关于Electrical Net的讨论

    在高速设计中，经常要求等长设计，并且一般都要求接入匹配电阻。由于接入的匹配电阻，本来应该是一根导线，变成了2个网线，这给做约束带来的困难。若接入的是一个匹配电阻，mentor会自动识别将这两段网线认为是同一个electrical net，如下图说明：

Electrical Net

An electrical net is a combination of two or more nets according to the following rules:

1. There is a series element, such as a resistor between the nets.

2. One of the nets contains a digital pin (input, output, <！--Element not supported - Type: 8 Name: #comment-->tri-state<！--Element not supported - Type: 8 Name: #comment-->, <！--Element not supported - Type: 8 Name: #comment-->bi-directional<！--Element not supported - Type: 8 Name: #comment-->, open emitter, open collector, open drain).

Restriction: Supply nets are ignored.

Naming Electrical Nets

One of the net names in the electrical net is used to name the electrical net using the following rules:

1. It is chosen from those nets that have a driver.

2. If no net has a driver, nets with I/O pins are used.

3. The net name that occurs first alphabetically in the list derived from either rule 1 or 2 is used to name the electrical net. For example, if nets J1-2, B5-23 and L4-23 comprise your electrical net the electrical net is named B5-23.

Result: Any time an electrical net is displayed in this application the ^^^ character string is added to the end of the net name (e.g. U2-8^^^).

    但是，一般总线上，为了节省PCB面积，一般都采用排阻。很明显，排阻是8个腿或16个腿，Mentor不会认为它是Series element。这样问题就来了，需要对每一个net做长度约束，否则若对排阻的两段做等长约束的话，则有些线会甩不开。

    问题：如何让Mentor识别类似排阻这样的器件，让其自动或手动识别是Electrical Net。

    我尝试过将排阻的symbol分开成单个symbol，如8腿的排阻分开成4个电阻，用4个slot来建part，但mentor还是无法识别。

在mentor pcb界面  有一个菜单工具，用于建立诸如电阻，电容，电感，电阻排的IBIS模型，建立一个电阻排模型，在ces中选择part选项卡，找到相应的电阻排，导入其ibis模型即可。