Debugger issues with NUCLEO-L476RG

Mbed Studio Bugs
6

It turns out that the pyocd.core.exceptions.TransferFaultError: SWD/JTAG Transfer Fault issue was not caused by Mbed Studio 1.3.1, but by the choice of the program to debug, namely the regular Blinky Mbed OS example, unmodified, using the Debug build profile. More specifically, it was the use of ThisThread::sleep_for() that was breaking everything.

Adding the following lines at the beginning of main() fixed the issue:

#ifdef MBED_DEBUG
    HAL_DBGMCU_EnableDBGSleepMode();
#endif

This function sets a bit required on this target to make the debug mode work properly with the WFI instruction:

/**
  * @brief  Enable the Debug Module during SLEEP mode.
  * @retval None
  */
void HAL_DBGMCU_EnableDBGSleepMode(void)
{
  SET_BIT(DBGMCU->CR, DBGMCU_CR_DBG_SLEEP);
}

The fact that this change is required in Mbed Studio does not seem normal to me, for two reasons:

  • One would expect to be able to run or debug this example on all Mbed-OS-supported platforms without having to modify it. It would be more natural if this bit were set by Mbed Studio (using Mbed OS or pyOCD) either if the Debug build profile is selected or when the debugger is started.
  • According to this page of the Mbed documentation, the sleep mode is supposed to be disabled for the Debug build profile, but this is actually not the case: the mbed-os-example-blinky\BUILD\NUCLEO_L476RG\ARMC6\.profile* files set DEVICE_SLEEP=1 for this build profile.

This hardware requirement applies to many other Cortex-M targets supported by Mbed OS. I don’t know how Mbed Studio handles it for these other targets.

I will edit the title of this topic to reflect my latest findings.

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