The ON/OFF button

The Nucleo board has a user programmable push button. We will now use it as an ON/OFF button for the alien voice effect.

Configuration

The idea is to use the push button to call an asynchronous routine in our code. To do that, we need to configure the button to trigger an interrupt and then we need to catch the interrupt in our code.

Go into CubeMX by clicking on the ioc file in your alien voice project; in the left panel click on "System > NVIC" and enable the line "EXTI line 4 to 15" by checking the corresponding checkmark. The pin PC13 is linked to EXTI13 in the hardware of the microcontroller. Interrupts are used because they provide a very fast access to the core of the system and thus a very fast reaction.

Still in CubeMX, verify that the label for pin PA5 is "LD2" and the label for pin PC13 is "B1".

Add the following state variable to the USER CODE BEGIN PV section

char user_button = 0;  /* user button status */

and add the following interrupt handler to the USER CODE BEGIN 0 section:

void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin) {
  if (GPIO_Pin == B1_Pin) {
    // blue button pressed
    if (user_button) {
      user_button = 0;
      // turn off LED
      HAL_GPIO_WritePin(LD2_GPIO_Port, LD2_Pin, GPIO_PIN_RESET);
    } else {
      user_button = 1;
      // turn on LED
      HAL_GPIO_WritePin(LD2_GPIO_Port, LD2_Pin, GPIO_PIN_SET);
    }
  }
}

The interrupt handler toggles the variable user_button and switches the LED on when its value is true.

TASK 1: Modify the alien voiceProcessfunction so that it switches between a passthrough and the alien voice.

Benchmarking

Now that we have an ON/OFF button, we can use the benchmarking timer we defined before to see how expensive it is to compute the alien voice.

TASK 2: Add the timing macros to the Process function and use the push button to compare execution times.

Solution

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Task 1

We don't want to check the user_button status variable every time we process a sample, so we will place the logic at the DMA interrupt level, before we process a data buffer. First, rename the function that implements the alien voice form Process to VoiceEffect. Then modify the function prototypes between the /* USER CODE BEGIN PFP */tags like so:

void VoiceEffect(int16_t *pIn, int16_t *pOut, uint16_t size);

void Process(int16_t *pIn, int16_t *pOut, uint16_t size) {
  if (user_button == 1) {
    VoiceEffect(pIn, pOut, size);
  } else { // just pass through
    for (uint16_t i = 0; i < size; pIn += 2, i += 2) {
      *pOut++ = *pIn;
      *pOut++ = *pIn;
    }
  }
}

Task 2

The modified Process function is trivial since we just need to add the timing macros before and after the code:

void Process(int16_t *pIn, int16_t *pOut, uint16_t size) {
  START_TIMER

  if (user_button == 1) {
    VoiceEffect(pIn, pOut, size);
  } else {
    // just pass through
    for (uint16_t i = 0; i < size; pIn += 2, i += 2) {
      *pOut++ = *pIn;
      *pOut++ = *pIn;
    }
  }

  STOP_TIMER
}

You should find that, while the passthrough requires approximately 33 microseconds, the alien voice effect requires 94 microseconds.