1)what is the maximum operating frequency of APB?
There isn't one. The AMBA protocols never specify max frequencies because that frequency will depend on the silicon library you are targetting, the complexity of the system you are designing, and how much effort you put into synthesis (not just going for simple timing budgets).
so basically it means the frequency of the AMBA is depends on the slave we are connected.
No, it means the maximum frequency you can achieve depends on the complexity of the system, and how hard you try with the synthesis tools.
The actual slave has little to do with the maximum frequency as it is only the bus interface on the slave that needs to run at PCLK, which shouldn't be too complex logic.
So look at how many slaves you have on your bus, and the address map for those slaves, and that will determine the address decode logic and slave response MUX logic complexity, both of which being combinatorial blocks will have a greater impact on achievable frequency than just a slave bus interface design alone.
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