总线周期4个阶段

定时方式

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/b53c6996-41b5-435d-bc0b-d11b1fdae9cd/Untitled.png

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/21e5baca-69af-42cd-a2ba-fbaa1b675f22/Untitled.png

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/de46bb23-fd84-444a-b9f3-253f7842e6e8/Untitled.png