Incorrect SPI clock frequency

[Sirac]

Hello,

I'm trying to get data from an external ADC (18bits) with an STM32l476RG nucleo board. There is a lot of overhead and I will improve it later but even the frequency clock is not fast enough.

I don't understand why but the maximum clock frequency I get is 16Mhz (normally 40Mhz) and th signal is very durty. The systeme core clock is 80Mhz (trough the PLL and HSE) and the SPI prescaler is 2. I use cubeMx to configure the MCU:

The system clock is configured like that:

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{
 
  RCC_OscInitTypeDef RCC_OscInitStruct;
  RCC_ClkInitTypeDef RCC_ClkInitStruct;
  RCC_PeriphCLKInitTypeDef PeriphClkInit;
 
    /**Initializes the CPU, AHB and APB busses clocks 
    */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
  RCC_OscInitStruct.HSEState = RCC_HSE_ON;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
  RCC_OscInitStruct.PLL.PLLM = 2;
  RCC_OscInitStruct.PLL.PLLN = 16;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV7;
  RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV2;
  RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV2;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }
 
    /**Initializes the CPU, AHB and APB busses clocks 
    */
  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
                              |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
 
  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_4) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }
 
  PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_USART2;
  PeriphClkInit.Usart2ClockSelection = RCC_USART2CLKSOURCE_PCLK1;
  if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }
 
    /**Configure the main internal regulator output voltage 
    */
  if (HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }
 
    /**Configure the Systick interrupt time 
    */
  HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
 
    /**Configure the Systick 
    */
  HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
 
  /* SysTick_IRQn interrupt configuration */
  HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
}

The SPI is configure as Master and receive only :

static void MX_SPI1_Init(void)
{
 
  /* SPI1 parameter configuration*/
  hspi1.Instance = SPI1;
  hspi1.Init.Mode = SPI_MODE_MASTER;
  hspi1.Init.Direction = SPI_DIRECTION_2LINES_RXONLY;
  hspi1.Init.DataSize = SPI_DATASIZE_9BIT;
  hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
  hspi1.Init.CLKPhase = SPI_PHASE_1EDGE;
  hspi1.Init.NSS = SPI_NSS_SOFT;
  hspi1.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_2;
  hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
  hspi1.Init.TIMode = SPI_TIMODE_DISABLE;
  hspi1.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
  hspi1.Init.CRCPolynomial = 7;
  hspi1.Init.CRCLength = SPI_CRC_LENGTH_DATASIZE;
  hspi1.Init.NSSPMode = SPI_NSS_PULSE_ENABLE;
  if (HAL_SPI_Init(&hspi1) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }
 
}

Any idea about this lack of speed ?

[waclawek.jan]

> the signal is very dirty

What do you exactly mean by this? How do you observe the clock? If you use a LA or oscilloscope, do you have adequate bandwidth/sampling rate?

JW

[Sirac]

There is a large overshoot on the falling and rising edge. I use an 350Mhz probe correctly calibrated and an 400Mhz oscilloscope. Please, see the picture.

[waclawek.jan]

Try decreasing the GPIO drive (as set in GPIO_OSPEEDR).

Ringing and overshoots are high-frequency artefacts on mismatched tracks/load. Note, that a few ns edge means significant frequency content well into the GHz region.

JW

[Bob S]

What @Community member​ said regarding drive strength. Though often that ringing you see on the scope is caused as much by a "long" ground lead on your scope probe as it is actual ringing (even with a 350MHz bandwidth probe, or SPECIALLY with a 350MHz probe).

Also - your SystemClock_Config() sets the PLL parameters for a 32MHz clock from an 8MHz crystal. That is why you are getting a 16MHz SPI clock. Change to M=1, N=20 to get 80 MHz SYSCLK and 40MHz SPI clock

[T J]

16MHz 40HMhz ? are huge clocking rates...

The ringing is the ground probe length and lack of a solid ground pin. as suggested by @Bob S​ 

at those frequencies the SPI will be running warm.

did you add a resistor in series with the clock pin ? at least 27R maybe 100R would be good.

that will lower the power and clip the high energy transistion current spikes. as suggested by @Community member​ 

try to thicken the ground track significantly on the next PCB,

[ from the Scope probe, cut the ground wire down to 1cm, connect it to the SPIchip bypass capacitor ground pin for the best image]

[Sirac]

Thank you for your answer.

"32MHz clock from an 8MHz"

The clock come from a 20Mhz crystal so I get 80Mhz.

I'm agree with you and I'm not really surprised with the poo quality of the signal. What I don't understand is more about the frequency. I should get something very ugly but at 40Mhz. I know it's a lot but it's juste to set up correctly the SPI.

I tried with the discovery board with the exact same config and I get a correct 40Mhz SPI clock but on this one no. I will try on an other pin to see what happen.

[Sirac · Accepted solution]

I find the mistake:

firstly RCC_ClkInitStruct.APB2CLKDivider was configured with RCC_HCLK_DIV2,

and the hardware was configured with the 8Mhz of the stlink MCO and not with the crystal....

So only my fault about this issue...

Thank you for answers.