How to constrain a large number of clocks and child clocks as asynchronous to each other?

Hi,

To constrain the timing of the clocks and their child clocks, you can use the "set_clock_groups" command in the SDC file. This command allows you to group clocks together and specify the relationship between them. In your case, you can group each parent clock and its child clocks together and specify that they are synchronous to each other.

For example, to group clk1 and its child clocks together, you can use the following command:

set_clock_groups -group [get_clocks -include_generated_clocks clk1] -synchronous

This command groups all clocks related to clk1 together, including its child clocks, and specifies that they are synchronous to all other clock groups. You can use similar commands to group clk2 and its child clocks together and clk3 and its child clocks together.

Using the "set_clock_groups" command allows you to constrain the timing of your clocks without having to write down every single combination. The command automatically considers all possible combinations within a clock group and constrains them accordingly.

set_clock_groups -name "clk1_group" -group [get_clocks -include_generated_clocks -of [get_ports clk1]] -synchronous
set_clock_groups -name "clk2_group" -group [get_clocks -include_generated_clocks -of [get_ports clk2]] -synchronous
set_clock_groups -name "clk3_group" -group [get_clocks -include_generated_clocks -of [get_ports clk3]] -synchronous

set_clock_groups -name "all_clocks" -group {clk1_group clk2_group clk3_group} -asynchronous

In this example, we create a clock group for each parent clock and its child clocks, and then add these clock groups to a virtual clock group named "all_clocks". This ensures that all clocks in the same group are asynchronous with each other.

--
Ramesh

Great answer! Thank you very much.

Can you elaborate the difference between these two variants?

-group [get_clocks -include_generated_clocks clk1]

-group [get_clocks -include_generated_clocks -of [get_ports clk1]]

The first variant uses the command:

set_clock_groups -group [get_clocks -include_generated_clocks clk1]

In this variant, the "get_clocks" command is used to get all the clocks in the design that are related to the clock "clk1", including the generated clocks derived from "clk1". 

The second variant uses the command:

set_clock_groups -group [get_clocks -include_generated_clocks -of [get_ports clk1]]

In this variant, the "get_ports" command is used to get the port object for "clk1", and then the "get_clocks" command is used to get all the clocks related to this port, including the generated clocks. 

The main difference between the two variants is in how the clock list is generated. The first variant gets all the clocks related to "clk1", regardless of where they are located in the design hierarchy. The second variant gets all the clocks related to the port object for "clk1", which may include clocks from other parts of the design hierarchy if they are connected to the same port as "clk1".

Fantastic explaination, thank you very much.

I added the following lines to my SDC file (the numbers in front are the line number):

59: set_clock_group -name "clk1_group" -group [get_clocks -include_generated_clocks -of [get_ports clk1]] -synchronous
60: set_clock_group -name "clk2_group" -group [get_clocks -include_generated_clocks -of [get_ports clk2]] -synchronous
61: set_clock_group -name "clk3_group" -group [get_clocks -include_generated_clocks -of [get_ports clk3]] -synchronous

62: set_clock_group -name "all_clocks" -group {clk1_group clk2_group clk3_group} -asynchronous

Unfortunately, when reading in the SDC file I get some errors (I am using Innovus 21.10):

**ERROR: (IMPTCM-67):    Option <patterns> and -of_objects are mutually exclusive. Check if more than one options that are mutually exclusive were given or if an invalid option was given.
**ERROR: (TCLCMD-1009):    Conflicting options asynchronous and exclusive (File [...], Line 59).

**ERROR: (IMPTCM-67):    Option <patterns> and -of_objects are mutually exclusive. Check if more than one options that are mutually exclusive were given or if an invalid option was given.
**ERROR: (TCLCMD-1009):    Conflicting options asynchronous and exclusive (File [...], Line 60).

**ERROR: (IMPTCM-67):    Option <patterns> and -of_objects are mutually exclusive. Check if more than one options that are mutually exclusive were given or if an invalid option was given.
**ERROR: (TCLCMD-1009):    Conflicting options asynchronous and exclusive (File [...], Line 61).

**WARN: (TCLCMD-513):    The software could not find a matching object of the specified type for the pattern 'clk1_group' (File [...], Line 62).

**WARN: (TCLCMD-513):    The software could not find a matching object of the specified type for the pattern 'clk2_group' (File [...], Line 62).

**WARN: (TCLCMD-513):    The software could not find a matching object of the specified type for the pattern 'clk3_group' (File [...], Line 62).

**ERROR: (TCLCMD-917):    Cannot find 'clocks' that match 'clk1_group clk2_group clk3_group' (File [...], Line 62).

Did I do something wrong?

Update:

Using

set_clock_group -name "all_clocks" -group [list [get_clock_groups clk_group] [get_clock_groups sclk_a_group] [get_clock_groups sclk_raw_group]] -asynchronous

also does not work. It yields a

**ERROR: (TCLCMD-917):    Cannot find 'clocks' that match '' (File [...] Line 62).

error message.