Black Box Clocking - Black Box Clocking - 2026.1 English - UG1483

To import a multirate module, you must tell Vitis Model Composer information about the module's clocking in the configuration M-function. Model Composer treats clock and clock enables differently than other types of ports. A clock port on an imported module must always be accompanied by a clock enable port (and vice versa). In other words, you must define clock and clock enables as a pair, and they must exist as a pair in the imported module. This is true for both single rate and multirate designs.

Although clock and clock enables must exist as pairs, Model Composer drives all clock ports on your imported module with the FPGA system clock. The FPGA system clock generates clock enable signals that drive the clock enable ports.

SysgenBlockDescriptor provides a method, addClkCEPair, which allows you to define clock and clock enable information for a black box. This method accepts three parameters. The first parameter defines the name of the clock port (as it appears in the module). The second parameter defines the name of the clock enable port (also as it appears in the module).

The port names of a clock and clock enable pair must follow the naming conventions provided below:

• The clock port must contain the substring clk
• The clock enable must contain the substring ce
• The strings containing the substrings clk and ce must be the same (for example, my_clk_1 and my_ce_1).

The third parameter defines the rate relationship between the clock and the clock enable port. The rate parameter is not a Simulink sample rate. Instead, this parameter tells Model Composer the relationship between the clock sample period, and the desired clock enable sample period. The rate parameter is an integer value that defines the ratio between the clock rate and the corresponding clock enable rate.

For example, assume you have a clock enable port named ce_3 that requires a period three times larger than the system clock period. The following function call establishes this clock enable port:

addClkCEPair('clk_3','ce_3',3); 

When Model Composer compiles a black box into hardware, it produces the appropriate clock enable signals for your module, and automatically wires them up to the appropriate clock enable ports.