During the timing analysis, tools verifies whether logic paths meets all the constraints defined in the SDC (Synopsys design constraints) and reports violation if any logic paths doesn’t meet the required timings. And the tools are not intelligent enough to find which logic path was true or false path. Therefore, we must inform the tool which are the false paths before performing the timing analysis. Few cases are mentioned below.
During the Functional mode the select line of mux is tied to 0 and tool should not perform the timing analysis on this path (marked in orange) which will be active during DFT test mode. This information must be provided to the following command
set_flase_path -to <list of end points>
We need to add false path on the pins which are tied to low or high (static signals). As they are static signals, timing checks are not necessary.
set_false_path -from [get_ports A]
If two clocks are not related to each other (Asynchronous) then we must define these paths as false path. In this case to avoid any setup/hold violations at capturing registers some synchronizers technique must be employed. Few of the techniques are
- Two flop Synchronizers
- Handshaking protocol
set_false_path -from CLKA -to CLKB
In case of the two flip flop synchronizers as show in above figure timing checks are not necessary between the Launching flop and the 1st stage of the synchronizers. Therefore, we have to consider the signal to Flop FF2 as false path