[SOLVED] How do we set max fanout for clock nets

[stevenv07]

Hello everyone,
Could you show me how to set max fanout for clock nets in cadence innovus 18.1?

Thanks so much!

 

[riti]

I think that should be given in design constraints file known as SDC.

 

[ThisIsNotSam]

look for the ccopt_* commands, they control how the clock tree is implemented.

designers will typically adjust insertion delay, play with clock tree size and power.. fanout is not a typical first tier concern.

 

[ranaya]

Hello everyone,
Could you show me how to set max fanout for clock nets in cadence innovus 18.1?

Thanks so much!

An (rudimentary) example setup for a CT looks like this :

set_ccopt_property routing_top_min_fanout 10000
set_ccopt_property buffer_cells { CKBUFM8R CKBUFM6R CKBUFM4R CKBUFM3R CKBUFM2R CKBUFM1R }
set_ccopt_property inverter_cells { CKINVM4R CKINVM3R CKINVM2R CKINVM1R }
set_ccopt_property delay_cells { DEL1M1R DEL1M4R DEL2M1R DEL2M4R DEL3M1R DEL3M4R DEL4M1R DEL4M4R }
set_ccopt_property clock_gating_cells { LAGCESM2RA LAGCESM4RA LAGCESM6RA LAGCESM8RA LAGCESM12RA LAGCESM16RA LAGCESM24RA LAGCESM32RA LAGCESM40RA LAGCESM48RA }
# Include this setting to use inverters in preference to buffers
#set_ccopt_property -net_type trunk target_max_trans 2ns
#set_ccopt_property -net_type leaf target_max_trans 500ps
set_ccopt_property use_inverters true
set_ccopt_property target_max_trans 1ns
set_ccopt_property target_skew 1ns
create_ccopt_clock_tree_spec
ccopt_design

For CT, target_max_fanout can be set with set_ccopt_property command (As @ThisIsNotSam stated). If you set it globally in *.sdc, the most stringent one will be applied during CTS. Moreover, you can set it up differently for top/trunk and leaf nets.