how do I select an STM32 for low external interrupt time?

I have a circuit which needs to respond in around 0.5uS to an external interrupt. I built the circuit with an STM32F031K6 and a 20MHz oscillator set to run on the 2x PLL, giving a 40MHz clock. I was surprised to see that although one clock cycle would be 25nS, i could only toggle a pin at 300nS - im not exactly sure why it takes so long, i have some experience with 8 bit AVRs and although I wouldn't expect it to run in one clock cycle, 12 seems slow. The external interrupt takes 3uS to respond. how can i choose a chip to meet my requirement of 0.5uS?

I'm just assuming that i need to change the chip, if anyone has advice on how i might reduce the response time that would also be great

my full code is here, this is a blank program generated by cube, i stripped out some of the generated commenting to make it easier to read



int main(void)
{
  MX_GPIO_Init();
  MX_ADC_Init();

  while (1)
  {
  }
}

void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

  /** Initializes the CPU, AHB and APB busses clocks 
  */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI14|RCC_OSCILLATORTYPE_HSE;
  RCC_OscInitStruct.HSEState = RCC_HSE_ON;
  RCC_OscInitStruct.HSI14State = RCC_HSI14_ON;
  RCC_OscInitStruct.HSI14CalibrationValue = 16;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
  RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL2;
  RCC_OscInitStruct.PLL.PREDIV = RCC_PREDIV_DIV1;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    Error_Handler();
  }
  /** Initializes the CPU, AHB and APB busses clocks 
  */
  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
                              |RCC_CLOCKTYPE_PCLK1;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_1) != HAL_OK)
  {
    Error_Handler();
  }
}

static void MX_ADC_Init(void)
{
  ADC_ChannelConfTypeDef sConfig = {0};

  /** Configure the global features of the ADC (Clock, Resolution, Data Alignment and number of conversion) 
  */
  hadc.Instance = ADC1;
  hadc.Init.ClockPrescaler = ADC_CLOCK_ASYNC_DIV1;
  hadc.Init.Resolution = ADC_RESOLUTION_12B;
  hadc.Init.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc.Init.ScanConvMode = ADC_SCAN_DIRECTION_FORWARD;
  hadc.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
  hadc.Init.LowPowerAutoWait = DISABLE;
  hadc.Init.LowPowerAutoPowerOff = DISABLE;
  hadc.Init.ContinuousConvMode = DISABLE;
  hadc.Init.DiscontinuousConvMode = DISABLE;
  hadc.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc.Init.DMAContinuousRequests = DISABLE;
  hadc.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  if (HAL_ADC_Init(&hadc) != HAL_OK)
  {
    Error_Handler();
  }
  /** Configure for the selected ADC regular channel to be converted. 
  */
  sConfig.Channel = ADC_CHANNEL_0;
  sConfig.Rank = ADC_RANK_CHANNEL_NUMBER;
  sConfig.SamplingTime = ADC_SAMPLETIME_1CYCLE_5;
  if (HAL_ADC_ConfigChannel(&hadc, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
  /** Configure for the selected ADC regular channel to be converted. 
  */
  sConfig.Channel = ADC_CHANNEL_1;
  if (HAL_ADC_ConfigChannel(&hadc, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
}

static void MX_GPIO_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct = {0};

  /* GPIO Ports Clock Enable */
  __HAL_RCC_GPIOF_CLK_ENABLE();
  __HAL_RCC_GPIOA_CLK_ENABLE();
  __HAL_RCC_GPIOB_CLK_ENABLE();

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOB, GPIO_PIN_1|GPIO_PIN_3|GPIO_PIN_4|GPIO_PIN_5 
                          |GPIO_PIN_6|GPIO_PIN_7, GPIO_PIN_RESET);

  /*Configure GPIO pins : PA2 PA11 */
  GPIO_InitStruct.Pin = GPIO_PIN_2|GPIO_PIN_11;
  GPIO_InitStruct.Mode = GPIO_MODE_IT_RISING_FALLING;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*Configure GPIO pins : PA3 PA4 PA12 PA15 */
  GPIO_InitStruct.Pin = GPIO_PIN_3|GPIO_PIN_4|GPIO_PIN_12|GPIO_PIN_15;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*Configure GPIO pins : PA6 PA7 */
  GPIO_InitStruct.Pin = GPIO_PIN_6|GPIO_PIN_7;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF1_TIM3;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*Configure GPIO pin : PB0 */
  GPIO_InitStruct.Pin = GPIO_PIN_0;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF1_TIM3;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  /*Configure GPIO pins : PB1 PB3 PB4 PB5 
                           PB6 PB7 */
  GPIO_InitStruct.Pin = GPIO_PIN_1|GPIO_PIN_3|GPIO_PIN_4|GPIO_PIN_5 
                          |GPIO_PIN_6|GPIO_PIN_7;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  /*Configure GPIO pins : PA8 PA9 PA10 */
  GPIO_InitStruct.Pin = GPIO_PIN_8|GPIO_PIN_9|GPIO_PIN_10;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF2_TIM1;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /* EXTI interrupt init*/
  HAL_NVIC_SetPriority(EXTI2_3_IRQn, 0, 0);
  HAL_NVIC_EnableIRQ(EXTI2_3_IRQn);

  HAL_NVIC_SetPriority(EXTI4_15_IRQn, 0, 0);
  HAL_NVIC_EnableIRQ(EXTI4_15_IRQn);

}

void Error_Handler(void)
{
}

#ifdef  USE_FULL_ASSERT
/**
  * @brief  Reports the name of the source file and the source line number
  *         where the assert_param error has occurred.
  * @param  file: pointer to the source file name
  * @param  line: assert_param error line source number
  * @retval None
  */
void assert_failed(uint8_t *file, uint32_t line)
{ 
  /* USER CODE BEGIN 6 */
  /* User can add his own implementation to report the file name and line number,
     tex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
  /* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

void EXTI2_3_IRQHandler(void)
{
  HAL_GPIO_EXTI_IRQHandler(GPIO_PIN_2);
}

void EXTI4_15_IRQHandler(void)
{
  HAL_GPIO_EXTI_IRQHandler(GPIO_PIN_11);
  GPIOB->ODR ^= 1<<1;
}

Solution

First of all, I recommend having a look at this ARM blog post for an in-depth introduction to interrupt latency of ARM Cortex-M processors.

As mentioned by @Colin the interrupt latency of a STM32F0 MCU with a Cortex-M0 core is 16 clock cycles starting when the signal on the EXTI line is asserted until entering the IRQ Handler with code reacting to the event. This clock cycle count cannot be reduced by firmware. When selecting a MCU with a Cortex-M3 or M4 core (e.g. a STM32F3), this required number of clock cycles drops to 12. The resulting latency still depends on the clock frequency of the core. Selecting a STM32 MCU with higher max. clock rate allows for faster reaction times:

STM32F0: up to 48 MHz Cortex M0 => ISR enter latency 333 ns
STM32G0: up to 64 MHz Cortex M0+ => ISR enter latency 234 ns
STM32F3: up to 72 Mhz Cortex M4 => ISR enter latency 166 ns
STM32G4: up to 170 Mhz Cortex M4 => ISR enter latency 70 ns

These calculation do not solve your problem though, because we have to consider as well what happens after the MCU entered the service routine. Several things come to my mind here:

Backup of additional registers depending on the complexity of the ISR
Code for an application specific reaction to the event (e.g. toggling an GPIO)
Wait-states for FLASH / RAM / Peripheral accesses. The higher the core clock, the more wait-states are typically needed because external parts are clocked at lower frequency.
Code for acknowledging / clearing the interrupt request

The last point can be postponed behind the application specific response an thus does not necessarily count to the reaction time, but all other points can have a significant impact. In order to fulfill your requirement with a cost-efficient STM32 MCU (I suppose you have selected the STM32F0 for this reason) you need to have good control over the number of instructions in the ISR. I do not recommend to use assembler here, but you should not rely on the CubeMX HAL implementation.

Since you say you need only little code in the ISR lets do a quick estimation:

Saving two additional registers on the stack => 2 instructions
Toggle a GPIO with a read-modify-write sequence => 3 instructions

Let's assume that each instruction takes 2 cycles, we need another 10 clock cycles. Using this best-case scenario we can have a look at our list with rather low-cost STM32 MCU's again:

STM32F0: 16 + 10 cycles at 48 MHz => 541 ns
STM32G0: 15 + 10 cycles at 64 MHz => 390 ns
STM32F3: 12 + 10 cycles at 72 MHz => 300 ns
STM32G4: 12 + 10 cycles at 170 MHz => 130 ns

With these numbers, a Cortex M0/M0+ looks not like the right choice. You will better go for a M3/M4 core with at least 64 MHz clock rate. I think the new G4 could be a good solution.

Anyway, I strongly recommend evaluating the real-world performance with the real-world requirement, since there are too many factors that can affect above latency calculations.