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Output data via SWO without debugger ?

Martin Maurer
Note: This was originally posted on 15th November 2010 at http://forums.arm.com

Hello,

i am using a Cortex-M3 (in my case a TI Stellaris LM3S9B92 Rev B1).
I would like to know, if it is possible to output data via SWO without a debugger ?
Do i need any clocks (via SWD) from outside (normally generated by the debugger) to output data via SWO (UART, not Manchester) ?

I already tried the following from inside Cortex-M3:

In setup:

#define TRCENA       0x01000000

#define DEMCR     (*((volatile unsigned long *)(0xE000EDFC)))
#define TRACE_ENABLE_REGISTER     (*((volatile unsigned long *)(0xE0000E00)))
#define TRACE_CONTROL_REGISTER       (*((volatile unsigned long *)(0xE0000E80)))
#define ASYNC_CLOCK_PRESCALER_REGISTER  (*((volatile unsigned long *)(0xE0040010)))
#define SELECTED_PIN_CONTROL_REGISTER   (*((volatile unsigned long *)(0xE00400F0)))

             DEMCR = DEMCR | TRCENA;

             TRACE_ENABLE_REGISTER = 0xFFFFFFFF;

             TRACE_CONTROL_REGISTER = 0x00000001;

             ASYNC_CLOCK_PRESCALER_REGISTER = 10;

             SELECTED_PIN_CONTROL_REGISTER = 0x00000002;

In main loop:

   if (DEMCR & TRCENA)
   {
     while (ITM_Port32(0) == 0);
     ITM_Port8(0) = 0x41;
   }


But i don't see any activity on SWO, nor is the line switched to output at all...

Best regards,

Martin
Parents
  • Note: This was originally posted on 17th November 2010 at http://forums.arm.com

    Hello Joseph,

    first i want to say a big thank you, with your help i meanwhile have UART data on SWO.


    Yes, if the debug interface is in JTAG mode, there is a switching sequence to switch it to Serial Wire model.
    (http://infocenter.ar...d/Chdhfbhc.html)


    I was able to switch to SWD via Keil uVision4 (via start debug in SWD mode). When i reach SWD, i see data coming out of SWO.
    For testing purposes I call every 1 ms the following code:

         while (ITM_Port32(4) == 0);
         ITM_Port32(4) = field;

         while (ITM_Port32(8) == 0);
         ITM_Port32(8) = field;

    On my logic analyzer (decoding UART data, 7,27 MBaud = 80 MHz / 11) i see the following content (always 16 byte in a packet every 1 ms)

    01 23 F4 F5 F4 F5 42 F5 F4 F5 01 F5 00 00 00 3E

    I have an a 8 bit ctr which i fill 4 times into each byte of field (which is 32 bits wide, 4 * 8 bit = 32 bits), so i expected to see e.g. F4 F4 F4 F4 or F5 F5 F5 F5 as content.
    Because data bit 0 seems to be some kind of random, could it be, that i write to ITM_Port, just when it is not allowed (wrong timing) ?
    Is there some other register than i used inside the while, which i must use to synchronize ?

    23 seems to be SWIT, according to http://infocenter.ar...H.html#Chdhdjaa
    meaning Port is 4, length of data is 4, which is correct.
    But second transmission surprises me:
    42 meaning Port is 8, ok, but length is 2 bytes.

    Do you know what is the meaning of 01 at the beginning ? And it looks like again after 10 bytes. I read somewhere that FIFO is only 10 bytes. Could it have to do something with it ?

    Is the overall frame format somewhere explained ? What is the meaning of the 3 bytes 00 and following 3E ?

    How do i find the start of data ? Is it (only) via sync packet, which i must switch on or is there another mechanism ?

    I also tried to output to only one port. I see the following content:

    01 03 xx xx xx xx 01 00 00 00 00 00 00 00 00 02

    Again 16 byte packet, again same question on 01 at beginning and now after 6 bytes, again some 00 and some final byte.


    May I ask you why do you want to use the ITM without a debugger? :-)
    (I suppose you could use a UART for text output?)


    We already use the UART for debugging, but think the speed and built-in features (like the timestamp or channel info) could give us additional benefit for tracing.

    Again many thanks for your help.

    Best regards,

    Martin
Reply
  • Note: This was originally posted on 17th November 2010 at http://forums.arm.com

    Hello Joseph,

    first i want to say a big thank you, with your help i meanwhile have UART data on SWO.


    Yes, if the debug interface is in JTAG mode, there is a switching sequence to switch it to Serial Wire model.
    (http://infocenter.ar...d/Chdhfbhc.html)


    I was able to switch to SWD via Keil uVision4 (via start debug in SWD mode). When i reach SWD, i see data coming out of SWO.
    For testing purposes I call every 1 ms the following code:

         while (ITM_Port32(4) == 0);
         ITM_Port32(4) = field;

         while (ITM_Port32(8) == 0);
         ITM_Port32(8) = field;

    On my logic analyzer (decoding UART data, 7,27 MBaud = 80 MHz / 11) i see the following content (always 16 byte in a packet every 1 ms)

    01 23 F4 F5 F4 F5 42 F5 F4 F5 01 F5 00 00 00 3E

    I have an a 8 bit ctr which i fill 4 times into each byte of field (which is 32 bits wide, 4 * 8 bit = 32 bits), so i expected to see e.g. F4 F4 F4 F4 or F5 F5 F5 F5 as content.
    Because data bit 0 seems to be some kind of random, could it be, that i write to ITM_Port, just when it is not allowed (wrong timing) ?
    Is there some other register than i used inside the while, which i must use to synchronize ?

    23 seems to be SWIT, according to http://infocenter.ar...H.html#Chdhdjaa
    meaning Port is 4, length of data is 4, which is correct.
    But second transmission surprises me:
    42 meaning Port is 8, ok, but length is 2 bytes.

    Do you know what is the meaning of 01 at the beginning ? And it looks like again after 10 bytes. I read somewhere that FIFO is only 10 bytes. Could it have to do something with it ?

    Is the overall frame format somewhere explained ? What is the meaning of the 3 bytes 00 and following 3E ?

    How do i find the start of data ? Is it (only) via sync packet, which i must switch on or is there another mechanism ?

    I also tried to output to only one port. I see the following content:

    01 03 xx xx xx xx 01 00 00 00 00 00 00 00 00 02

    Again 16 byte packet, again same question on 01 at beginning and now after 6 bytes, again some 00 and some final byte.


    May I ask you why do you want to use the ITM without a debugger? :-)
    (I suppose you could use a UART for text output?)


    We already use the UART for debugging, but think the speed and built-in features (like the timestamp or channel info) could give us additional benefit for tracing.

    Again many thanks for your help.

    Best regards,

    Martin
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