Xilinx System Generator V2.1 Reference Manual page 100

Xilinx System Generator v2.1 Reference Guide
The block parameters dialog box can be invoked by double-clicking the icon in your
Simulink model.
Figure 3-69: Gateway Out block parameters dialog box
Parameters specific to the Gateway Out block are:
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IOB Timing Constraint: In hardware, a Gateway Out is realized as a set of
input/output buffers (IOBs). There are three ways to constrain the timing on
IOBs. They are None , Data Rate , and Data Rate, Set 'FAST'
Attribute .
If None is selected, no timing constraints for the IOBs are put in the user
constraint file (.ucf) produced by System Generator. This means the paths from
the IOBs to synchronous elements are not constrained.
If Data Rate is selected, the IOBs are constrained at the data rate at which the
IOBs operate. The rate is determined by System Clock Period provided on the
System Generator block and the sample rate of the Gateway relative to the other
sample periods in the design. For example, the following OFFSET = OUT
constraints are generated for a Gateway Out named 'Dout' that is running at the
system period of 10 ns:
# Offset out constraints
NET "Dout<0>" OFFSET = OUT : 10.0 : AFTER "clk";
NET "Dout<1>" OFFSET = OUT : 10.0 : AFTER "clk";
NET "Dout<2>" OFFSET = OUT : 10.0 : AFTER "clk";
NET "Dout_valid" OFFSET = OUT : 10.0 : AFTER "clk";
NET "Dout_valid" FAST;
It should be noted there is a valid bit that accompanies the data signal. It is
constrained at the same rate. For more information concerning the valid bit, refer
to the Hardware Handshaking section in Chapter 1 of this manual.
If Data Rate, Set 'FAST' Attribute is selected, the OFFSET = OUT
constraints described above are produced. In addition, a FAST slew rate attribute
is generated for each IOB. This reduces delay but increases noise and power
consumption. For the previous example, the following additional attributes are
added to the .ucf file
NET "Dout<0>" FAST;
NET "Dout<1>" FAST;
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