Design Specification
A counter having 2 input clocks, clk_a running at 100 MHz and clk_B running at 200MHz.
there are clock divider for clk_A in design.
Counter having inputs clk_A , clk_B , reset, count_start1 ,count_start2 and output count_val
1.count_start1 clocking with clk_A
2.count_start2 clocking with clk_B
3.count_val clocking with clk_A
4.reset is synchronous and clocked with clk_A
below is the sample SDC file - counter.sdc
At the top of SDC , below optional commands
# create source clock
create_clock -name clka -period 10 [get_ports clk_A]
create_clock -name clkb -period 5 [get_ports clk_B]
#create generated clock
create_generate_clock -divide_by 2 -source [get_ports clk_A] -name clk_DIV_A [get_pins <pin_name>]
# set clock uncertainty (setup) , here 10% or more depend on clock frequency and technology library
set_clock_uncertainty -setup <10% of clk_A> [get_clocks clk_A]
set_clock_uncertainty -setup <10% of clk_B> [get_clocks clk_B ]
#set clock uncertainty (hold) , Hold doesn't depend on clock frequency.
# $hold_margin - 50 ps
set_clock_uncertainty -hold $hold_margin [all_clocks]
## set clock latency if required
# declare false path between clk_A and clk_B
set_clock_groups -logically_exclusive [get_clocks clk_A clk_B]
or set_false_path -from clk_A -to clk_B
set_false_path -from clk_B -to clk_A
## Declare input delays and output delays on ports
set_input_delay -clock clk_A -max {0.5*clk_A period} [get_ports count_start1 ]
set_input_delay -clock clk_A min 0 [get_ports count_start1]
set_input_delay -clock clk_B -max {0.5*clk_A period} [get_ports count_start2 ]
set_input_delay -clock clk_B min 0 [get_ports count_start2]
set_output_delay -clock clk_A min 0 [get_ports count_val]
#declare multicycle paths
set_multicycle_path <declare multicycle path if any in design>
##Below are the constraints helps to setup the environments
set_case_analysis 0 [get_pins counter/heir_a/sel_a]
set_driving_cell -lib_cell BUFX9 [all_inputs]
set_fanout_load 8 [all_outputs]
set_load 50 [all_outputs] ## set the value of capacitive load on pin or net in design
# set the specified ports/pins at logic zero or logic one (this is similar to set_case_analysis)
set_logic_zero [get_ports <>]
set_logic_one [get_pins <> ]
set_max_area ## if there is any area specific requirement
set_max_capacitance 0.1 [all_outputs]
set_max_transition 0.2 [all_pins] ## specify the max transition time on pins or ports
set_port_fanout_number 5 [get_ports <PORT>] ## set maximum fanout of a port
commands which I have not defined in above sdc -
set_min_delay -from PATH_A -to PATH_B 1 ## set minimum delay 1 ns from path A to B
set_max_delay -from PATH_A -to PATH_B 2 ## set maximum delay 2 ns from path A to B
There are many other commands which may be used in design but those are more specific to the requirement, design implementation , and technology specific. Above all sdc commands are generic commands.
Let me know if there are anything I missed here or any question , please feel free to ask.
Thanks
Rahul Jain
