Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
8 years ago

How to set clock crossings correctly

I was wondering the best way to do these clock crossings. I have a map_cdc module that is used for clock crossing, and is instantiated in multiple places for different clock domain crossings. I has a .qip file and inside that I have a .sdc which sets these constraints:

set_min_delay -from [get_registers {*|map_cdc:*|PulseSync:in_clock_sync|In_to_Out_Val}] -to [get_registers {*|map_cdc:*|PulseSync:*|In_to_Out_Val_r[0]}] -100

set_max_delay -from [get_registers {*|map_cdc:*|PulseSync:in_clock_sync|In_to_Out_Val}] -to [get_registers {*|map_cdc:*|PulseSync:*|In_to_Out_Val_r[0]}] 100

set_max_skew -from [get_registers {*|map_cdc:*|PulseSync:in_clock_sync|In_to_Out_Val}] -to [get_registers {*|map_cdc:*|PulseSync:*|In_to_Out_Val_r[0]}] -get_skew_value_from_clock_period src_clock_period -skew_value_multiplier 0.8

set_net_delay -from [get_registers {*|map_cdc:*|PulseSync:in_clock_sync|In_to_Out_Val}] -to [get_registers {*|map_cdc:*|PulseSync:*|In_to_Out_Val_r[0]}] -max -get_value_from_clock_period dst_clock_period -value_multiplier 1

The problem is when I instantiate it multiple times, the constraints overwrite each other (since I have wildcards), which is a problem since I get get values from src_clock_period/dst_clock_period.

In the "SCFIFO and DCFIFO IP Cores User Guide" it shows using the variable $hier_path in order to get the full path to keepers to apply timing constraints to. I was thinking this is what I should use in my constraints so that each time the .qip file is included I can make the .sdc file only apply to that instance (with something like [get_keepers $hier_path|map_cdc:..).) But when I try use this in my sdc files I get the error "hier_path no such variable".

Do I need to set this variable somewhere in the .qip file? I can't find any information on it. Or does someone have a better solution? I feel like this should be a common problem.

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    I have the same problem. I am wondering how to set the $hier_path variable which is mentioned in the DCFIFO user guide.

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    What about something like this to get the hierarchy?: 

    
set inst 
# set inst_list 
set inst_list {}
foreach_in_collection c $inst {
	lappend inst_list 
}
set hierarchy_list {}
#  find all hierarchies
foreach each_inst $inst_list {
#  extract the hierarchy
	regexp {^(.*)\|*$} $each_inst matched hier_path 
	if {$hier_path ni $hierarchy_list} {
		lappend hierarchy_list $hier_path
	}
}
foreach each_inst $hierarchy_list {
	set_max_skew  ...
	set_min_delay ...
	set_max_delay ...
	set_net_delay ...
}