set_ false_ path

false path It refers to the logical existence , But there is no need to analyze timing path, Use set_false_path To ignore timing The process of analysis and optimization , Reduce run time, Put more resources on optimizing the real critical path On .

1 set_false_path Application

1.1 point-to-point timing exception

commonly set_false_path You need to specify the -from/-to perhaps -through.

 set_false_path -from [get_pins FFB1/CP] -to [get_pins FFB2/D]

Set to false path Of path, The tool will not consider its timing constraints , however delay It still has to be calculated , Because of this false path Combinatorial logic on may be applied to other path.

If you pass through a certain point path Need to set false, You can also use set_disable_timing.

set_disable_timing [get_pins pin_name]
 Equivalent to ：
set_false_path -through [get_pins pin_name]

set_disable_timing All time series analysis at this point will be removed , Naturally, there is no need to calculate the delay , The effect is better than false path More extreme , You also need to be more careful when applying . 

1.2 asynchronous clocks

false path Another application scenario of is to set the asynchronous clock , But now the setting of asynchronous clock is mostly used set_clock_group -asynchronous The command , because set_false_path Calculation crosstalk The default setting is false path Of clock Treat it as a synchronous clock , This brings a problem , namely crosstalk The calculation is too optimistic . To understand this problem, we need to know corsstalk Calculation principle of , Two adjacent net,corsstalk Time is equal to their timing window Length of coincidence , But the exception is , If two net It belongs to asynchronous clock ,STA The analysis will think that the two belong to “infinite timing window”, Literally translated as infinite timing window, There is no need to consider timing window The overlapping part of , It is equivalent to complete coincidence , Think about it all the time corsstalk.

In addition to the above reasons , The convenience of writing is still set_clock_groups better , Set asynchronous set_clock_groups Equivalent false path as follows ： 

set_clock_groups -asynchronous -group clockA -group clockB

set_false_path -from clockA -to clockB
set_false_path -from clockB -to clockA

If you have to deal with multiple clock domian Asynchronous clock declaration of , that set_clock_group -asynchronous The convenience of is even more obvious .

set_clock_groups -asynchronous -group {clockA clockB } -group {clockC clockD}

2 Exception Type Priority 

frequently-used timing exception And the priority order is as follows ：

For example, when set_max_delay And set_false_path When the conflict , Higher priority set_max_delay In the conflict part, the usage scenario will fail . The following example ：

set_max_delay -from A 5.1
set_false_path -to B

here ,set_max_delay -from A -to B The setting of will be invalid , because -to B Is set to false path 了 .