CMSIS Debug Support

Cortex-M3 ITM Debug Access

The Cortex-M3 incorporates the Instrumented Trace Macrocell (ITM) that provides together with the Serial Viewer Output trace capabilities for the microcontroller system. The ITM has 32 communication channels which are able to transmit 32 / 16 / 8 bit values; two ITM communication channels are used by CMSIS to output the following information:

ITM Channel 0: used for printf-style output via the debug interface.
ITM Channel 31: is reserved for RTOS kernel awareness debugging.

Debug IN / OUT functions

CMSIS provides following debug functions:

ITM_SendChar (uses ITM channel 0)
ITM_ReceiveChar (uses global variable)
ITM_CheckChar (uses global variable)

ITM_SendChar

ITM_SendChar is used to transmit a character over ITM channel 0 from the microcontroller system to the debug system.
Only a 8 bit value is transmitted.

static __INLINE uint32_t ITM_SendChar (uint32_t ch)
{
  /* check if debugger connected and ITM channel enabled for tracing */
  if ((CoreDebug->DEMCR & CoreDebug_DEMCR_TRCENA)  &&
      (ITM->TCR & ITM_TCR_ITMENA)                  &&
      (ITM->TER & (1UL << 0))  ) 
  {
    while (ITM->PORT[0].u32 == 0);
    ITM->PORT[0].u8 = (uint8_t)ch;
  }  
  return (ch);
}

ITM_ReceiveChar

ITM communication channel is only capable for OUT direction. For IN direction a globel variable is used. A simple mechansim detects if a character is received. The project to test need to be build with debug information.

The globale variable ITM_RxBuffer is used to transmit a 8 bit value from debug system to microcontroller system. ITM_RxBuffer is 32 bit wide to enshure a proper handshake.

extern volatile int ITM_RxBuffer;                    /* variable to receive characters                             */

A dedicated bit pattern is used to determin if ITM_RxBuffer is empty or contains a valid value.

#define             ITM_RXBUFFER_EMPTY    0x5AA55AA5 /* value identifying ITM_RxBuffer is ready for next character */

ITM_ReceiveChar is used to receive a 8 bit value from the debug system. The function is nonblocking. It returns the received character or '-1' if no character was available.

static __INLINE int ITM_ReceiveChar (void) {
  int ch = -1;                               /* no character available */

  if (ITM_RxBuffer != ITM_RXBUFFER_EMPTY) {
    ch = ITM_RxBuffer;
    ITM_RxBuffer = ITM_RXBUFFER_EMPTY;       /* ready for next character */
  }
  
  return (ch); 
}

ITM_CheckChar

ITM_CheckChar is used to check if a character is received.

static __INLINE int ITM_CheckChar (void) {

  if (ITM_RxBuffer == ITM_RXBUFFER_EMPTY) {
    return (0);                                 /* no character available */
  } else {
    return (1);                                 /*    character available */
  }
}

ITM Debug Support in uVision

uVision uses in a debug session the Debug (printf) Viewer window to display the debug data.

Direction microcontroller system -> uVision:

Characters received via ITM communication channel 0 are written in a printf style to Debug (printf) Viewer window.

Direction uVision -> microcontroller system:

Check if ITM_RxBuffer variable is available (only performed once).
Read character from Debug (printf) Viewer window.
If ITM_RxBuffer empty write character to ITM_RxBuffer.

Note

Current solution does not use a buffer machanism for trasmitting the characters.

RTX Kernel awareness in uVision

uVision / RTX are using a simple and efficient solution for RTX Kernel awareness. No format overhead is necessary.
uVsion debugger decodes the RTX events via the 32 / 16 / 8 bit ITM write access to ITM communication channel 31.

Following RTX events are traced:

Task Create / Delete event
1. 32 bit access. Task start address is transmitted
2. 16 bit access. Task ID and Create/Delete flag are transmitted
  High byte holds Create/Delete flag, Low byte holds TASK ID.
Task switch event
1. 8 bit access. Task ID of current task is transmitted

Note

Other RTOS information could be retrieved via memory read access in a polling mode manner.