[flight control development basic tutorial 3] crazy shell · open source formation UAV - serial port (basic transceiver)

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A serial port （ Basic transceiver ）
    
    

 

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    One 、 Serial port introduction
    
    In the field of embedded development, serial port usually refers to UART,UART That is, universal asynchronous transceiver （Universal Asynchronous Receiver/Transmitter). It's a universal serial data bus , For asynchronous communication . The bus communicates in both directions , Can achieve full duplex transmission and reception .
    UART It is often used for data transmission between two single-chip computers , As shown in the figure below .
    
    
    chart 2
    The communication process is shown in the figure below .
    

 

 

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    STM32F103 Provides 3 Serial port , Support synchronous single line communication and half duplex single line communication 、 Support LIN、 Support modem operation 、 Smart card protocol and IrDA SIR ENDEC standard 、 have DMA etc. . The pins that can be configured as serial ports are limited , Therefore, when designing the hardware circuit, we need to pay attention to which pins can be configured as serial port pins , As shown in the following table .
    Serial port pin mapping
    
    
    chart 4
    
    
    
    Two 、UART Serial communication protocol
    
    In the process of serial communication, a frame of data contains the start bit 、 Data bits 、 Stop bits and optional parity bits , In addition to knowing these concepts , You also need to understand the concept of baud rate .
    （1） Start bit , When no data is sent , The data line is in logic “1” state ; Let's start with a logic “0” The signal , Indicates the start of character transfer ;
    （2） Data bits , Immediately after the start bit 8 Data . Start at the lowest level ;
    （3） Parity bit , Generally do not use ;
    （4） Stop bit , It's usually 1 position （ It can also be 1.5 Bits or 2 position ）, High level . As a sign of the end ;
    （5） Baud rate , Baud rate indicates the number of symbols transmitted per second , Is a measure of data transfer rate .
    As shown in the figure below , It is the frame composition of a frame of serial port data .
    

chart 5
    3、 ... and 、UART Serial port register
    
    STM32F103 There are many registers related to serial port , Here we will only talk about a few important registers .
    （1）USART_SR： Status register , As shown in the figure below ：

chart 6
    The bits that need to be understood are ：TXE： Send register is empty ;TC： Send complete ;RXNE: The read data register is not empty ;ORE： Overflow error ;FE: Frame error ;PE: Parity error .
    （2）USART_DR： This register has only low 8 Bit effective , And it has read and write operations , Whether the data in this register is to be sent or received depends on the operation performed “ Read ” still “ write in ”. As shown in the figure below ：
    
    
    chart 7
    
    （3）USART_BRR： Baud rate register , Used to set the baud rate of serial communication , As shown in the figure below ：
    
    
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    The corresponding value will be set in the register ,DIV_Mantissa[11:0] Used to set the integer part of the frequency division coefficient ,DIV_Fraction[3,0] Used to set the fractional part of the frequency division coefficient .
    The baud rate calculation formula of serial port is as follows ：
    
    
    The formula 1
    
    there fPCLKx(x=1、2) It's a clock for peripherals (PCLK1 be used for USART2、3、4、5, PCLK2 be used for USART1, here STM32F103CBT6 No serial port, no explanation ).USARTDIV Is an unsigned floating-point number . Assume that the baud rate of the serial port is 115200, Serial port is used 1, The clock of the serial port is PCLK2 The clock of （ namely APB2 Bus clock ） by 72MHz, As shown in the figure below .
    
    
    chart 9
    According to the formula, we can get :
    USARTDIV=72000000/(115200*16)=39.0625
    
    So the decimal part DIV_Fraction=0.0625*16=1=0x01.
    
    
    Integral part DIV_Mantissa=39=0x27.
    
    Four 、 Serial port basic transceiver experiment
    
    Serial port transceiver experiment use STM32 Serial port 1 namely UART1, Connect the computer through the serial port module , Use the serial port debugging assistant to communicate ,STM32 After receiving the data, send the received data back to the computer . The serial port pin is shown in the figure below ：
    
    
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    The two serial port pins are respectively connected with PA9 And PA10 Pin to pin , Then we are right PA9 And PA10 Pin and serial port module are configured , Configuration code （ By calling the official library ）.
    The idea of coding is as follows ：
    
    Programming ideas
    
    
    surface 1
    The serial port initialization code is as follows ：
    
    
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    The serial port interrupt function code is as follows ：
    
    
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    Set the serial port 1 The baud rate of is 115200, The code is as follows ：
    
    
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    preservation 、 compile 、 Download code , Put the on the flight control USART1 Unplug the connected visual module first , Then connect. USB Serial port module , As shown in the figure below ：
    
    
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    Then I'm putting USB Plug the serial transfer module into the computer USB mouth , Open the serial port debugging assistant on the computer , Find the corresponding COM mouth （ The premise is that USB Serial port drive ）, Set the baud rate to 115200, open , Enter... In the input box of the serial port debugging assistant “www.fengke.club”, Point send , Then you can see that the serial port debugging assistant receives a message from STM32 Postback message of , It's also “www.fengke.club”, As shown in the figure below .
    
    
    chart 15
    
    
  

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