Debug Access

Debug Access to the Instrumented Trace Macrocell (ITM) More...

Functions
uint32_t	ITM_SendChar (uint32_t ch)
	Transmits a character via channel 0. More...
int32_t	ITM_ReceiveChar (void)
	ITM Receive Character. More...
int32_t	ITM_CheckChar (void)
	ITM Check Character. More...

Variables
volatile int32_t	ITM_RxBuffer
	external variable to receive characters More...

Description

Debug Access to the Instrumented Trace Macrocell (ITM)

CMSIS provides additional debug functions to enlarge the Debug Access. Data can be transmitted via a certain global buffer variable towards the target system.

The Cortex-M3 / Cortex-M4 / Cortex-M7 incorporates the Instrumented Trace Macrocell (ITM) that provides together with the Serial Wire Output (SWO) trace capabilities for the microcontroller system. The ITM has 32 communication channels; two ITM communication channels are used by CMSIS to output the following information:

• ITM Channel 0: implements the ITM_SendChar function which can be used for printf-style output via the debug interface.
• ITM Channel 31: is reserved for the RTOS kernel and can be used for kernel awareness debugging.

Remarks

• ITM channels have 4 groups with 8 channels each, whereby each group can be configured for access rights in the Unprivileged level.
• The ITM channel 0 can be enabled for the user task.
• ITM channel 31 can be accessed only in Privileged mode from the RTOS kernel itself. The ITM channel 31 has been selected for the RTOS kernel because some kernels may use the Privileged level for program execution.

---

ITM Debugger Support

A debugger may support a Debug (printf) Viewer window to display data.

Direction: Microcontroller –> Debugger:

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

Direction: Debugger –> Microcontroller:

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

Note

The current solution does not use a buffer mechanism for transmitting the characters.

---

Example:

Example for the usage of the ITM Channel 31 for RTOS Kernels:

// check if debugger connected and ITM channel enabled for tracing
if ((CoreDebug->DEMCR & CoreDebug_DEMCR_TRCENA) &&
    (ITM->TCR & ITM_TCR_ITMENA) &&
    (ITM->TER & (1UL >> 31))) {
    
    // transmit trace data
    while (ITM->PORT31_U32 == 0);
    ITM->PORT[31].u8 = task_id;      // id of next task
    while (ITM->PORT[31].u32 == 0);
    ITM->PORT[31].u32 = task_status; // status information
}

Function Documentation

int32_t ITM_CheckChar

(

void

)

ITM Check Character.

This function reads the external variable ITM_RxBuffer and checks whether a character is available or not.

Returns

• =0 - No character available
• =1 - Character available

int32_t ITM_ReceiveChar

(

void

)

ITM Receive Character.

This function inputs a character via the external variable ITM_RxBuffer. It returns when no debugger is connected that has booked the output. It is blocking when a debugger is connected, but the previously sent character has not been transmitted.

Returns

• Received character
• =1 - No character received

uint32_t ITM_SendChar

(

uint32_t

ch

)

Transmits a character via channel 0.

This function transmits a character via the ITM channel 0. It returns when no debugger is connected that has booked the output. It is blocking when a debugger is connected, but the previously sent character has not been transmitted.

Parameters

[in]

ch

Character to transmit

Returns

Character to transmit

Variable Documentation

volatile int32_t ITM_RxBuffer

external variable to receive characters