Duty cycle variation of inter-clock timing paths

In the post, duty cycle variation, we understood what duty cycle variation is, and how to apply for intra-clock timing paths. But of similar importance is duty cycle variation as applied to inter-clock timing paths. Let us discuss these cases one-by-one:

Root clock to root-inverted clock: Inverted clock is same as root clock in frequency, with phase inverted. So, duty cycle variation needs to be applied for following cases:

• Root rise edge -> generated rise edge
• Root fall edge -> generated fall edge
• Generated rise edge -> Root rise edge
• Generated fall edge -> Root fall edge

Following commands will be needed to be applied:

set_clock_uncertainty -rise_from root_clk -rise_to gen_clk <duty_cycle> 

set_clock_uncertainty -fall_from root_clk -fall_to gen_clk <duty_cycle>

set_clock_uncertainty -rise_from gen_clk -rise_to root_clk <duty_cycle>

set_clock_uncertainty -fall_from gen_clk -fall_to root_clk <duty_cycle>

Root clock to odd 50% divided clock: In this scenario, we need to apply extra uncertainty for the following cases:

• Root rise edge -> Generated fall edge
• Root fall edge -> Generated rise edge
• Generated rise edge -> Root fall edge
• Generated fall edge -> Root rise edge

Following commands will need to be applied for this case:

set_clock_uncertainty -rise_from root_clk -fall_to gen_clk <duty cycle>

set_clock_uncertainty -fall_from root_clk -rise_to gen_clk <duty cycle>

set_clock_uncertainty -rise_from gen_clk -fall_to root_clk <duty cycle>

set_clock_uncertainty -fall_from gen_clk -rise_to root_clk <duty cycle>

Root clock to even 50% divided clock: In this case, we need to apply duty cycle uncertainty for the following cases:

• Root fall edge -> Generated rise edge
• Root fall edge -> Generated fall edge
• Generated rise edge -> Root fall edge
• Generated fall edge -> Root fall edge

Below figure shows these cases for a 50% divided clock from root clock.

 

So, the rule of thumb is same. Wherever there is a timing path wherein both rising and falling edges of root clock are involved, duty cycle variation will come into play. If you just keep this basic thing into mind, duty cycle variation will never haunt you. :-)