create_generated_clock

[Fraserbc]

Hi,

I have two issues with create_generated_clock. The first is when trying to use the master_clock argument without providing source is errors out with missing -source argument., but according to this it should work.

My second issue is that it requires source to be a pin or a port as it errors with master_pin type 'Net' is not a pin or port., not allowing nets to be used.

[vijayank88]

@Fraserbc Refer following doc: https://github.com/The-OpenROAD-Project/OpenSTA/blob/master/doc/OpenSTA.pdf

[Fraserbc]

I've just read through it again and can't see anything showing how to create a generated clock using a net as a source. Is there any workaround?

[vijayank88]

@Fraserbc Can you attach test case with .sdc file you're facing the issue?

[Fraserbc]

Here's an example of what I'm trying to do

# Create the first clock
create_generated_clock -name CLK1 -source [get_ports clk_in] \
        -divide_by 2 [get_nets CLK_DIV_2]

create_generated_clock -name CLK2 -source [get_nets CLK_DIV_2] \
        -divide_by 2 [get_nets CLK_DIV_4]

I want to be able to create a generated clock with the master being another generated clock, without the source being a pin.

[vijayank88]

@Fraserbc Can you share source verilog file?

[get_nets CLK_DIV_2] unable to resolve during the process i think.

By providing pin list it will work. Example:

create_generated_clock -name gclk1 -source clk1 -divide_by 4 r1/Q

[jjcherry56]

Nets are not supported for generated clock sources. It is actually pretty strange and counter to the documentation (that I wrote) that nets are supported for the clock root. The net is converted to the driver pin(s) of the net. The SDC to do the equivalent is [get_pins -of_object [get_net CLK_DIV_2] -filter "direction == output"]

[Fraserbc]

@jjcherry56 Thanks so much! That's exactly what I was looking for. By any chance do you have any good places to learn about SDC constraints, specifically things like that [get_pins -of_object [get_net CLK_DIV_2] -filter "direction == output"]?