Unable to run STM32F401 at 84 MHz Sysclk (works fine at 48 MHz)

[Robin7331]

Hello guys :waving_hand:

I am having a STM32F401CDU6TR on a custom board and I am not able to run it at 84 MHz SYSCLK. The source is an external 8 MHz crystal with two 22pF caps as seen here:

​

I created the project in CubeMX as a Makefile project. I only use CubeMX for configuring the GPIOs as well as the clock. The actual code is written and flashed via VSCode. Works great so far.

Here is my current 84 MHz clock configuration in CubeMX:

And this is my RCC configuration:

​

When I flash this code I always end up in a HardFault_Handler before the SystemClock_Config(); function returns.

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};
 
  /** Configure the main internal regulator output voltage
  */
  __HAL_RCC_PWR_CLK_ENABLE();
  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE3);
  /** Initializes the RCC Oscillators according to the specified parameters
  * in the RCC_OscInitTypeDef structure.
  */
  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 = 8;
  RCC_OscInitStruct.PLL.PLLN = 336;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV4;
  RCC_OscInitStruct.PLL.PLLQ = 7;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    Error_Handler();
  }
  /** Initializes the CPU, AHB and APB buses 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_DIV2;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
 
  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK)
  {
    Error_Handler();
  }
}

When I activate MCO1 I can measure the HSE and PLLCLK signals with the expected frequency.

When I set the SYSCLK to 48 MHz as seen here everything works as expected.

 

I have no idea what is going on here 🤷�?♂�?

Thanks for your help!

Cheers,

Robin

[waclawek.jan]

> When I flash this code I always end up in a HardFault_Handler before the SystemClock_Config(); function returns.

Single-step that code, best in disasm view (i.e. instruction by instruction) to find out where it goes into the HardFault.

Is FLASH set appropriately to your VDD and clock? Is VDD stable enough? Do you have adequate decoupling?

Try the same code on a "known good" board such as Nucleo or Disco.

Is your crystal 8MHz?

JW

[Andrew Neil]

"8 MHz crystal with two 22pF caps"

It actually shows 20pF

and the crystal is labelled "16pF"

Is it actually oscillating at all? If it is, is it at 8MHz?

[Robin7331]

Thanks for taking time! Here are my answers.

> Single-step that code, best in disasm view (i.e. instruction by instruction) to find out where it goes into the HardFault.

Will do that later this day. Thanks.

> Is FLASH set appropriately to your VDD and clock?

I am not sure what you mean by that? The voltage of the flash? How do I set that?

> Is VDD stable enough?

I am not sure what is considered stable. I'll do some measurements later while powering on the board.

I am looking for a voltage drop on that line? And or jitter?

> Do you have adequate decoupling?

I placed 100n decoupling caps across all VDD/GND supply pins around the MCU as close to the MCU as possible.

I might be able to refactor this so the caps are closer. But when I take a look at a nucleo board those seem to be even further away from the MCU.

But I have to admit that my decoupling design could be better. Do you think that is the issue?

Here's a picture of the board layout with the 3v3 net highlighted. Back copper is hidden. Its a 4 layer board.

​

> It actually shows 20pF

You're right. I meant to say 20 pF. Thats what I have installed.

> Is it actually oscillating at all? If it is, is it at 8MHz?

It is oscillating at exactly 8 MHz. The only thing that bothers me is the voltage peak to peak is a little low, right?

Might also be because I am introducing an error when measuring with my 10x probe.

I measured at one of the crystals leads.

​

[Tesla DeLorean]

HSE looks fine

W​hat's going on with VCAP? Check voltage and capacitor value.

Should see 1.25V​

[Robin7331]

> W​hat's going on with VCAP? Check voltage and capacitor value.

The value is 2.2n as seen here

​

Thats what I see when measuring on the MCU facing side of the C9. Seems fine, right?

​

[Tesla DeLorean · Accepted solution]

Should be at least 2.2 uF, it is a bulk capacitor for the internal LDO

I'll double check the specs, the single up case might be 4.7 uF

[Tesla DeLorean]

[Robin7331]

Oh you are right!

Thats clearly a bug.

Thanks for pointing it out. Will swap that for a 4.7uF one.

[waclawek.jan]

I hate how Clive always beats us all in this game... must be the inferior quality of my crystal cube... 😉

Jan