How to change system clock during run time?

[GunkutA]

Hello, for decreasing the power coonsumption I want to change HSI to LSI. This is the HSI clock configurations for 72 Mhz:

void SystemClock_Config(void)
{
	RCC_OscInitTypeDef RCC_OscInitStruct = {0};
	RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};
	RCC_PeriphCLKInitTypeDef PeriphClkInit = {0};
 
	/** Initializes the CPU, AHB and APB busses clocks
	 */
	RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
	RCC_OscInitStruct.HSEState = RCC_HSE_ON;
	RCC_OscInitStruct.HSEPredivValue = RCC_HSE_PREDIV_DIV1;
	RCC_OscInitStruct.HSIState = RCC_HSI_ON;
	RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
	RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
	RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL9;
	if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
	{
		Error_Handler();
	}
	/** 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_DIV2;
	RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
 
	if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK)
	{
		Error_Handler();
	}
	PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_ADC|RCC_PERIPHCLK_USB;
	PeriphClkInit.AdcClockSelection = RCC_ADCPCLK2_DIV6;
	PeriphClkInit.UsbClockSelection = RCC_USBCLKSOURCE_PLL_DIV1_5;
	if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
	{
		Error_Handler();
	}
}

And I want to enable LSI then disable the HSI as follows:

	//Turn on the LSI
	RCC->CSR|=RCC_CSR_LSION;
	//Turn off the HSI
	RCC->CR&=~(RCC_CR_HSION);

Is it a correct way to change the system clock to 40 kHz? Or do I any others things that must be done? Any I am not sure from the order. Should I first disable HSI then turn on the LSI or should I first turn on the LSI then turn off the HSI?

[waclawek.jan]

Which STM32? Not all models allow to use LSI as system clock.

Generally, you

• enable the new clock
• wait until stable
• switch the system clock source in RCC_CFGR.SW
• wait until the switch is accomplished (by polling RCC_CFGR.SWS)
• then you can switch off the original clock

Meantime, you might want to change settings of those peripherals which are clock-dependent, e.g. UART baudrate.

Read the RCC chapter in RM.

JW

[GunkutA]

Thank you, that is STM32F10x

[GunkutA]

Yes it does not let LSI being used as the system clock, unfortunately.

[Uwe Bonnes]

You must also care for wait states and maybe other settings, like voltage regulators.