There are several ways in which a Model Composer hardware co-simulation block can be synchronized with its associated FPGA hardware. In single-step clock mode, the FPGA is in effect clocked from Simulink, whereas in free-running clock mode, the FPGA runs off an internal clock, and is sampled asynchronously when Simulink wakes up the hardware co-simulation block.
In single-step clock mode, the hardware is kept in lock step with the software simulation. This is achieved by providing a single clock pulse (or some number of clock pulses if the FPGA is over-clocked with respect to the input/output rates) to the hardware for each simulation cycle. In this mode, the hardware co-simulation block is bit-true and cycle-true to the original model.
Because the hardware co-simulation block is in effect producing the clock signal for the FPGA hardware only when Simulink awakes it, the overhead associated with the rest of the Simulink model's simulation, and the communication overhead (e.g. bus latency) between Simulink and the FPGA board can significantly limit the performance achieved by the hardware. As long as the amount of computation inside the FPGA is significant with respect to the communication overhead (e.g. the amount of logic is large, or the hardware is significantly over-clocked), the hardware will provide significant simulation speed-up.
In free-running clock mode, the hardware runs asynchronously relative to the software simulation. Unlike the single-step clock mode, where Simulink effectively generates the FPGA clock, in free-running mode, the hardware clock runs continuously inside the FPGA itself. In this mode, simulation is not bit and cycle true to the original model, because Simulink is only sampling the internal state of the hardware at the times when Simulink awakes the hardware co-simulation block. The FPGA port I/O is no longer synchronized with events in Simulink. When an event occurs on a Simulink port, the value is either read from or written to the corresponding port in hardware at that time. However, since an unknown number of clock cycles have elapsed in hardware between port events, the current state of the hardware cannot be reconciled to the original Model Composer model. For many streaming applications, this is in fact highly desirable, as it allows the FPGA to work at full speed, synchronizing only periodically to Simulink.
In free-running mode, you must build explicit synchronization mechanisms into the Model Composer model. A simple example is a status register, exposed as an output port on the hardware co-simulation block, which is set in hardware when a condition is met. The rest of the Model Composer model can poll the status register to determine the state of the hardware.
Selecting the Clock Mode
Not every hardware board supports a free-running clock. However, for those that do, the parameters dialog box for the hardware co-simulation block provides a means to select the desired clocking mode. You may change the co-simulation clocking mode before simulation starts by selecting either the Single stepped or Free running radio button for Clock Source in the parameters dialog box.
For a description of a way to programmatically turn on or off a free-running clock using M-Hardware Cosim, see the description of the Run operation under in M-Hwcosim MATLAB Class.