STM32 serial port register library function configuration method

author ： A big cat 1201
special column ：《STM32 Study 》
Maxim ： You just try to , Leave the rest to time ！

describe

Communication is required between devices , For example, between single-chip microcomputer and single-chip microcomputer , Between SCM and computer, etc , There are two ways of communication , Serial communication and parallel communication .

Serial communication and parallel communication

Let's talk about the difference between the two

Parallel communication

• - Transmission principle ： All bits of data are transmitted at the same time .
• - advantage ： Fast
• - shortcoming ： It takes up a lot of pin resources

Suppose that the transmission between two devices is a 8 A data , The above figure shows the connection mode of the data lines of the two devices , Eight wires are needed , Two devices account for eight each IO mouth , every last IO Port corresponds to a bit of data .

serial communication

• - Transmission principle ： Data transmission in bit order .
• - advantage ： Less pin resources
• - shortcoming ： Relatively slow

It is also transmission 8 A data , Only one data line is needed to realize data transmission . The other is a common ground .

‍ Serial communication mode

1. Serial communication is also divided into ：

Simplex ：

• Data transmission only supports data transmission in one direction

Half duplex ：

• Allows data to be transmitted in both directions , however , At some point , Only data can be transmitted in one direction , It's actually a kind of direction switching simplex communication ;

Full duplex system ：

• Allows data to be transmitted in both directions at the same time , therefore , Full duplex communication is a combination of two simplex communication modes , It requires that both the transmitting device and the receiving device have independent receiving and transmitting capabilities .

2. According to the data transmission mode, serial communication is divided into ：

Synchronous communication ： With clock synchronous signal transmission .

• -SPI,IIC communication interface

asynchronous communication ： No clock synchronization .

• -UART( Universal asynchronous transceiver ), Single bus

Common serial communication interface ：

ad locum , Ben meow only talks about asynchronous communication .
Serial asynchronous communication has the following characteristics ：

Full duplex asynchronous communication :

• Fractional baud rate generator system , Provide accurate baud rate .
  - Send and receive common programmable baud rate , Up to 4.5Mbits/s
• Programmable data word length （8 Bits or 9 position ）;
• Configurable stop bits （ Support 1 perhaps 2 Bit stop bit ）;
• Configurable use DMA Multi buffer communication .
• Separate transmitter and receiver enable bits .
• Detection marks ：① Accept buffer ② Send buffer empty ③ End of transmission flag
• Multiple interrupt sources with flags . Trigger interrupt .
• other ： Calibration control , Four error detection flags .

When using asynchronous communication , Parameters need to be set ：

• Start bit
• Data bits （8 Bits or 9 position ）
• Parity bit （ The first 9 position ）
• Stop bit （1,1.5,2 position ）
• Baud rate setting

It looks like this

Such a data format is called a frame format .

Serial communication wiring

The large capacity STM32F10x Series of chips , contain 3 individual USART（ Universal synchronous asynchronous transceiver ） and 2 individual UART（ Universal asynchronous transceiver ）, in other words , As a serial port for asynchronous communication, there can be 5 individual , Their port multiplexing is shown in the figure below ：

RXD Is the receiver pin ,TXD Is the sender pin .
When wiring , One side RXD With the other party TXD Connected to a , also TXD With the other party RXD Connected to a .

Serial communication structure block diagram

The figure above is the whole structure block diagram of serial communication .

Serial communication process

The red frame in the block diagram is the data receiving and generating process of serial communication .

Data transmission ：

• When CPU When you want to send a data , First write the data into the transmit data register （TDR） in , Then send the shift register bit by bit , Until the stop bit is encountered, the transmission ends .

Data reception ：

• When external data is transmitted ,CPU Need to receive data , At this time, the data is received bit by bit in the receive shift register , Then it is transferred to the receiving data register （RDR） in ,CPU Then read data from this register .

Registers involved in serial communication （ Skipping ）

The blue box in the structure block diagram is the register owned by the serial port .

1. Status register (USART_SR)

This is it 32 Bit distribution , Only 0 To 9 Bit in use , Other bits are reserved , Each bit used represents a state of serial communication .

This is what it uses 10 A bit represents the situation of meaning .
In the process of use, some library functions are usually used to query the status of serial communication .

2. Data register (USART_DR)

This is it 32 Bit distribution , Only low 8 position , The rest are reserved , the 8 Bits can be read and written . Whether sending data or receiving data is for this 8 Bit to operate .

This is a 8 Bit details .

3. Control register 1(USART_CR1)

This is it 32 Bit usage , Only low 14 Bit in use , Other bits are reserved .

This is the specific control of each of them .

4. Control register 2(USART_CR2)

This is it 32 Bit usage , GM only uses low 14 position , Other bits are reserved .

This is the specific control of each of them .

5. Control register 3(USART_CR3)

This is it 32 Bit usage , Only the first 11 position , Other bits are reserved .

This is the specific control of each of them .

6. Baud ratio register (USART_BRR)

This is it 32 Bit usage , Only low 16 Bit in use , Other bits are reserved .0 To 3 Bit is the fractional part of baud rate ,4 To 15 Bit is the integer part of baud rate .

This is the specific meaning of its use of bits .
above 6 A register is a register we often use , Of course, there are other registers , When it comes to use, Ben meow will talk about it in detail .

Baud rate setting

The green frame in the serial connection structure block diagram is used to set the baud rate of serial communication .

USART1 It uses f_PCLK2, Its general frequency is 72MHZ, Others USART It uses f_PCLK1, The general frequency is 36MHZ.
The baud rate is f_PCLKx Divided by USARTDIV（ Division coefficient ）, Divided by 16 To come out .
USARTDIV Through register USART_BRR Set the value obtained , It can be a decimal .

‍ Calculation of baud rate

This is the calculation formula of baud rate .
Now let's receive how to pass USARTDIV Get serial port USART_BRR Value in register .

Suppose our serial port 1 The value of baud rate to be set is 115200, and PCLK2 The clock is 72MHZ.

• According to the formula above ：
  USARTDIV=72000000/(115200*16)=39.0625

So get ：

• DIV_Fractino = 16*0.0625 = 1 = 0X01;（ The fractional part ）
  DIV_Mantissa = 39 = 0X27;（ Integral part ）

So now we have USART_BRR The value in the register is 0X271. Just set the serial port 1 Of BRR The value of the register is 0X271 You can get 115200 Baud rate .

Pay attention here , The decimal part is multiplied by 16 And then put in USART_BRR Register low 4 In a .

General steps of serial port configuration

1. Serial clock enable ,GPIO Clock enable

In this meow's article Port reuse and remapping The reuse of serial port has been explained in detail in the article , Here we use it directly .

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA,ENABLE);//PA9 and PA10 It's serial 1 Multiplexing port 
RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1,ENABLE);// A serial port 1 Mounted on APB2 On the bus 

2. GPIO Port mode settings

Due to the port multiplexing function used , So you need to configure , That is to say IO The port is initialized , Also in Port reuse and remapping The reuse of serial port has been explained in detail in the article , Here we use it directly .

GPIO_InitTypeDef GPIO_InitStructe;
GPIO_InitStructe.GPIO_Mode=GPIO_Mode_AF_PP;// Push pull multiplex output 
GPIO_InitStructe.GPIO_Pin=GPIO_Pin_9;//PA9 Pin multiplexing is TX
GPIO_InitStructe.GPIO_Speed=GPIO_Speed_10MHz;// Speed doesn't matter 
GPIO_Init(GPIOA,&GPIO_InitStructe);//PA9 Pin multiplexing is USART1 Of TX Pin 
	
GPIO_InitStructe.GPIO_Mode=GPIO_Mode_IN_FLOATING;// Floating input 
GPIO_InitStructe.GPIO_Pin=GPIO_Pin_10;//PA10 Pin multiplexing is RX
GPIO_InitStructe.GPIO_Speed=GPIO_Speed_10MHz;// Speed doesn't matter 
GPIO_Init(GPIOA,&GPIO_InitStructe);//PA10 Pin multiplexing is USART1 Of RX Pin 

The above code will USART1 Of TXD and RXD The pins are multiplexed to PA9 and PA10 On .

3. Initialization of serial port parameters

USART_InitStructe.USART_BaudRate=115200;// The baud rate is set to 115200
USART_InitStructe.USART_HardwareFlowControl=USART_HardwareFlowControl_None;// No hardware flow control 
USART_InitStructe.USART_Mode=USART_Mode_Tx|USART_Mode_Rx;//USART1 Send and receive pins 
USART_InitStructe.USART_Parity=USART_Parity_No;// No parity bit 
USART_InitStructe.USART_StopBits=USART_StopBits_1;// Stop bit is 1 position 
USART_InitStructe.USART_WordLength=USART_WordLength_8b;// Data bit 8 position , No parity bit 
USART_Init(USART1,&USART_InitStructe);// Serial port initialization 

The above code completes the initialization of the serial port .

4. Turn on interrupt and initialize NVIC（ If you need to turn on interrupt, you need this step ）

In this meow's article NVIC Interrupt priority management The middle level of the article talked about the grouping and use of interrupts .

First, you need to group interrupts in the main function

NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);// Interrupt grouping uses grouping 2

After the grouping is completed, we need to USART1 Interrupt enable , And set the priority

USART_ITConfig(USART1,USART_IT_RXNE,ENABLE);// Yes USART1 Receive interrupt enable of 

The control register above 1(USART_CR1) There is a detailed introduction in the introduction .
Then set the interrupt priority

NVIC_InitTypeDef NVIC_InitStructe;
NVIC_InitStructe.NVIC_IRQChannel=USART1_IRQn;// The interrupt type is USART1 interrupt 
NVIC_InitStructe.NVIC_IRQChannelCmd=ENABLE;// Can make 
NVIC_InitStructe.NVIC_IRQChannelPreemptionPriority=1;// The preemption priority is set to 1
NVIC_InitStructe.NVIC_IRQChannelSubPriority=1;// The response priority is set to 1
NVIC_Init(&NVIC_InitStructe);// Interrupt priority initialization 

5. Enable serial port

After all initialization settings are completed, enable the serial port , That is, open the serial port

USART_Cmd(USART1,ENABLE);// Enable serial port , That is, open the serial port 

7. Write interrupt handling functions

Now the serial port has been opened , We need to write the function after entering the interrupt , That is, what to do after entering the interrupt

void USART1_IRQHandler(void)
{
    
	u8 ret = 0;
	// Determine whether it is a receive interrupt 
	if(USART_GetITStatus(USART1,USART_IT_RXNE))
	{
    
		ret = USART_ReceiveData(USART1);// Read the value in the data register after receiving interrupt 
		USART_SendData(USART1,ret);// Send the received value to the computer 
	}
}

The format of the function name of the interrupt handler function must follow this , Because it has been stipulated in the startup document , The return type is empty

After entering the interrupt , It is possible that the transmission is interrupted , It may also be a receive interrupt , So we need to use

USART_GetITStatus(USART1,USART_IT_RXNE)

This function determines whether the interrupt is a receive interrupt , If so, go back 1, If the return value is 0, The description is sending interrupt , Then the contents in the interrupt processing function will not be executed , Continue to wait for .
The received value is the value we input to the single chip microcomputer on the computer , Then feed this back to the computer .

8. Wait for the interrupt to occur

int main(void)
{
    
	NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);// Interrupt grouping uses grouping 2
	my_USART_Init();// Carry out a series of initialization of the serial port 
	while(1);// Waiting for the interruption to occur 
}

above 8 The first step is the whole register library function configuration process of serial port asynchronous communication , After the configuration is completed, you can use

Of course, there are some library functions that are not mentioned in the above process , For example, serial port transmission status acquisition

FlagStatus USART_GetFlagStatus(USART_TypeDef* USARTx, uint16_t USART_FLAG);
void USART_ClearITPendingBit(USART_TypeDef* USARTx, uint16_t USART_IT);

wait , These functions will be explained in detail later when they are used .

Effect display

Burn the code to STM32 In the after , Open the serial debugging assistant

Set the baud rate , Stop bit , Data bits , The check bit and other information are set as the same as those written in our program .

Send information in the send window

The corresponding information is received in the receiving window

The following is the diagram of the whole interface

Because single chip microcomputer has no phenomenon , So the status of the development board is not shown here .

summary

The configuration of serial port register and serial communication mainly depends on the use of library functions , Be clear about when to use what library functions , What is its role , As for which register is specifically operated , stay ST After the official provides the corresponding library functions, the understanding of registers is not so important , Just have an impression , In case you can find it when you need it . As long as we strictly follow the configuration process mentioned above, it will be easy to configure , As for the specific use of serial communication , In the later serial communication experiment, we will talk about .