Xilinx System Generator V2.1 Reference Manual page 135

The ce2_group contains the blocks operating at twice the system period, i.e., the input
register and the up sampler. Here are the corresponding constraints.
The ce3_group operates at three times the system period. It contains the down
sampler and the output register, and its constraints are the following.
Group to group constraints establish the relative speeds of the groups. Here are the
constraints that relate the speed of ce2_group to ce1_group.
Port timing requirements can be set in the parameter dialog boxes for Gateway In and
Out blocks. These requirements are translated into port constraints like those shown
below. In this example, the 3-bit DIN input is constrained to operate at its gateway's
sample rate (corresponding to a period of 20 ns). The 'FAST' attributes indicate that
the ports should be implemented using hardware resources that reduce delay. (The
delay is reduced, but at a cost of increased noise and power consumption.) The
Din_valid lines constrain the companion valid signal that accompanies DIN. For
more information concerning valid signals, see the Hardware Handshaking section.
Checking the Specify IOB Location Constraints option for a Gateway In or Gateway
Out block allows port locations to be specified. The locations must be entered as a
Using the System Generator Constraints Files
# ce2_group and inner group constraint
INST "InReg" TNM = "ce2_group";
INST "Up_Sample" TNM = "ce2_group";
TIMESPEC "TS_ce2_group_to_ce2_group" = FROM "ce2_group" TO
"ce2_group" "TS_clk" * 2;
# ce3_group and inner group constraint
INST "Down_Sample" TNM = "ce3_group";
INST "OutReg" TNM = "ce3_group";
TIMESPEC "TS_ce3_group_to_ce3_group" = FROM "ce3_group" TO
"ce3_group" "TS_clk" * 3;
# Group-to-group constraints
TIMESPEC "TS_ce1_group_to_ce2_group" = FROM "ce1_group" TO
"ce2_group" "TS_clk" * 1;
TIMESPEC "TS_ce1_group_to_ce3_group" = FROM "ce1_group" TO
"ce3_group" "TS_clk" * 1;
TIMESPEC "TS_ce2_group_to_ce1_group" = FROM "ce2_group" TO
"ce1_group" "TS_clk" * 1;
TIMESPEC "TS_ce2_group_to_ce3_group" = FROM "ce2_group" TO
"ce3_group" "TS_clk" * 2;
TIMESPEC "TS_ce3_group_to_ce1_group" = FROM "ce3_group" TO
"ce1_group" "TS_clk" * 1;
TIMESPEC "TS_ce3_group_to_ce2_group" = FROM "ce3_group" TO
"ce2_group" "TS_clk" * 2;
# Offset in constraints
NET "Din<0>" OFFSET = IN : 20.0 : BEFORE "clk";
NET "Din<1>" OFFSET = IN : 20.0 : BEFORE "clk";
NET "Din<2>" OFFSET = IN : 20.0 : BEFORE "clk";
NET "Din_valid" OFFSET = IN : 20.0 : BEFORE "clk";
NET "Din<0>" FAST;
NET "Din<1>" FAST;
NET "Din<2>" FAST;
NET "Din_valid" FAST;
System Generator Software Features
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