STM32U535 random hard fault when running at 160 MHz

[Fabrice_GIRARDOT]

Hello.

On a custom board with STM32U535CEU6TR, I can't have HCLK set to 160 MHz. At that speed, I get random hard faults. If I set HCLK to 80 MHz, then everything is fine, no more random hard faults.

On the same board, with STM32U535CEU6QTR; no problem with HCLK set to 160 MHz.

Here is the MCU power supply (same for both boards revision) :

Clock initialization is borrowed from CubeMX generated code :

/* Init HAL */
HAL_Init();

/* Configure the internal regulator output voltage */
__HAL_RCC_PWR_CLK_ENABLE();
HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1);

/* Initialize main PLL */
RCC_OscInitTypeDef RCC_OscInitStruct = { 0, };
RCC_OscInitStruct.OscillatorType      = RCC_OSCILLATORTYPE_HSI;
RCC_OscInitStruct.HSIState            = RCC_HSI_ON;
RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
RCC_OscInitStruct.PLL.PLLState        = RCC_PLL_ON;
RCC_OscInitStruct.PLL.PLLSource       = RCC_PLLSOURCE_HSI;
RCC_OscInitStruct.PLL.PLLMBOOST       = RCC_PLLMBOOST_DIV1;
RCC_OscInitStruct.PLL.PLLM            = 4;
RCC_OscInitStruct.PLL.PLLN            = 40;
RCC_OscInitStruct.PLL.PLLP            = 2;
RCC_OscInitStruct.PLL.PLLQ            = 2;
RCC_OscInitStruct.PLL.PLLR            = 1;
RCC_OscInitStruct.PLL.PLLRGE          = RCC_PLLVCIRANGE_0;
RCC_OscInitStruct.PLL.PLLFRACN        = 0;
HAL_RCC_OscConfig(&RCC_OscInitStruct);

/* Connect HCLK SYSCLK PCLK1 PCLK2 and PCLK3 to PLL output */
RCC_ClkInitTypeDef RCC_ClkInitStruct = { 0, };
RCC_ClkInitStruct.ClockType      = RCC_CLOCKTYPE_HCLK  | RCC_CLOCKTYPE_SYSCLK |
                                   RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2  | RCC_CLOCKTYPE_PCLK3;
RCC_ClkInitStruct.SYSCLKSource   = RCC_SYSCLKSOURCE_PLLCLK;
RCC_ClkInitStruct.AHBCLKDivider  = RCC_SYSCLK_DIV1;
RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
RCC_ClkInitStruct.APB3CLKDivider = RCC_HCLK_DIV1;
HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_4);

With STM32U535CEU6TR, if I change this line ...

RCC_ClkInitStruct.AHBCLKDivider  = RCC_SYSCLK_DIV1;

... to this line ...

RCC_ClkInitStruct.AHBCLKDivider  = RCC_SYSCLK_DIV2;

... then I don't get random hard faults, but I don't have the expected clock frequencies I need.

For what I know, difference between Q and non-Q versions of this MCU is the non-Q version has no embedded SMPS voltage regulator. I'm not changing it, not calling HAL_PWREx_ConfigSupply(), so I assume it is set to the default reset value "LDO" (PWR_CR3 bit REGSEL).

I monitored with an oscilloscope the MCU 3V3 power supply and can't identify any issue with it.

Any help would be greatly appreciated.

[Tesla DeLorean · Accepted solution]

Check VCAP parts actually placed.

[gbm]

Check if BOOSTEN bit is set in PWR->VOSR.

I have just realized that I run my U545 at 240 MHz by mistake, without any problems.

[Fabrice_GIRARDOT]

Hi, and thanks for your reply.

VCAP capacitor is close to MCU :

On the actual board, it is present :

Thanks to your hint, I discovered that this capacitor is 10 µF while datasheet says 4.7 µF.

Changed to 4.7 µF and voilà ! MCU is now running fine at 160 MHz ! \o/

Thanks a lot !

[gbm]

I would rather suspect a cold joint corrected as a side effect of your fixes. Usually, but not always, bigger capacitance is better in these places.