Issue in transmitting data over UART in STM32F103C8 ('Blue Pill') - c

Question

I am beginner in embedded systems. I am trying to write data on UART2 of STM32F103C8 (i. e., the Blue Pill board) and want to see the data in one of the ports of my computer using an FTDI adapter, which is connected to UART2 of the STM32F103C8 board. But on my console I receive some random square block instead of the character which I want to transmit.
Here is my code written in Keil IDE.
#include "stm32f10x.h" // Device header
void usart2_init(void);
void USART_write(int data);
void delayMs(int delay);
int main(void)
{
usart2_init();
while(1)
{
USART_write('A');
delayMs(5000);
}
}
void usart2_init(void)
{
// Enable clock source for USART2
RCC->APB1ENR |= 0x20000; // 0b 0000 0000 0000 0010 0000 0000 0000 0000
RCC->APB2ENR |= 0x4;
GPIOA->CRL |= 0x900; // Set PA2 as TX pin (AF)
USART2->BRR = 0x341; // Setting Baudrate to 9600 #8 MHz.
USART2->CR1 |= 0x00008; // Enable TX only
USART2->CR1 |= 0x02000; // Enable USART module by setting bit 13 to 1 i
USART->CR1 register
}
void USART_write(int data)
{
// We need to wait until Tx buffer is empty for sending data.
while(!(USART2->SR & 0x0080)); // 0x0080
USART2->DR = (data & 0xFF);
}
void delayMs(int delay)
{
int i;
for( ; delay>0 ; delay--)
{
for(i=0; i<3195; i++)
{
}
}
}
Below I attached the screenshot while debugging.
Click here to see screenshot.
You can see the unwanted square block instead of character instead of the characters I want to transmit. In the image you can also see the UART registers and their values.
I am using ST-LINK2 to upload the firmware.
Am I missing some information or doing some mistake while dealing with FTDI and Tera Term? This is my Tera Term configuration:
Baud rate = 9600
Data = 8 bit
Parity = none
Stop bit = 1
Control flow = none
How can I fix this?

Answer 1

When working with those devices you should have a careful reading of datasheets/reference manuals and possibly applicable AN (application notes) widely available from the producer site.
In the specific case, the STM32F10x family works generally with a system clock of 72 MHz generated by an internal PLL oscillator which use the external crystal, having frequency of 8 MHz on Blue Pill, as the reference for the PLL (phase-locked loop) circuitry that controls it. The 8 MHz crystal oscillator is called HSE, which stands for High Speed External oscillator.
I said 'generally' because the user can select different system clock frequencies by programming the specific prescalers and internal clock circuitry, which can become very complicated at first sight (and even at second…).
Now assuming the standard configuration the peripheral interfaces, where the system clock is selected at 72 MHz and the AHB prescaler to 1, the two peripheral clocks (PCLCKx) are set to 36 MHz (PCLCK1: prescaler = 2) and 72 MHz (PCLCK2), respectively.
On STM32F10x chips, only USART1 is clocked from PCLCK2, and all other are clocked from PCKLK1 (maximum 36 MHz).
So your device presumably is clocked at 36 MHz. To have a rate of 9600 baud, we need an overall divisor of 36 MHz / (9600 * 16) = 234.375.
The baud rate generator can handle fractional divisions by considering the integer and fractional parts separately. We get:
DIV_Fraction = 16 * 0.375 = 6 = 0x06
DIV_Mantissa = 234 = 0xEA
USART_BRR = (DIV_Mantissa << 4) | DIV_Fraction = 0xEA6
The notation 0dxx is taken from ST documentation and stands for decimal representation.
Conclusion
To operate on those devices, read carefully the documentation before starting to program, use helping libraries if possible. Personally I prefer libopencm3 over standard HAL libraries, which simplifies the use.

Answer 2

this will work man, i tested it
void init_Usart(void)
{
// Enable clock source for USART2
RCC->APB1ENR |= 0x20000; // 0b 0000 0000 0000 0010 0000 0000 0000 0000
RCC->APB2ENR |= 0x4;
GPIOA->CRL |= 0x900; // Set PA2 as TX pin (AF)
USART2->BRR = 0xEA6; // Setting Baudrate to 9600 #8 MHz.
USART2->CR1 |= 0x00008; // Enable TX only
USART2->CR1 |= 0x02000; // Enable USART module
}
void USART_write(int data)
{
// We need to wait until Tx buffer is empty for sending data.
while(!(USART2->SR & 0x0080)); // 0x0080
USART2->DR = (data & 0xFF);
}