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AXI4 Narrow and possibly unaligned READ

af_23

Hi.

I have a 16-byte AXI4 data bus.

I want to read 3 bytes, and there's a limitation to only use INCR burst. 

I know that AXI only supports 1,2,4,8, etc byte-size bursts, but I have another module to receive the data from AXI and extract only the desired 3 bytes.

Length 0 (1 beat) is enough obviously.

Let's say the read address is 0x4a7a.

My question is, what is the correct ARSIZE for this read request?

I originally thought that SIZE = 0x2 (4 bytes) is the answer, exemplified below:

this Access is not aligned.

0xF 0xE 0xD 0xC 0xB 0xA 0x9 0x8 0x7 0x6 0x5 0x4 0x3 0x2 0x1 0x0
V V V V

But I guess that SIZE = 0x3 (8 bytes) is also possible:

0xF 0xE 0xD 0xC 0xB 0xA 0x9 0x8 0x7 0x6 0x5 0x4 0x3 0x2 0x1 0x0
V V V V V V

this Access is not aligned as well

Is there one better than the other? is one of them illegal?

Thanks

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  • 0 Arm Employee Badge in reply to Brice Collier

    Note that nowhere does any spec say that "narrow burst is not recommended", so the start of the above post is misleading !

    Obviously if you use narrow transfers they are not utilising the full width of the data bus, so are not maximising possible bandwidth, but they are supported for the very reason the original poster was asking about, when data narrower than the data bus width is required.

    And yes, AXI4-lite does not support narrow transfers as there is no AxSIZE, but again that isn't relevant here as the original post was asking about AXI4.

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  • 0 Arm Employee Badge in reply to Brice Collier

    Note that nowhere does any spec say that "narrow burst is not recommended", so the start of the above post is misleading !

    Obviously if you use narrow transfers they are not utilising the full width of the data bus, so are not maximising possible bandwidth, but they are supported for the very reason the original poster was asking about, when data narrower than the data bus width is required.

    And yes, AXI4-lite does not support narrow transfers as there is no AxSIZE, but again that isn't relevant here as the original post was asking about AXI4.

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