Intel® Quartus® Prime Standard Edition User Guide: Timing Analyzer

ID 683068
Date 2/21/2024
Document Table of Contents Deriving PLL Clocks

The Derive PLL Clocks (derive_pll_clocks) constraint automatically creates clocks for each output of any PLL in your design. derive_pll_clocks detects your current PLL settings and automatically creates generated clocks on the outputs of every PLL by calling the create_generated_clock command.

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] \

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:

Figure 50. Simple PLL Design

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: Deriving PLL Clocks:
Info: create_generated_clock -source pll_inst|altpll_component|pll|inclk[0] -divide_by 2 -name
pll_inst|altpll_component|pll|clk[0] pll_inst|altpll_component|pll|clk[0]

The input clock pin of the PLL is the node pll_inst|altpll_component|pll|inclk[0] 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[0].

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[0]), and one for the secondary input clock (for example, inclk[1]). Create exclusive clock groups for the primary and secondary output clocks since they are not active simultaneously.