ID 683068
Date 9/24/2018
Public

## 2.3.4.5.1. Exclusive Clock Groups (-exclusive)

Use the -exclusive option to declare that two clocks are mutually exclusive. You can declare clocks as mutually exclusive if you define multiple clocks are for the same node. This case occurs for multiplexed clocks.

For example, a input port may be clocked by either a 25-MHz or a 50-MHz clock. To constrain this port, create two clocks on the port, and then create clock groups that declare that the clocks do not coexist in the design. Declaring the clocks as mutually exclusive eliminates clock transfers that are derived between the 25-MHz clock and the 50-MHz clock.

Figure 51. Clock Mux with Synchronous Path Across the Mux
create_clock -period 40 -name clk_a [get_ports {port_a}]
create_clock -add -period 20 -name clk_b [get_ports {clk_a}]
set_clock_groups -exclusive -group {clk_a} -group {clk_b}