有人用过Bus-holder吗？

xdnc

请教一个问题：现在tsmc N28 工艺里提供了 Bus-holder，综合时默认是don't use 属性。这个一般是谁加？前端/综合/pr哪个步骤加？好像以前的工艺里没有这个东西的，都不知道怎么用。

另附wiki名词解释。。

A bus-holder (or Bus-keeper) is a weak latch circuit which holds last value on a tri-state bus.

The circuit is basically a delay element with the output connected back to the input through a relatively high impedance. This is usually achieved with twoinverters connected back to back. The resistor drives the bus weakly; therefore other circuits can override the value of the bus when they are not in tri-state mode.

Bus-holders are used to prevent CMOS gate inputs from getting floating values when they are connected to tri-stated nets. Otherwise both transistors in the gate could get turned on, thus shorting the power supply and ground, which would destroy the CMOS gate. This is prevented by the bus-holder pulling the input to the last valid logic level (0 or 1) on the net. The circuit is usually placed in parallel with the tri-stated net.

shycat

一直都有的。如果前面的逻辑可能输出为Z态，或者说断电了，断开了，用这个可以维持一个稳定的电平，减少leakage

dandan58

楼上正解。之前的一个设计中也用到了bus_holder，用于存在三态的逻辑中，保持住之前的状态

kay443

一直都有吗？不知道怎么用..

教父

这个是在前端综合调用该单元库的时候，有don't use 属性存在，工具就不会调用该单元，就像你在外面设置的don't_use某一类单元CELL 一样，但是当你确定要调用该单元CELL的时候， 你可以在综合脚本里面移除该属性。

核桃树

所以bus keeper 就是在PAD上的一个辅助电路，用于避免在高阻态时，PAD悬空，导致上拉和下拉mos进入都打开的状况，出现漏电，时间久了就会损坏gate，bus keeper是有两个晶体管串成环形，首尾相连，使PAD上保持初始的高电平或者低电平。