SDC Commands¶
The following subset of SDC syntax is supported by VPR.
create_clock¶
Creates a netlist or virtual clock.
Assigns a desired period (in nanoseconds) and waveform to one or more clocks in the netlist (if the –name option is omitted) or to a single virtual clock (used to constrain input and outputs to a clock external to the design).
Netlist clocks can be referred to using regular expressions, while the virtual clock name is taken as-is.
Example Usage:
#Create a netlist clock
create_clock -period <float> <netlist clock list or regexes>
#Create a virtual clock
create_clock -period <float> -name <virtual clock name>
#Create a netlist clock with custom waveform/duty-cycle
create_clock -period <float> -waveform {rising_edge falling_edge} <netlist clock list or regexes>
Omitting the waveform creates a clock with a rising edge at 0 and a falling edge at the half period, and is equivalent to using -waveform {0 <period/2>}.
Non-50% duty cycles are supported but behave no differently than 50% duty cycles, since falling edges are not used in analysis.
If a virtual clock is assigned using a create_clock command, it must be referenced elsewhere in a set_input_delay or set_output_delay constraint.
Note
One of -name or <netlist clock list or regexes> must be specified.
Warning
If a netlist clock is not specified with a :sdc:command:`create_clock` command, paths to and from that clock domain will not be analysed.
set_clock_groups¶
Specifies the relationship between groups of clocks. May be used with netlist or virtual clocks in any combination.
Since VPR supports only the -exclusive option, a :sdc:command:`set_clock_groups` constraint is equivalent to a :sdc:command:`set_false_path` constraint (see below) between each clock in one group and each clock in another.
For example, the following sets of commands are equivalent:
#Do not analyze any timing paths between clk and clk2, or between
#clk and clk3
set_clock_groups -exclusive -group {clk} -group {clk2 clk3}
and
set_false_path -from [get_clocks {clk}] -to [get_clocks {clk2 clk3}]
set_false_path -from [get_clocks {clk2 clk3}] -to [get_clocks {clk}]
set_false_path¶
Cuts timing paths unidirectionally from each clock in -from to each clock in –to.
Otherwise equivalent to :sdc:command:`set_clock_groups`.
Example Usage:
#Do not analyze paths launched from clk and captured by clk2 or clk3
set_false_path -from [get_clocks {clk}] -to [get_clocks {clk2 clk3}]
#Do not analyze paths launched from clk2 or clk3 and captured by clk
set_false_path -from [get_clocks {clk2 clk3}] -to [get_clocks {clk}]
Note
False paths are supported between entire clock domains, but not between individual registers.
set_max_delay/set_min_delay¶
Overrides the default setup (max) or hold (min) timing constraint calculated using the information from :sdc:command:`create_clock` with a user-specified delay.
Example Usage:
#Specify a maximum delay of 17 from input_clk to output_clk
set_max_delay 17 -from [get_clocks {input_clk}] -to [get_clocks {output_clk}]
#Specify a minimum delay of 2 from input_clk to output_clk
set_min_delay 2 -from [get_clocks {input_clk}] -to [get_clocks {output_clk}]
Note
Max/Min delays are supported between entire clock domains, but not between individual netlist elements.
set_multicycle_path¶
Sets how many clock cycles elapse between the launch and capture edges for setup and hold checks.
The default the setup mutlicycle value is 1 (i.e. the capture setup check is performed against the edge one cycle after the launch edge).
The default hold multicycle is one less than the setup multicycle path (e.g. the capture hold check occurs in the same cycle as the launch edge for the default setup multicycle).
Example Usage:
#Create a 4 cycle setup check, and 0 cycle hold check from clkA to clkB
set_multicycle_path -from [get_clocks {clkA}] -to [get_clocks {clkB}] 4
#Create a 3 cycle setup check from clk to clk2
# Note that this moves the default hold check to be 2 cycles
set_multicycle_path -setup -from [get_clocks {clk}] -to [get_clocks {clk2}] 3
#Create a 0 cycle hold check from clk to clk2
# Note that this moves the default hold check back to it's original
# position before the previous setup setup_multicycle_path was applied
set_multicycle_path -hold -from [get_clocks {clk}] -to [get_clocks {clk2}] 2
#Create a multicycle to a specific pin
set_multicycle_path -to [get_pins {my_inst.in\[0\]}] 2
Note
Multicycles are supported between entire clock domains, and ending at specific registers.
Note
If neither -setup nor -hold the setup multicycle is set to path_multiplier and the hold multicycle offset to 0.
Note
Only a single -to option can be specified (either clocks or pins, but not both).
set_input_delay/set_output_delay¶
Use set_input_delay if you want timing paths from input I/Os analyzed, and set_output_delay if you want timing paths to output I/Os analyzed.
Note
If these commands are not specified in your SDC, paths from and to I/Os will not be timing analyzed.
These commands constrain each I/O pad specified after get_ports to be timing-equivalent to a register clocked on the clock specified after -clock.
This can be either a clock signal in your design or a virtual clock that does not exist in the design but which is used only to specify the timing of I/Os.
The specified delays are added to I/O timing paths and can be used to model board level delays.
For single-clock circuits, -clock can be wildcarded using * to refer to the single netlist clock, although this is not supported in standard SDC.
This allows a single SDC command to constrain I/Os in all single-clock circuits.
Example Usage:
#Set a maximum input delay of 0.5 (relative to input_clk) on
#ports in1, in2 and in3
set_input_delay -clock input_clk -max 0.5 [get_ports {in1 in2 in3}]
#Set a minimum output delay of 1.0 (relative to output_clk) on
#all ports matching starting with 'out*'
set_output_delay -clock output_clk -min 1 [get_ports {out*}]
#Set both the maximum and minimum output delay to 0.3 for all I/Os
#in the design
set_output_delay -clock clk2 0.3 [get_ports {*}]
set_clock_uncertainty¶
Sets the clock uncertainty between clock domains. This is typically used to model uncertainty in the clock arrival times due to clock jitter.
Example Usage:
#Sets the clock uncertainty between all clock domain pairs to 0.025
set_clock_uncertainty 0.025
#Sets the clock uncertainty from 'clk' to all other clock domains to 0.05
set_clock_uncertainty -from [get_clocks {clk}] 0.05
#Sets the clock uncertainty from 'clk' to 'clk2' to 0.75
set_clock_uncertainty -from [get_clocks {clk}] -to [get_clocks {clk2}] 0.75
set_clock_latency¶
Sets the latency of a clock. VPR automatically calculates on-chip clock network delay, and so only source latency is supported.
Source clock latency corresponds to the delay from the true clock source (e.g. off-chip clock generator) to the on-chip clock definition point.
#Sets the source clock latency of 'clk' to 1.0
set_clock_latency -source 1.0 [get_clocks {clk}]
set_disable_timing¶
Disables timing between a pair of connected pins in the netlist. This is typically used to manually break combinational loops.
#Disables the timing edge between the pins 'FFA.Q[0]' and 'to_FFD.in[0]' on
set_disable_timing -from [get_pins {FFA.Q\\[0\\]}] -to [get_pins {to_FFD.in\\[0\\]}]
Special Characters¶
SDC Examples¶
The following are sample SDC files for common non-default cases (assuming netlist clock domains clk and clk2).
A¶
Cut I/Os and analyse only register-to-register paths, including paths between clock domains; optimize to run as fast as possible.
create_clock -period 0 *
B¶
Same as A, but with paths between clock domains cut. Separate target frequencies are specified.
create_clock -period 2 clk
create_clock -period 3 clk2
set_clock_groups -exclusive -group {clk} -group {clk2}
C¶
Same as B, but with paths to and from I/Os now analyzed. This is the same as the multi-clock default, but with custom period constraints.
create_clock -period 2 clk
create_clock -period 3 clk2
create_clock -period 3.5 -name virtual_io_clock
set_clock_groups -exclusive -group {clk} -group {clk2}
set_input_delay -clock virtual_io_clock -max 0 [get_ports {*}]
set_output_delay -clock virtual_io_clock -max 0 [get_ports {*}]
D¶
Changing the phase between clocks, and accounting for delay through I/Os with set_input/output delay constraints.
#Custom waveform rising edge at 1.25, falling at 2.75
create_clock -period 3 -waveform {1.25 2.75} clk
create_clock -period 2 clk2
create_clock -period 2.5 -name virtual_io_clock
set_input_delay -clock virtual_io_clock -max 1 [get_ports {*}]
set_output_delay -clock virtual_io_clock -max 0.5 [get_ports {*}]
E¶
Sample using many supported SDC commands. Inputs and outputs are constrained on separate virtual clocks.
create_clock -period 3 -waveform {1.25 2.75} clk
create_clock -period 2 clk2
create_clock -period 1 -name input_clk
create_clock -period 0 -name output_clk
set_clock_groups -exclusive -group input_clk -group clk2
set_false_path -from [get_clocks {clk}] -to [get_clocks {output_clk}]
set_max_delay 17 -from [get_clocks {input_clk}] -to [get_clocks {output_clk}]
set_multicycle_path -setup -from [get_clocks {clk}] -to [get_clocks {clk2}] 3
set_input_delay -clock input_clk -max 0.5 [get_ports {in1 in2 in3}]
set_output_delay -clock output_clk -max 1 [get_ports {out*}]
F¶
Sample using all remaining SDC commands.
create_clock -period 3 -waveform {1.25 2.75} clk
create_clock -period 2 clk2
create_clock -period 1 -name input_clk
create_clock -period 0 -name output_clk
set_clock_latency -source 1.0 [get_clocks{clk}]
#if neither early nor late is specified then the latency applies to early paths
set_clock_groups -exclusive -group input_clk -group clk2
set_false_path -from [get_clocks{clk}] -to [get_clocks{output_clk}]
set_input_delay -clock input_clk -max 0.5 [get_ports{in1 in2 in3}]
set_output_delay -clock output_clk -min 1 [get_ports{out*}]
set_max_delay 17 -from [get_clocks{input_clk}] -to [get_clocks{output_clk}]
set_min_delay 2 -from [get_clocks{input_clk}] -to [get_clocks{output_clk}]
set_multicycle_path -setup -from [get_clocks{clk}] -to [get_clocks{clk2}] 3
#For multicycle_path, if setup is specified then hold is also implicity specified
set_clock_uncertainty -from [get_clocks{clk}] -to [get_clocks{clk2}] 0.75
#For set_clock_uncertainty, if neither setup nor hold is unspecified then uncertainty is applied to both
set_disable_timing -from [get_pins {FFA.Q\\[0\\]}] -to [get_pins {to_FFD.in\\[0\\]}]