ARM Technical Support Knowledge Articles

Why is there a 1KB restriction in AHB?

Applies to: AHB

Scenario

1. What is the calculation behind this 1KB boundary.
2. Can we have more than 1KB space allocated to each slave.

Answer

The 1KB restriction you refer to is not a restriction on maximum slave size but a constraint within AHB that says that a burst must not cross a 1KB boundary. The limit is designed to prevent bursts crossing from one device to another and to give a reasonable trade-off between burst size and efficiency. In practise, this means that a master must ALWAYS break a burst that would otherwise cross the 1KB boundary and restart it with a non-sequential transfer, thus:

Address: 0x3F0 0x3F4 0x3F8 0x3FC 0x400 0x404 0x408
Transfer: NSEQ SEQ    SEQ   SEQ    NSEQ  SEQ   SEQ

There is no upper limit to the slave footprint - ARM PrimeCells, for instance, are 4KB

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