly... Has anyone a solution for me? Thanks