原创 Hierarchical Wire Load Models(层次结构化的线载模型)

 2010-7-28 20:44  7547 11 11 分类: FPGA/CPLD

Hierarchical Wire Load Models(Comes from DC user guide)
层次结构化的线载模型（来自DC用户手册）
DC supports three modes for determining which wire load model to use for nets that cross hierarchical boundaries:
DC支持三种模式，来决定如何选择用于跨层次边界的网线的线载模型。
Top - Top模式

DC models nets as if the design has no hierarchy and uses the wire load model specified for the top level of the design hierarchy for all nets in a design and its subdesigns. The tool ignores any wire load models set on subdesigns with the set_wire_load_model command.
此模式下，DC认为没有层次结构来对网线建模，所有网线使用顶层设计的线载模型。DC工具忽略下层子模块所设定的线载模型。
Use top mdoe if you plan to flatten the design at a higher level of hierarchy before layout.

如果打算在布图前在高层次结构上打平设计，请使用此模式。

Enclosed - Enclosed 模式

DC uses the wire load model of the smallest design that fully encloseds the net. If the design enclosing the net has no wire load model, the tool traverses the design hierearchy upward until it finds a wire load model. Enclosed mode is more accurate than top mode when cells in the same design are placed in a contiguous region during layout.
DC使用包含某网线的最小的设计所定义的线载模型对此网线建模。如果此设计没有定义线载模型，DC工具向其上一层设计查找线载模型，直到找到线载模型的定义。如果在布图时将同一设计的单元摆放在一块相连的区域，Enclosed模式要比top模式更精确。
Use enclosed mode if the design are placed in physical hierarchies.

如果在布图时摆放设计具有物理层次结构，请使用此模式。

Segmented - Segmented 模式

DC determines the wire load models of each segment of a net by the design encompassing the segment. Nets crossing hierarchical boundaries are divided into segments. For each net segment, DC uses the wire load model of the design containing the segment. If the design contains a segment that has no wire load model, the tool traverses the design hierarchy upward until it finds a wire load model.
DC为每一个网线按照设计包含的关系分成很多段，每一段网线使用线载模型。跨层次设计边界的网线会被分成若干段。对每一网线段，DC使用包含此段的设计所定义的线载模型。如果包含此网线段的设计没有定义线载模型，DC查找上一层次设计的线载模型，直到找到线载模型。

Use segmented mdoe if the wire load models in your technology have been characterized with net segments.

如果你的制程支持网线分段的特性，请使用此模式。

Following Figure shows a sample design with a cross-hierarchy net, cross_net. The top level of the hierarchy(design TOP) has a wire load model of 50x50. The next level of hierarchy(design MID) has a wire load model of 40x40. The leaf-level designs, A and B, have wire load models of 20x20 and 30x30, respectively.