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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 af_23

    To read back the 3 bytes you want, at addresses 0xA-0xC, you will either need to use a 3-beat transaction (ARLEN=0x2) with ARSIZE=0x0 (8-bit), or a 2-beat transaction (ARLEN=0x1) with ARSIZE=0x1 (16-bit), or a 1-beat transaction (ARLEN=0x0) with ARSIZE=0x3 (64-bit).

    Only the first of these options results in an aligned transaction which only accesses the bytes you want. The other two options will read back more data than is required, for your receiving module to then extract the valid bytes it wants.

    I'm guessing it is the single transfer 64-bit width solution you want, which will access addresses 0x8-0xF.

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  • 0 Arm Employee Badge in reply to af_23

    To read back the 3 bytes you want, at addresses 0xA-0xC, you will either need to use a 3-beat transaction (ARLEN=0x2) with ARSIZE=0x0 (8-bit), or a 2-beat transaction (ARLEN=0x1) with ARSIZE=0x1 (16-bit), or a 1-beat transaction (ARLEN=0x0) with ARSIZE=0x3 (64-bit).

    Only the first of these options results in an aligned transaction which only accesses the bytes you want. The other two options will read back more data than is required, for your receiving module to then extract the valid bytes it wants.

    I'm guessing it is the single transfer 64-bit width solution you want, which will access addresses 0x8-0xF.

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