The introduction of serial communication protocol and RS-232 and the working principle of USB/TTL to 232 module

1. Introduction

Serial communication is a very common serial communication mode between devices. Because it is simple and convenient, most electronic devices support this communication mode. Electronic engineers often use this communication mode to output debugging information when debugging equipment.
In computer science, most complex problems can be simplified by layering. For example, the chip is divided into core layer and on-chip peripherals; STM32
The standard library is the software layer between registers and user code. We also understand the communication protocol in a hierarchical way. The most basic thing is to divide it into physical layer and protocol layer.

nameComposition functionphysical layerIt has the characteristics of mechanical and electronic functional parts to ensure the transmission of original data in physical mediaprotocol layerSpecify the communication logic and unify the data packaging and unpacking standards of both sending and receiving parties.

There are many standards and variants in the physical layer of serial communication. The following mainly explains the RS-232 standard

2.RS-232 standard

RS-232 standard mainly specifies the purpose of signal, communication interface and signal level standard

Common communication structures between RS-232 standard serial port devices:

It can be seen from the figure that the two communication devices are connected through DB9 interface and use RS-232 standard for signal transmission.

Then why do you need a level conversion chip?

The reason is very simple, because the controller cannot directly recognize the signal of RS-232 level standard, so the signal can only be converted into the level signal of "TTL standard" recognized by the controller through "level conversion chip". Next, what is the difference between the level of "RS232 standard" and "TTL standard"

(1) Difference between RS232 level and TTL level

According to different level standards used in communication, serial communication can be divided into TTL standard and RS-232 standard

Standard nameLogic 1Logic 0TLL2.4V~5V0~0.5VRS-232-15V~3V+3V~+15V

It is not difficult to see from the table that the logic voltages divided by the two standards are different. TTL level standard is often used in electronic circuits. Ideally, 5V is used to represent binary logic 1 and 0V is used to represent logic 0; In order to increase the long-distance transmission and anti-interference ability of serial communication, it uses - 15V to represent logic 1 and + 15V to represent logic 0

The following figure shows the comparison when RS232 and TTL level calibration are used to represent the same signal

(2) RS-232 signal line

RS-232 serial port standard is commonly used for communication between computer, routing and modulation (MODEN, commonly known as "cat"). In this communication system, the equipment is divided into Data Terminal Equipment DTE (computer, routing) and Data Communication Equipment DCE (modem)
Desktop computers generally have RS-232 standard COM port (also known as DB9 interface), in which the pin type outgoing signal line is called the male head, and the hole type outgoing signal line is called the female head. The male connector is generally led out in the computer, while the female connector is generally led out in the modem equipment. It can be connected with the computer by using the serial port cable in the figure above. During communication, the signal transmitted in the serial port line is modulated using the RS-232 standard explained above.

The following describes the names and basic functions of each interface

nameSymbolData directionexplainCarrier detectionDCDDTE→DCEIt is used for DTE to inform the other party whether the machine has received the carrier signal of the other partyreceive data RXDDTE←DCEData receiving signal, i.e. inputsend dataTXDDTE→DCEData transmission signal, i.e. outputData terminalDTRDTE→DCEUsed by DTE to inform the other party whether the machine is readySignallyGNDThe ground potential between the two communication devices may be different, which will affect the level signal of the transceiver. Therefore, the ground connection must be used between the two serial port devices, that is, common groundData equipmentDSRDTE←DCEIt is used by DCE to inform the other party whether the machine is in standby stateRequest sendRTSDTE→DCEDTE requests DCE to send data to DCEAllow sendingCTSDTE←DCEDCE responds to the RTS sending request of the other party and tells the other party whether it can send dataRing indicate RIDTE←DCEIndicates that the DCE terminal and the line have been connected

Note: the TXD and RXD between the two devices should be cross connected

2, USB to serial port

1. Basic working principle

USB to serial port is to realize the conversion from computer USB interface to physical serial port. You can add serial ports for computers or other USB hosts without serial ports. Using USB to serial port devices is equivalent to turning the traditional serial port devices into plug and play USB devices.

After the USB host detects that the USB to serial port device is inserted, it will first reset the device, and then start the USB enumeration process. During USB enumeration, device descriptors, configuration descriptors, interface descriptors, etc. will be obtained. The descriptor will contain the manufacturer ID, device ID and Class class of the USB device. The operating system will match the corresponding USB device driver for the device according to this information.

The realization of USB virtual serial port depends on the USB to serial port driver in the system, which is generally directly provided by the manufacturer. The CDC serial port driver of the operating system can also be used. The driver is mainly divided into two functions. One is to register the USB device driver to complete the control and data communication of the USB device. The other is to register the serial port driver to provide the corresponding implementation method for the serial port application layer.

List of drive data flows corresponding to serial port transceiver:

Occurrence or receptionData flow directionSerial port transmissionSerial port application sends data → USB serial port driver obtains data → driver sends data to USB serial port device through USB channel → USB serial port device receives data and sends it through serial portSerial port receivingThe USB serial port device receives the serial port data → packages the serial port data through USB and uploads it to the USB host → the USB serial port driver obtains the serial port data uploaded through USB → the driver saves the data in the serial port buffer and provides it to the serial port application for reading

2.CH340 module introduction

Physical map
Internal structure diagram

Note: RXD receiving terminal is connected to external TX transmitting terminal, and TXD transmitting terminal is connected to external RX receiving terminal

3, Instance

1. Example description

Complete a USART serial port communication program of STM32 (query mode is OK, interrupt mode is not required temporarily). Requirements:

Set the baud rate to 115200, 1 stop bit and no check bit;
STM32 system continuously sends "hello windows!" to the upper computer (win10). Win10 uses the "serial port assistant" tool to receive.

2. Example process

(1) Create project

Open KEIL5 to create a new project, right-click source and add a new file
Because it is an assembly language, you choose to create Asm files
Creation complete

(2) Code part

code:

;RCC Register address mapping RCC_BASE EQU 0x40021000 RCC_CR EQU (RCC_BASE + 0x00) RCC_CFGR EQU (RCC_BASE + 0x04) RCC_CIR EQU (RCC_BASE + 0x08) RCC_APB2RSTR EQU (RCC_BASE + 0x0C) RCC_APB1RSTR EQU (RCC_BASE + 0x10) RCC_AHBENR EQU (RCC_BASE + 0x14) RCC_APB2ENR EQU (RCC_BASE + 0x18) RCC_APB1ENR EQU (RCC_BASE + 0x1C) RCC_BDCR EQU (RCC_BASE + 0x20) RCC_CSR EQU (RCC_BASE + 0x24) ;AFIO Register address mapping AFIO_BASE EQU 0x40010000 AFIO_EVCR EQU (AFIO_BASE + 0x00) AFIO_MAPR EQU (AFIO_BASE + 0x04) AFIO_EXTICR1 EQU (AFIO_BASE + 0x08) AFIO_EXTICR2 EQU (AFIO_BASE + 0x0C) AFIO_EXTICR3 EQU (AFIO_BASE + 0x10) AFIO_EXTICR4 EQU (AFIO_BASE + 0x14) ;GPIOA Register address mapping GPIOA_BASE EQU 0x40010800 GPIOA_CRL EQU (GPIOA_BASE + 0x00) GPIOA_CRH EQU (GPIOA_BASE + 0x04) GPIOA_IDR EQU (GPIOA_BASE + 0x08) GPIOA_ODR EQU (GPIOA_BASE + 0x0C) GPIOA_BSRR EQU (GPIOA_BASE + 0x10) GPIOA_BRR EQU (GPIOA_BASE + 0x14) GPIOA_LCKR EQU (GPIOA_BASE + 0x18) ;GPIO C Mouth control GPIOC_BASE EQU 0x40011000 GPIOC_CRL EQU (GPIOC_BASE + 0x00) GPIOC_CRH EQU (GPIOC_BASE + 0x04) GPIOC_IDR EQU (GPIOC_BASE + 0x08) GPIOC_ODR EQU (GPIOC_BASE + 0x0C) GPIOC_BSRR EQU (GPIOC_BASE + 0x10) GPIOC_BRR EQU (GPIOC_BASE + 0x14) GPIOC_LCKR EQU (GPIOC_BASE + 0x18) ;Serial port 1 control USART1_BASE EQU 0x40013800 USART1_SR EQU (USART1_BASE + 0x00) USART1_DR EQU (USART1_BASE + 0x04) USART1_BRR EQU (USART1_BASE + 0x08) USART1_CR1 EQU (USART1_BASE + 0x0c) USART1_CR2 EQU (USART1_BASE + 0x10) USART1_CR3 EQU (USART1_BASE + 0x14) USART1_GTPR EQU (USART1_BASE + 0x18) ;NVIC Register address NVIC_BASE EQU 0xE000E000 NVIC_SETEN EQU (NVIC_BASE + 0x0010) ;SETENA Starting address of register array NVIC_IRQPRI EQU (NVIC_BASE + 0x0400) ;Start address of interrupt priority register array NVIC_VECTTBL EQU (NVIC_BASE + 0x0D08) ;Address of vector table offset register NVIC_AIRCR EQU (NVIC_BASE + 0x0D0C) ;Address of application interrupt and reset control register SETENA0 EQU 0xE000E100 SETENA1 EQU 0xE000E104 ;SysTick Register address SysTick_BASE EQU 0xE000E010 SYSTICKCSR EQU (SysTick_BASE + 0x00) SYSTICKRVR EQU (SysTick_BASE + 0x04) ;FLASH Buffer register address image FLASH_ACR EQU 0x40022000 ;SCB_BASE EQU (SCS_BASE + 0x0D00) MSP_TOP EQU 0x20005000 ;Starting value of main stack PSP_TOP EQU 0x20004E00 ;Process stack start value BitAlias_BASE EQU 0x22000000 ;Bit alias area start address Flag1 EQU 0x20000200 b_flas EQU (BitAlias_BASE + (0x200*32) + (0*4)) ;Bit address b_05s EQU (BitAlias_BASE + (0x200*32) + (1*4)) ;Bit address DlyI EQU 0x20000204 DlyJ EQU 0x20000208 DlyK EQU 0x2000020C SysTim EQU 0x20000210 ;Constant definition Bit0 EQU 0x00000001 Bit1 EQU 0x00000002 Bit2 EQU 0x00000004 Bit3 EQU 0x00000008 Bit4 EQU 0x00000010 Bit5 EQU 0x00000020 Bit6 EQU 0x00000040 Bit7 EQU 0x00000080 Bit8 EQU 0x00000100 Bit9 EQU 0x00000200 Bit10 EQU 0x00000400 Bit11 EQU 0x00000800 Bit12 EQU 0x00001000 Bit13 EQU 0x00002000 Bit14 EQU 0x00004000 Bit15 EQU 0x00008000 Bit16 EQU 0x00010000 Bit17 EQU 0x00020000 Bit18 EQU 0x00040000 Bit19 EQU 0x00080000 Bit20 EQU 0x00100000 Bit21 EQU 0x00200000 Bit22 EQU 0x00400000 Bit23 EQU 0x00800000 Bit24 EQU 0x01000000 Bit25 EQU 0x02000000 Bit26 EQU 0x04000000 Bit27 EQU 0x08000000 Bit28 EQU 0x10000000 Bit29 EQU 0x20000000 Bit30 EQU 0x40000000 Bit31 EQU 0x80000000 ;Vector table AREA RESET, DATA, READONLY DCD MSP_TOP ;Initialize main stack DCD Start ;Reset vector DCD NMI_Handler ;NMI Handler DCD HardFault_Handler ;Hard Fault Handler DCD 0 DCD 0 DCD 0 DCD 0 DCD 0 DCD 0 DCD 0 DCD 0 DCD 0 DCD 0 DCD 0 DCD SysTick_Handler ;SysTick Handler SPACE 20 ;Reserved space 20 bytes ;Code snippet AREA |.text|, CODE, READONLY ;Main program start ENTRY ;Instructs the program to execute from here Start ;Clock system settings ldr r0, =RCC_CR ldr r1, [r0] orr r1, #Bit16 str r1, [r0] ;Enable external crystal oscillator ;Start external 8 M Crystal oscillator ClkOk ldr r1, [r0] ands r1, #Bit17 beq ClkOk ;Wait for the external crystal oscillator to be ready ldr r1,[r0] orr r1,#Bit17 str r1,[r0] ;FLASH Buffer ldr r0, =FLASH_ACR mov r1, #0x00000032 str r1, [r0] ;set up PLL The PLL magnification is 7,HSE Input no frequency division ldr r0, =RCC_CFGR ldr r1, [r0] orr r1, #(Bit18 :OR: Bit19 :OR: Bit20 :OR: Bit16 :OR: Bit14) orr r1, #Bit10 str r1, [r0] ;start-up PLL Phase locked loop ldr r0, =RCC_CR ldr r1, [r0] orr r1, #Bit24 str r1, [r0] PllOk ldr r1, [r0] ands r1, #Bit25 beq PllOk ;choice PLL Clock as system clock ldr r0, =RCC_CFGR ldr r1, [r0] orr r1, #(Bit18 :OR: Bit19 :OR: Bit20 :OR: Bit16 :OR: Bit14) orr r1, #Bit10 orr r1, #Bit1 str r1, [r0] ;other RCC Related settings ldr r0, =RCC_APB2ENR mov r1, #(Bit14 :OR: Bit4 :OR: Bit2) str r1, [r0] ;IO port settings ldr r0, =GPIOC_CRL ldr r1, [r0] orr r1, #(Bit28 :OR: Bit29) ;PC.7 Output mode,Maximum speed 50 MHz and r1, #(~Bit30 & ~Bit31) ;PC.7 Universal push-pull output mode str r1, [r0] ;PA9 Serial port 0 transmitting pin ldr r0, =GPIOA_CRH ldr r1, [r0] orr r1, #(Bit4 :OR: Bit5) ;PA.9 Output mode,Maximum speed 50 MHz orr r1, #Bit7 and r1, #~Bit6 ;10: Multiplexing function push-pull output mode str r1, [r0] ldr r0, =USART1_BRR mov r1, #0x271 str r1, [r0] ;Configure baud rate-> 115200 ldr r0, =USART1_CR1 mov r1, #0x200c str r1, [r0] ;USART Module total enable send and receive enable ;71 02 00 00 2c 20 00 00 ;AFIO Parameter setting ;Systick Parameter setting ldr r0, =SYSTICKRVR ;Systick Initial installation value mov r1, #9000 str r1, [r0] ldr r0, =SYSTICKCSR ;set up,start-up Systick mov r1, #0x03 str r1, [r0] ;NVIC ;ldr r0, =SETENA0 ;mov r1, 0x00800000 ;str r1, [r0] ;ldr r0, =SETENA1 ;mov r1, #0x00000100 ;str r1, [r0] ;Switch to user level line program mode ldr r0, =PSP_TOP ;Initialize thread stack msr psp, r0 mov r0, #3 msr control, r0 ;initialization SRAM register mov r1, #0 ldr r0, =Flag1 str r1, [r0] ldr r0, =DlyI str r1, [r0] ldr r0, =DlyJ str r1, [r0] ldr r0, =DlyK str r1, [r0] ldr r0, =SysTim str r1, [r0] ;Main cycle main ldr r0, =Flag1 ldr r1, [r0] tst r1, #Bit1 ;SysTick Generate 0.5s,Set bit 1 beq main ;0.5s The flag is not set yet ;0.5s The flag has been set ldr r0, =b_05s ;Bit band operation reset 0.5s sign mov r1, #0 str r1, [r0] bl LedFlas mov r0, #'H' bl send_a_char mov r0, #'e' bl send_a_char mov r0, #'l' bl send_a_char mov r0, #'l' bl send_a_char mov r0, #'o' bl send_a_char mov r0, #' ' bl send_a_char mov r0, #'w' bl send_a_char mov r0, #'o' bl send_a_char mov r0, #'r' bl send_a_char mov r0, #'l' bl send_a_char mov r0, #'d' bl send_a_char mov r0, #'\n' bl send_a_char b main ;Subroutine serial port 1 sends a character send_a_char push ldr r2, =USART1_DR str r0, [r2] b1 ldr r2, =USART1_SR ldr r2, [r2] tst r2, #0x40 beq b1 ;Send complete(Transmission complete)wait for pop bx lr ;subroutine led twinkle LedFlas push ldr r0, =Flag1 ldr r1, [r0] tst r1, #Bit0 ;bit0 Flashing flag bit beq ONLED ;Open for 0 led lamp ;Off for 1 led lamp ldr r0, =b_flas mov r1, #0 str r1, [r0] ;The flashing flag position is 0,The next status is on ;PC.7 Output 0 ldr r0, =GPIOC_BRR ldr r1, [r0] orr r1, #Bit7 str r1, [r0] b LedEx ONLED ;Open for 0 led lamp ldr r0, =b_flas mov r1, #1 str r1, [r0] ;The flashing flag position is 1,The next status is off ;PC.7 Output 1 ldr r0, =GPIOC_BSRR ldr r1, [r0] orr r1, #Bit7 str r1, [r0] LedEx pop bx lr ;Abnormal program NMI_Handler bx lr HardFault_Handler bx lr SysTick_Handler ldr r0, =SysTim ldr r1, [r0] add r1, #1 str r1, [r0] cmp r1, #500 bcc TickExit mov r1, #0 str r1, [r0] ldr r0, =b_05s ;The clock tick counter is set to 0 when it is greater than or equal to 500 times of clearing.5s Flag bit ;Bit band operation set 1 mov r1, #1 str r1, [r0] TickExit bx lr ALIGN ;By using zero or null instructions NOP fill,Aligns the current position with a specified boundary END

There was no error compiling

(3) Burning program

Download succeeded

(4) Serial port debugging

Next, open the upper computer and serial communication assistant. I use SSCOM. Click to open the communication port in the upper left corner and select CH340
Click to open the serial port

3. Result display

Display received Hello world, successful!

4, Summary

This blog introduces the serial port protocol of STM32F103 single chip microcomputer, introduces the RS-232 standard, explains the reason why RSS-232 is converted to TLL and its principle, and compares the two levels. This paper expounds the basic principle of USB transfer serial port, takes CH340 module as an example, inputs Hello World to the host computer through the serial port, and applies the theoretical learning to practice, hoping to help readers. At the same time, it also hopes that readers can try to experiment by themselves. Only through the experiment can they really know and master knowledge. If there are deficiencies or omissions in the blog, I hope you can point out

5, References

USB to serial port CH340 wiring method
Principle and application of USB to serial port
Creating STM32 assembler based on MDK: serial port output Hello world