Generated clocks are driven inside the design by special cells called Clock Modifying Blocks (for example, an MMCM), or by some user logic.
Generated clocks are associated with a master clock. The
create_generated_clock command considers the start point of the
master clock. The master clock can be a primary clock or another generated clock.
Generated clock properties are directly derived from their master clock. Instead of specifying their period or waveform, you must describe how the modifying circuitry transforms the master clock.
The relationship between a master clock and a generated clock can be any of the following:
- A simple frequency division
- A simple frequency multiplication
- A combination of a frequency multiplication and division in order to obtain a non-integral ratio (usually done by MMCM and PLL)
- A phase shift or a waveform inversion
- A duty cycle transformation
- A combination of all the above
Recommended: Define all primary clocks first. They are
needed for defining the generated clocks.
Note: To compute the latency for the generated clock, the
tool traces both sequential and combinational paths between the source pin of the
generated clock and the source pin of the master clock. In some cases, it might be
desirable to only trace through combinational paths to calculate the generated clock
latency. You can do this using the
-combinational command line
option.