Xilinx Constraints Guide

Chapter 4: Xilinx Constraints
Modifying Sets
Similarly, the Q and R primitives are in their own H_SET set because they are not linked
through element L to any other design elements. The lowest common ancestor for their
H_SET set is L, which gives it the name A/D/L/h_set. After the flattening, NGDBuild
attaches the sets to the primitives shown in the following tale.
Set Primitives
H_SET=A/h_set F, G, H, O, P, J, K
H_SET=A/D/L/h_set Q, R
H_SET=A/E/h_set N
Consider a situation in which a set is created at the top of the design. There is no lowest
common ancestor if macro A also has an RLOC constraint, since A is at the top of the
design and has no ancestor. In this case, the base name h_set has no hierarchically
qualified prefix, and the name of the H_SET set is simply h_set.
The RLOC constraint assigns a primitive an RLOC value (the row and column numbers
with the optional extensions), specifies its membership in a set, and links together
elements at different levels of the hierarchy. In the Three H_SET Sets example, the RLOC
constraint on macros C and D links together all the objects with RLOC constraints below
them. An RLOC constraint is also used to modify the RLOC values of constraints below
it in the hierarchy. In other words, RLOC values of elements affect the RLOC values of
all other member elements of the same H_SET set that lie below the given element
in the design hierarchy.
When the design is flattened, the XY values of an RLOC constraint on an element are
added to the XY values of the RLOC constraints of the set members below it in the
hierarchy. This feature allows you to modify existing RLOC values in submodules
and macros without changing the previously assigned RLOC values on the primitive
symbols.
The following sections describe the effect of the hierarchy on set modification.
Adding RLOC Values Down the Hierarchy Example (Slice-Based
XY Designations)
This example illustrates the process of adding RLOC values down the hierarchy. The
row and column values between the parentheses show the addition function performed
Constraints Guide
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