Cannot restart Output Compare

[NickO]

I am using output compare with TIM3 on a STM32H755

 

I am able to use Output Compare to generate a single pulse on pin PC7 using TIM3, Channel 2.

I am using Active Level on Match.

• I call HAL_TIM_OC_Start(&htim3, TIM_CHANNEL_2) once
• After 0.5s, it detects a match (which is good), and my pin drops and stays low (I have negative polarity set).

However,  I can find no way to restart the process.

I have tried resetting the timer calling HAL_TIM_OC_Stop(&htim3, TIM_CHANNEL_2) and HAL_TIM_OC_Start(&htim3, TIM_CHANNEL_2), but the pin remains low. I've even tried calling the initialisation code again.

 

I assume I am missing something fundamental here?

 

Settings: (see screen shot attached )

Auto-reload preload: Disable

Mode: Active Level on Match

Output Compare Preload: Disable

CH Polarity: Low

 

[Sarra.S]

Hello @NickO, 

You can consider one pulse mode, this mode stops the counter at the next update event, and you might need to reconfigure or reset this setting to restart the pulse generation! 

Otherwise, could you share the timer configuration?

 

[NickO]

Hi

 

Thank you for responding.

 

I did look at one-pulse mode, but I struggled to get it to work. I may need to find an example as reading the reference guide didn't help. What I actually need is a re-triggerable one-pulse (the thinking being that it needs to be constantly tr-triggered to sustain the the pin in a high state).

As I could not get it to work, I achieve this by using output capture, where producer threads are pushing data into a queue. With each new sample, the counter is reset to zero (thus preventing a match) as follows:

__HAL_TIM_SET_COUNTER(&htim3, 0);

If the producer thread was to freeze (or miss the deadline), the output drops to zero and stays there, which is the intended behaviour. However, should I wish to reset this, it seems "challenging"

 

As for my current approach, here is the initialisation code (ignore the reference to tim8):

void MX_TIM3_Init(void)
{

 /* USER CODE BEGIN TIM3_Init 0 */

 /* USER CODE END TIM3_Init 0 */

 TIM_ClockConfigTypeDef sClockSourceConfig = {0};
 TIM_MasterConfigTypeDef sMasterConfig = {0};
 TIM_OC_InitTypeDef sConfigOC = {0};

 /* USER CODE BEGIN TIM3_Init 1 */

 /* USER CODE END TIM3_Init 1 */
 htim3.Instance = TIM3;
 htim3.Init.Prescaler = 50000-1;
 htim3.Init.CounterMode = TIM_COUNTERMODE_UP;
 htim3.Init.Period = 4000-1;
 htim3.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;
 htim3.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_DISABLE;
 if (HAL_TIM_Base_Init(&htim3) != HAL_OK)
 {
 Error_Handler();
 }
 sClockSourceConfig.ClockSource = TIM_CLOCKSOURCE_INTERNAL;
 if (HAL_TIM_ConfigClockSource(&htim3, &sClockSourceConfig) != HAL_OK)
 {
 Error_Handler();
 }
 if (HAL_TIM_OC_Init(&htim3) != HAL_OK)
 {
 Error_Handler();
 }
 sMasterConfig.MasterOutputTrigger = TIM_TRGO_RESET;
 sMasterConfig.MasterSlaveMode = TIM_MASTERSLAVEMODE_DISABLE;
 if (HAL_TIMEx_MasterConfigSynchronization(&htim3, &sMasterConfig) != HAL_OK)
 {
 Error_Handler();
 }
 sConfigOC.OCMode = TIM_OCMODE_ACTIVE;
 sConfigOC.Pulse = 2000;
 sConfigOC.OCPolarity = TIM_OCPOLARITY_LOW;
 sConfigOC.OCFastMode = TIM_OCFAST_DISABLE;
 if (HAL_TIM_OC_ConfigChannel(&htim3, &sConfigOC, TIM_CHANNEL_2) != HAL_OK)
 {
 Error_Handler();
 }
 /* USER CODE BEGIN TIM3_Init 2 */

 /* USER CODE END TIM3_Init 2 */
 HAL_TIM_MspPostInit(&htim3);

}

 

And here:

void HAL_TIM_MspPostInit(TIM_HandleTypeDef* timHandle)
{

 GPIO_InitTypeDef GPIO_InitStruct = {0};
 if(timHandle->Instance==TIM3)
 {
 /* USER CODE BEGIN TIM3_MspPostInit 0 */

 /* USER CODE END TIM3_MspPostInit 0 */
 __HAL_RCC_GPIOC_CLK_ENABLE();
 /**TIM3 GPIO Configuration
 PC7 ------> TIM3_CH2
 */
 GPIO_InitStruct.Pin = 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_AF2_TIM3;
 HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

 /* USER CODE BEGIN TIM3_MspPostInit 1 */

 /* USER CODE END TIM3_MspPostInit 1 */
 }
 else if(timHandle->Instance==TIM8)
 {
 /* USER CODE BEGIN TIM8_MspPostInit 0 */

 /* USER CODE END TIM8_MspPostInit 0 */

 __HAL_RCC_GPIOC_CLK_ENABLE();
 /**TIM8 GPIO Configuration
 PC8 ------> TIM8_CH3
 */
 GPIO_InitStruct.Pin = GPIO_PIN_8;
 GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
 GPIO_InitStruct.Pull = GPIO_NOPULL;
 GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
 GPIO_InitStruct.Alternate = GPIO_AF3_TIM8;
 HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

 /* USER CODE BEGIN TIM8_MspPostInit 1 */

 /* USER CODE END TIM8_MspPostInit 1 */
 }

}

 

I start the timer using:

HAL_TIM_OC_Start(&htim3, TIM_CHANNEL_2);

 

[waclawek.jan]

As @Sarra.S said above, this works exactly as expected: once a match occurs, OCxRef goes Active, and any further match sets it again to Active i.e. does nothing.

If you don't want to use one of the PWM modes of TIMx_CHx  (possibly with running the timer in One-Pulse mode, TIMx_CR1.OPM=1), set the given OCxRef Inactive, you can use for example the Force inactive level i.e. set TIMx_CCMRx.OCxM to 0b100.

I don't think this is something you can simply click in CubeMX. See description of TIMx_CCMR.OCxM in the TIM chapter in RM.

JW

[NickO]

That makes a lot of sense now you say it. I will try that and confirm.
The key for me is that the output stays low, and that this is the correct and expected behaviour as you confirm, so thanks - that’s most reassuring. 
The motivation behind my question is less that I want to restart the process as such, but to understand what it would take to do so (part of assessing risk). I will reread the reference guide in light of this.