18.104.22.168. Deriving PLL Clocks
Create Base Clock for PLL input Clock Ports
If your design contains transceivers, LVDS transmitters, or LVDS receivers, use the derive_pll_clocks to constrain this logic in your design and create timing exceptions for those blocks.
create_clock -period 10.0 -name fpga_sys_clk [get_ports fpga_sys_clk] derive_pll_clocks
Include the derive_pll_clocks command in your .sdc file after any create_clock command. Each time the Timing Analyzer reads the .sdc file, the appropriate generated clock is created for each PLL output clock pin. If a clock exists on a PLL output before running derive_pll_clocks, the pre-existing clock has precedence, and an auto-generated clock is not created for that PLL output.
The following shows a simple PLL design with a register-to-register path:
The Timing Analyzer generates messages like the following example when you use the derive_pll_clocks command to constrain the PLL.
derive_pll_clocks Command Messages
Info: Info: Deriving PLL Clocks: Info: create_generated_clock -source pll_inst|altpll_component|pll|inclk -divide_by 2 -name pll_inst|altpll_component|pll|clk pll_inst|altpll_component|pll|clk Info:
The input clock pin of the PLL is the node pll_inst|altpll_component|pll|inclk which is the -source option. The name of the output clock of the PLL is the PLL output clock node, pll_inst|altpll_component|pll|clk.
If the PLL is in clock switchover mode, multiple clocks generate for the output clock of the PLL; one for the primary input clock (for example, inclk), and one for the secondary input clock (for example, inclk). Create exclusive clock groups for the primary and secondary output clocks since they are not active simultaneously.
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