The typical cases for using the false path constraint are:
- Ignoring timing on a path that is never active. For example, a path that crosses
two multiplexers that can never let the data propagate in a same clock cycle because
of the select pins connectivity.Figure 1. Path Cannot be Sensitized
set_false_path -through [get_pins MUX0/I0] -through [get_pins MUX1/I1] - Ignoring timing on an asynchronous CDC path.
- Ignoring static paths in the design. Some registers take a value once during the
initialization phase of the application and never toggle again. When they appear to
be on the critical path of the design, they can be ignored for timing to relax the
constraints on the implementation tools and help with timing closure. It is
sufficient to define a false path constraint from the static register only, without
explicitly specifying the paths endpoints. For example, the paths from a 32-bit
configuration register
config_reg[31..0]to the rest of the design can be ignored by adding the following false path constraint:set_false_path -from [get_cells config_reg[*]]