STM32L151C8T6 USART1 works from time to time

Hello.

I'm using STM32L151C8T6 for some simple task. In the beginning I used MSI clock and everything was fine. But I decided to switch to HSI and now I got problems using USART1, namely I think that the USART1 periphery isn't properly initialized due to some clock problem.

If I use the debugger and I'm watching the USART1 registers, then my program works fine. If I switch to monitor the CPU registers for example, then my program got stuck in my implementation of fputc(). All data for the USART1 registers is 0x00. I guess that something went wrong moving my code to HSI from MSI. Below is the code.

void main() {
	sysinit();
	Dummy_USART_Init(); // #include "usart.h"
	setupIWDG();
	IWDGReset = isIWDGReset();
 
	//some code to read/write variables from the EEPROM
 
	while(1) {
/*...*/
}
}
 

void sysinit() {
	RCC_DeInit();
	RCC_HSICmd(ENABLE); //16MHz, ADC requirement 	// TODO: The ADC HSI need at least 1.5V core!! check for bugs! BUGBUG
	while(RCC_GetFlagStatus(RCC_FLAG_HSIRDY) == RESET);
	RCC_SYSCLKConfig( RCC_SYSCLKSource_HSI ); // MSI, HSI, HSE or PLL
 while(RCC_GetSYSCLKSource() != 0x04);
	RCC_HCLKConfig( RCC_SYSCLK_Div1 ); // AHB clock = SYSCLK 
 
	RCC_PCLK1Config( RCC_HCLK_Div1 );
	RCC_PCLK2Config( RCC_HCLK_Div1 ); //RCC_HCLK_Div1: APB2 clock = HCLK (from AHB clock)
 
	//RCC_PLLConfig( RCC_PLLSource_HSI_Div2, RCC_PLLMul_6 );
	//RCC_PLLCmd( ENABLE );
	//RCC_SYSCLKConfig( RCC_SYSCLKSource_PLLCLK ); //use PLL as the SYSCLK clock source
 
	SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK); //MISC in the help file...
	NVIC_InitTypeDef NVIC_InitStructure;
	NVIC_InitStructure.NVIC_IRQChannel = SysTick_IRQn;
	NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
	NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
	NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
	NVIC_Init(&NVIC_InitStructure);
	
	RCC_ClocksTypeDef workingFreq;
	RCC_GetClocksFreq(&workingFreq);
	systicks = workingFreq.SYSCLK_Frequency / 1000;
	SysTick_Config(systicks); //ticks - number of ticks btw. 2 interrupts
	if(workingFreq.SYSCLK_Frequency > 1000000) {
		sysTicksPerMs = systicks/(workingFreq.SYSCLK_Frequency/1000);
	}
}

void Dummy_USART_Init() {
	RCC_AHBPeriphClockCmd( RCC_AHBPeriph_GPIOA, ENABLE );
	RCC_APB2PeriphClockCmd( RCC_APB2Periph_USART1, ENABLE );
 
	GPIO_InitTypeDef GPIO_InitStructure;
	
	GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
	GPIO_InitStructure.GPIO_OType = GPIO_OType_PP; //push-pull output
	GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9 | GPIO_Pin_10;
	GPIO_InitStructure.GPIO_Speed = GPIO_Speed_40MHz; //400kHz, 2,10,40 MHz
	GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
	GPIO_Init( GPIOA, &GPIO_InitStructure );
 
	GPIO_PinAFConfig( GPIOA, GPIO_PinSource9, GPIO_AF_USART1 );
	GPIO_PinAFConfig( GPIOA, GPIO_PinSource10, GPIO_AF_USART1 );
 
	USART_InitTypeDef USART_InitStructure;
	USART_InitStructure.USART_BaudRate = 115200;
	USART_InitStructure.USART_WordLength = USART_WordLength_8b;
	USART_InitStructure.USART_StopBits = USART_StopBits_1;
	USART_InitStructure.USART_Parity = USART_Parity_No;
	USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
	USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
	
	USART_DeInit( USART1 );
	USART_Init( USART1, &USART_InitStructure );
	USART_Cmd( USART1, ENABLE );
}

Any advises would be helpful!

Edit: here is the fputc()

int fputc(int ch, FILE * f) { //file is defined in stdio.h; set library configuration to FULL
 while( USART_GetFlagStatus(USART1, USART_FLAG_TXE) == RESET );
 USART_SendData( USART1, (unsigned int) ch );
 return ch;
}

 The TC and TXE flags in USART1 are always 0.