Periodic PC sample packets via DWT

[Scopi]

I am trying to persuade an stm32F412 to provide Periodic PC sample packets.

Reference manual RM0402 implies it is doable (Configure DWT as a PC sampler)

The "Cortex-M4 Technical Reference Manual r0p0" says 
"You can configure the DWT to generate PC samples at defined intervals" and DWT registers are described in the ARMv7M Architecture Reference Manual

That reference manual contains what I thought were reasonable instructions.

I can see the CYCCNT register changing value as expected.

I have confirmed that ITM and DWT registers can be written to. 

I'm obviously missing something!

The below code does generate ITM packets over the SWO interface of my stlink v2-mini e, picked up via openocd, so I believe that the serial and clock speeds are set correctly.

  DBGMCU->CR |=  DBGMCU_CR_TRACE_IOEN; 
  CoreDebug->DEMCR |=  CoreDebug_DEMCR_TRCENA_Msk;

  DWT->CYCCNT = 0;

  ITM->LAR   =  0xC5ACCE55;           // unlock the ITM registers
  ITM->TCR   |=  ITM_TCR_ITMENA_Msk; // enable the ITM
  ITM->TCR |= ITM_TCR_DWTENA_Msk;   //manual described as necessary
  ITM->TCR |= ITM_TCR_SYNCENA_Msk; //This plus DWT syntap should mean I see synch packets

  ITM->TPR   =  0xFFFFFFFF;      // enable unprivileged acc.
  ITM->TER   =  0xFFFFFFFF;      // enable all ports

  TPI->CSPSR =  1;               // port width = 1 bit 
  TPI->SPPR  =  2;               // UART encoding 
  TPI->ACPR  =  (5-1); 

  TPI->FFCR &= ~TPI_FFCR_EnFCont_Msk; // no formatter (unclear what this function really means, more reading required)
  //TPI->FFCR |= TPI_FFCR_EnFCont_Msk; //with this turned on, ITM does seem to emit more readable packets

  DWT->CTRL = 0;
  DWT->CTRL |= (0b1111 << DWT_CTRL_POSTINIT_Pos ); //Try to start the countdown from a high point
  DWT->CTRL |= DWT_CTRL_CYCTAP_Msk 
            | (0xFUL << DWT_CTRL_POSTPRESET_Pos);  //Reset countdown
  DWT->CTRL &= ~DWT_CTRL_CYCTAP_Msk;  //Changing how frequent the counter tics
  DWT->CTRL |= (DWT_CTRL_CYCCNTENA_Msk);  //enable counter
  DWT->CTRL |=(DWT_CTRL_PCSAMPLENA_Msk);  //enable pcsampling

I am trying to use the itm-tools code found at https://github.com/japaric/itm-tools to decode the packets. I have no reason to believe that tool is flawed; I can see it recognising other packet types.

[Scopi · Accepted solution]

Absolute PEBKAC issue here - although I was getting 'other packet types' recognised by the decoder, when I looked at the SWO line on the logic analyser it was clear there was a baudrate mismatch. Doubling the value in ACPR made everything work.