# Timing Analyzer Clock Multiplexer Examples

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The Timing Analyzer makes it easy to use Synopsys® Design Constraint (SDC) commands to constrain complex clock structures, such as multiplexed clocks. The following shows three example circuits and the appropriate SDC commands to constrain them.

Figure 1. Shows a simple register-to-register circuit clocked by the clk port.

Assume that the clk port is driven by an off-chip multiplexer that selects between two clocks, one with a 10 ns period and one with an 8 ns period. The following SDC commands show how to assign multiple clocks to the clk port. It also shows how to add an exception indicating that the two clocks will never be active at the same time in the FPGA.

# Create the two clocks on the port
create_clock -name clk_100 -period 10 [get_ports clk]
create_clock -name clk_125 -period 8 [get_ports clk] -add
# Set the two clocks as exclusive clocks
set_clock_groups -exclusive -group {clk_100} -group {clk_125}

Figure 2. Shows a simple register-to-register circuit with a clock multiplexer on the FPGA, with two clock ports: clkA and clkB.

Assume that the clkA port is driven by a clock with a 10 ns period, and that the clkB port is driven by a clock with an 8 ns period. The following SDC commands show how to assign the clocks. This example is similar to the previous example, but the clocks are assigned to separate ports.

# Create a clock on each port
create_clock -name clk_100 -period 10 [get_ports clkA]
create_clock -name clk_125 -period 8 [get_ports clkB] -add
# Set the two clocks as exclusive clocks
set_clock_groups -exclusive -group {clk_100} -group {clk_125}

Figure 3. Shows a more complex clocking circuit with linked clock multiplexers on the FPGA.

In this case, you must use the set_clock_groups command to indicate that clocks A and D, A and B, C and D, and B and C, can never be active at the same time.

create_clock -name A -period 10 [get_ports clkA]
create_clock -name B -period 8 [get_ports clkB]
create_clock -name C -period 8 [get_ports clkC]
create_clock -name D -period 10 [get_ports clkD]
# cut paths between clocks
set_clock_groups -exclusive -group {A C} -group {B D}