请教AMCC5920的问题，高人先谢了

我用AMCC5920作了一块PCI板，通过串行RAM对5920进行初始化，理论显示SCL线时钟应为65KHz，但我测试为0Hz，我查阅的相关资料如下：
SCL：Serial Clock. This clock provides timing for all transactions on the two-wire serial bus. The S5920 drives this signal when performing as a serial bus master. SCL operates at the maximum
allowable clock speed and enters the high Z state when FLT# is asserted or the serial bus is
inactive.

请问最后一句话the serial bus is inactive.是何意？ SCL为0Hz是由什么造成的？