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the usage of WSTRB signal

Dong Luo
Note: This was originally posted on 26th February 2009 at http://forums.arm.com

Hi All,
I was going through the AMBA AXI specs, but I have some questions about the usage of the WSTRB signal. In the middle of a burst, can some bits of WSTRB be low? Again, in the last transfer of a burst, can some bits of WSTRB be low? For example, assuming the data bus is 32-bit wide and in the middle of a burst, can I transfer data 0x11xx22xx(x means don't care) and set  WSTRB to be 0b1010 ? Thanks in advance.
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    Note: This was originally posted on 27th February 2009 at http://forums.arm.com

    Hi 0254081,

    > I was going through the AMBA AXI specs, but I have some questions
    > about the usage of the WSTRB signal. In the middle of a burst, can
    > some bits of WSTRB be low?

    Yes.

    > Again, in the last transfer of a burst, can some bits of WSTRB be low?

    Yes.

    > For example, assuming the data bus is 32-bit wide and in the middle
    > of a burst, can I transfer data 0x11xx22xx(x means don't care) and set 
    > WSTRB to be 0b1010 ?

    That depends on the value of AWSIZE.

    If AWSIZE is 3'b010, then yes, WSTRB could be 4'b1010.

    However if you were doing 16 bit transfers on a 32 bit wide burst, you could only assert WSTRB bits that apply to the half of the bus currently being used.

    JD
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  • 0
    Note: This was originally posted on 27th February 2009 at http://forums.arm.com

    Hi 0254081,

    > I was going through the AMBA AXI specs, but I have some questions
    > about the usage of the WSTRB signal. In the middle of a burst, can
    > some bits of WSTRB be low?

    Yes.

    > Again, in the last transfer of a burst, can some bits of WSTRB be low?

    Yes.

    > For example, assuming the data bus is 32-bit wide and in the middle
    > of a burst, can I transfer data 0x11xx22xx(x means don't care) and set 
    > WSTRB to be 0b1010 ?

    That depends on the value of AWSIZE.

    If AWSIZE is 3'b010, then yes, WSTRB could be 4'b1010.

    However if you were doing 16 bit transfers on a 32 bit wide burst, you could only assert WSTRB bits that apply to the half of the bus currently being used.

    JD
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