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[Cortex-M3] SWO Packet Parsing

Pietsuro Botsutoshi
Note: This was originally posted on 21st February 2012 at http://forums.arm.com

Hi,

I'm trying to play with SEGGER JLink device and ARM M3 micro in order to implement a SWV engine.
Currently I set a SWD protocol communication, and all other needed definitions for the SWO activation:

DEMCR    = 0x01000000
DWT_FUNCTION0 = 0x00000000
DWT_FUNCTION1 = 0x00000000
DWT_FUNCTION2 = 0x00000000
DWT_FUNCTION3 = 0x00000000

I would like to monitor a variable located at address 0x2000017C, a 2 bytes length interger that changes every 100us:

DWT_COMP0   = 0x2000017C
DWT_MASK0  = 0x00000001
DWT_FUNCTION0 = 0x00000023

SELECTED_PIN_PROTOCOL_REG = 0x00000002
TPIU_ACPR       = 0x0000008F
LOCK_ACCESS_REGISTER   = 0xC5ACCE55
ITM_TCR       = 0x0001000F
ITM_TPR       = 0x0000000F
ITM_TER       = 0x00000000
DWT_CTRL       = 0x400003FE
TPIU_FFCR       = 0x00000100

When I start my acquisition phase, I'm able to obtain the buffer content from the TPIU (see attachment):


00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 FC 00 00 00 05 8F 00 00 00 00 D0...

but I have difficulties regarding the parsing process: I'm not able to understand if the 28 bytes of 0s belong to the header that should terminates with only one 0b1.
The serie, in fact, terminates with 0xFC = 0b11111100, but I don't know if the first 1 bit (position [7]) belongs to the header definition, so if I have to discard it and consider the remaining value (0b1111100) value for the next data retrieving.

And after the header, how can I understand what type of info is following?

Thank you very much.
Parents
  • Arm Employee Badge
    Note: This was originally posted on 28th February 2012 at http://forums.arm.com

    Hi there,
    I guess you need to get back to Segger to try to get more information from them.
    Or you can try to capture the data using a logic analyzer and compare against this.

    One thing you can try first is to use ITM stimulus port to generate easy to identify data on the trace, and see how the buffer content looks like.
    Then you can ensure the trace setup is working correctly before decoding other packet types.

    regards,
    Joseph
Reply
  • Arm Employee Badge
    Note: This was originally posted on 28th February 2012 at http://forums.arm.com

    Hi there,
    I guess you need to get back to Segger to try to get more information from them.
    Or you can try to capture the data using a logic analyzer and compare against this.

    One thing you can try first is to use ITM stimulus port to generate easy to identify data on the trace, and see how the buffer content looks like.
    Then you can ensure the trace setup is working correctly before decoding other packet types.

    regards,
    Joseph
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