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SWO and variables reading

Pietsuro Botsutoshi
Note: This was originally posted on 27th February 2012 at http://forums.arm.com

Dear all,

I would like to create a SWV application, and my question is the following: how many variables can I read via SWO communication?

I mean, without considering the overflow packet that could be sent in case of huge information traffic, is there a physical limit regarding this topic?

If I've understood weel, DWT_COMPx registers, with x={1, .., 4} is used in order to store the variable address that I would like to monitor, and DWT_MASKx are also used in order to raise a valid new SWO packet.

Does this mean that it's possible to check only for four variables, or should I set the mask register in order to detect ranges of variables?
With the latter case I could be able to monitor a very large number of items.

Am I wrong or is this the right approach?

Thank you very much!
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  • Arm Employee Badge
    Note: This was originally posted on 9th March 2012 at http://forums.arm.com

    Hi,

    Sorry for the delay. It has been a very busy week.

    In your case, variable C and D are very close to each other, so we can set up one comparator to be trigger on both variables:

    First, set TRCENA bit in DEMCR to 1 to enable the trace subsystem including the DWT.

    • variable A, 2 bytes length @0x2000017C[list]
    • DWT_MASK0 = 1 (half word size)
    • DWT_COMP0 = 0x2000017C
    • DWT_FUNC0 - depends on what you want to capture from SWO, for example[list]
    • 0x02 - emit data on read/write
    • 0x03 - emit PC and data on read/write
    • see ARMv7-M architecture reference manual for details of this register
    [/list]
    • variable B, 1 byte length @0x20000190
      • DWT_MASK1 = 0 (byte size)
      • DWT_COMP1 = 0x20000190
      • DWT_FUNC1 -again, depends on what you want to capture
      • variable C, 2 bytes length @0x20000146, variable D, 1 byte length @0x20000144
        • DWT_MASK2 = 2 (word size, or 4 bytes)
        • DWT_COMP2 = 0x20000144 (cover 0x20000144 to 0x20000147)
        • DWT_FUNC2 -again, depends on what you want to capture
        • variable E, 2 bytes length @0x2000023E
          • DWT_MASK3 = 1 (halfword size)
          • DWT_COMP3 = 0x2000023E
          • DWT_FUNC3 -again, depends on what you want to capture[/list]You can increase MASK register to increase the capture range, but you need to make sure the address is aligned to the boundary of the corresponding size.
            Also, the more data address range to be capture by each comparator, you might end up with much more trace traffic and you will need to filter out unwanted information, and potentially lost information due to trace overflow.

            regards,
            Joseph
        Reply
        • Arm Employee Badge
          Note: This was originally posted on 9th March 2012 at http://forums.arm.com

          Hi,

          Sorry for the delay. It has been a very busy week.

          In your case, variable C and D are very close to each other, so we can set up one comparator to be trigger on both variables:

          First, set TRCENA bit in DEMCR to 1 to enable the trace subsystem including the DWT.

          • variable A, 2 bytes length @0x2000017C[list]
          • DWT_MASK0 = 1 (half word size)
          • DWT_COMP0 = 0x2000017C
          • DWT_FUNC0 - depends on what you want to capture from SWO, for example[list]
          • 0x02 - emit data on read/write
          • 0x03 - emit PC and data on read/write
          • see ARMv7-M architecture reference manual for details of this register
          [/list]
          • variable B, 1 byte length @0x20000190
            • DWT_MASK1 = 0 (byte size)
            • DWT_COMP1 = 0x20000190
            • DWT_FUNC1 -again, depends on what you want to capture
            • variable C, 2 bytes length @0x20000146, variable D, 1 byte length @0x20000144
              • DWT_MASK2 = 2 (word size, or 4 bytes)
              • DWT_COMP2 = 0x20000144 (cover 0x20000144 to 0x20000147)
              • DWT_FUNC2 -again, depends on what you want to capture
              • variable E, 2 bytes length @0x2000023E
                • DWT_MASK3 = 1 (halfword size)
                • DWT_COMP3 = 0x2000023E
                • DWT_FUNC3 -again, depends on what you want to capture[/list]You can increase MASK register to increase the capture range, but you need to make sure the address is aligned to the boundary of the corresponding size.
                  Also, the more data address range to be capture by each comparator, you might end up with much more trace traffic and you will need to filter out unwanted information, and potentially lost information due to trace overflow.

                  regards,
                  Joseph
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