ARM Technical Support Knowledge Articles

Why is there a 1KB restriction in AHB?

Applies to: AHB


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


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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