Ordering Constraints for Better Runtime - 2022.1 English

Vivado Design Suite User Guide: Using Constraints (UG903)

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2022.1 English

When loading the timing constraints in memory, the timing engine validates each new constraint and prints messages to flag potential problems. Some timing constraints partially invalidate the timing database (also referred as timing graph) and some other timing constraints require an up-to-date timing database in order to be properly applied. Once the timing database is out of date, subsequent timing updates are needed, for instance, to update auto-derivation clocks or to disable certain timing paths in the design. The XDC commands which query the clocks or which traverse the design to query netlist objects require an up-to-date timing database.

Interleaving constraints and commands that impact the timing database state can be runtime intensive as the timing information gets invalidated and updated multiple times.

For runtime optimization, Xilinx® recommends that you order the timing constraints and queries carefully. The table below lists the XDC constraints and commands that have an impact on the timing graph.

Table 1. XDC Constraints and Their Impact on the Timing Graph
Constraints with Impact on Timing Graph Constraints with No Impact on Timing Graph Constraints which Require Up-to-Date Timing Graph
create_clock set_bus_skew all_fanout
create_generated_clock set_clock_groups all_fanin
set_case_analysis set_clock_latency get_clocks
set_clock_sense set_false_path get_generated_clocks
set_clock_uncertainty set_input_delay all_clocks
set_disable_timing set_input_jitter Any constraint with the –clock option
set_external_delay set_min_delay
set_propagated_clock set_max_delay

One of the most runtime intensive combinations is set_disable_timing with all_fanout or all_fanin. Such combinations should be avoided. For example:

set_disable_timing –from <pin> -to [all_fanout …] set_disable_timing –from [all_fanin …] -to <pin>

Based on the table above, the optimal constraints order for runtime optimization is:

  1. XDC constraints set_disable_timing, set_case_analysis, and set_external_delay.
  2. Constraints that have an impact on the timing graph.
  3. Constraints that do not require timing graph updates.
Tip: When the same query is done in multiple places, it is recommended that you save the result of the query inside a Tcl variable and refer to that Tcl variable when it is needed.

For example, the following sequence of constraints is not optimal.

create_clock –name clk1
create_generated_clock –name genclk1 –master_clock [get_clocks -of [get_pins ...]] set_disable_timing ...
create_clock –name clk2
set_false_path -from [get_clocks -of [get_pins ff1/C]] set_case_analysis ...
create_clock –name clk3
set_max_delay -to [get_clocks -of [get_pins ff2/C]]

The following shows a more optimal and runtime efficient sequence.

set_disable_timing ... 
set_case_analysis ... 
create_clock –name clk1 create_clock –name clk2 
create_clock –name clk3
create_generated_clock –name genclk1 –master_clock [get_clocks -of [get_pins ...]] 
set_false_path -from [get_clocks -of [get_pins ff1/C]]
set_max_delay -to [get_clocks -of [get_pins ff2/C]]