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goes beyond basic constraints, focusing on efficient methods to manage the analysis. It provides practical examples of prioritizing different commands and demonstrates how tools maintain different types of paths, including clock paths, data paths, and asynchronous paths.
# Maximum output delay for external setup compliance set_output_delay -max 0.4 -clock SYS_CLK [get_ports OUT_DATA] # Minimum output delay for external hold compliance set_output_delay -min -0.1 -clock SYS_CLK [get_ports OUT_DATA] Use code with caution. 4. Timing Exceptions and Control
The 2021 guide is famous for its "Exception Handling" chapter. It categorizes exceptions by severity .
Operates at the High-Level Design (HDL) phase. It includes sharing common sub-expressions, resource sharing (e.g., sharing an adder across different conditional branches), and selecting optimal macro structures (like choosing a Carry-Lookahead Adder vs. a Ripple-Carry Adder based on timing pressure).
# Constrain output port 'data_out' with downstream setup/hold requirements set_output_delay -max 0.6 -clock sys_clk [get_ports data_out] set_output_delay -min -0.2 -clock sys_clk [get_ports data_out] Use code with caution. 4. Advanced Timing Exceptions
: Selects specific physical cells from the target technology library (.lib) that fulfill delay, power, and area targets. Critical Path Resynthesis
The 2021 guidelines emphasize that constraints should be . Over-constraining forces the tool to work unnecessarily hard, leading to bloated area and excessive power consumption. Under-constraining, conversely, leads to optimistic results that fail in silicon. 2. Defining the Clock Tree
The 2021 guide dedicates Chapter 8 to "Optimization for Area and Power under Timing Constraints."
Real-world clock networks suffer from physical imperfections. You must model these characteristics explicitly during synthesis before physical layout occurs:
The 2021 release did not just add new commands; it introduced a philosophical shift: . Historically, designers wrote loose constraints at the Register Transfer Level (RTL) and tightened them during physical design. The 2021 guide emphasizes signoff-quality constraints at the synthesis stage.
