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PSTRB signal indicates which byte lanes to update during a write transfer.
it shows that the bus contain valid data, when PSTRB[3:0]=1111.
why we need bus instead of single bit PSTRB signal?
The APB protocol doesn't support unaligned addresses. The APB tries to be as simple as possible, so complex access types are pointless complication.
So if you want to access an APB register, use a correctly aligned transfer (the alignment being defined by the width of the data bus implementation).