PWM DMA to drive WS281x LED generates inconsistent period and waveform

[nico23]

I'm trying to drive the PWM of an STM32G0 but the waveform generated had an inconsistent period between bit 0 and bit 1, and I can't understand why

I'm using TIMER 4 on Channel 2. On the main.c I'm initializing MX_DMA_Init

void MX_DMA_Init(void)
{
  /* DMA controller clock enable */
  __HAL_RCC_DMA1_CLK_ENABLE();

  /* DMA1_Channel2_3_IRQn interrupt configuration */
  HAL_NVIC_SetPriority(DMA1_Channel2_3_IRQn, 0, 0);
  HAL_NVIC_EnableIRQ(DMA1_Channel2_3_IRQn);
}

and TIM4 configured as 

void MX_TIM4_Init(void)
{
  /* USER CODE BEGIN TIM4_Init 0 */
  HAL_TIM_Base_DeInit(&htim4);
  /* USER CODE END TIM4_Init 0 */

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

  htim4.Instance = TIM4;
  htim4.Init.Prescaler = 0;
  htim4.Init.CounterMode = TIM_COUNTERMODE_UP;
  htim4.Init.Period = 60 - 1;
  htim4.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;
  htim4.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_DISABLE;
  if (HAL_TIM_Base_Init(&htim4) != HAL_OK)
  {
    Error_Handler();
  }
  sClockSourceConfig.ClockSource = TIM_CLOCKSOURCE_INTERNAL;
  if (HAL_TIM_ConfigClockSource(&htim4, &sClockSourceConfig) != HAL_OK)
  {
    Error_Handler();
  }
  if (HAL_TIM_PWM_Init(&htim4) != HAL_OK)
  {
    Error_Handler();
  }
  sMasterConfig.MasterOutputTrigger = TIM_TRGO_RESET;
  sMasterConfig.MasterSlaveMode = TIM_MASTERSLAVEMODE_DISABLE;
  if (HAL_TIMEx_MasterConfigSynchronization(&htim4, &sMasterConfig) != HAL_OK)
  {
    Error_Handler();
  }
  sConfigOC.OCMode = TIM_OCMODE_PWM1;
  sConfigOC.Pulse = 0;
  sConfigOC.OCPolarity = TIM_OCPOLARITY_HIGH;
  sConfigOC.OCFastMode = TIM_OCFAST_DISABLE;
  if (HAL_TIM_PWM_ConfigChannel(&htim4, &sConfigOC, TIM_CHANNEL_2) != HAL_OK)
  {
    Error_Handler();
  }
  
  /* USER CODE BEGIN TIM4_Init 2 */

  /* USER CODE END TIM4_Init 2 */
  HAL_TIM_MspPostInit(&htim4);
}

The HAL_TIM_MspPostInit has 

__HAL_RCC_GPIOD_CLK_ENABLE();
    /**TIM4 GPIO Configuration
    PD13     ------> TIM4_CH2
    PD14     ------> TIM4_CH3
    PD15     ------> TIM4_CH4
    */
    GPIO_InitStruct.Pin = OUT_uC3_Pin|OUT_uC2_Pin|OUT_uC1_Pin;
    GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
    GPIO_InitStruct.Pull = GPIO_NOPULL;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_HIGH;
    GPIO_InitStruct.Alternate = GPIO_AF2_TIM4;
    HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);

with 

#define OUT_uC3_Pin                     GPIO_PIN_13
#define OUT_uC3_GPIO_Port               GPIOD

The led_render function correctly fills the buffer with

    // Fill with alternating pattern: HI, LO, HI, LO...
    for(uint8_t i = 0; i < WR_BUF_LEN; i++) {
        wr_buf[i] = (i % 2) ? PWM_HI : PWM_LO;
    }
    
    HAL_TIM_PWM_Start_DMA(&htim4, TIM_CHANNEL_2, (uint32_t *) wr_buf, WR_BUF_LEN);

and in the .h

#define PWM_HI (38)
#define PWM_LO (19)
// LED parameters
#define NUM_BPP (3) // WS2812B
#define NUM_PIXELS (18)
#define NUM_BYTES (NUM_BPP * NUM_PIXELS)

But for some reason I'm seeing a the strange, not correct, waveform

Am I doing something wrong?

[waclawek.jan]

What is the expected waveform and how is the observed one different?

[EDIT] Assuming timer clock is equal to system clock, 60 cycles may be sufficient for DMA, but AFAIK Cube/HAL does not use the Update event to load TIMx_CCRx but the CC event, which may leave much less time, and result in DMA not feeding the TIMx_CCRx fast enough. I don't say this *is* the problem here (as I don't know what are the symptoms, see my first question), but may be a thing to consider. Try longer times, or higher system clock frequency. 

JW

[gbm]

What is the declaration of wr_buf and where is it placed?