在M480系列芯片手册中GPIO  PxPUSEL 的描述如下：
Note 1: Basically, the pull-up control and pull-down control has following behavior
limitation
The independent pull-up control register only valid when MODEn set as tri-state and
open-drain mode
The independent pull-down control register only valid when MODEn set as tri-state mode
When both pull-up pull-down is set as 1 at “tri-state” mode, keep I/O in tri-state mode


1，请问 tri-state 在 MODE 描述中没有，请问 tri-state 指的是什么模式？？
2，请问 这里说上拉控制只在 tri-state and open-drain mode 有效，下拉控制只在 tri-state mode 有效；那么 Input mode ，不能上下拉吗？？