Basic understanding of serial communication
The communication is divided into parallel communication and serial communication, and each bit of data in parallel communication is simultaneously transmitted, and byte-to-unit communication can be realized, but the communication line occupies resources and costs are high. Previously used P1 = 0x55, once assigned to the 8 pins of the P1 port, and signal output at the same time, similar to 8 lanes can pass 8 cars, this form is parallel, generally called P0, P1, P2 and P3 are 4 sets of parallel buses of 51 single-chip microcomputers.
Serial communication is a lane, one can only pass through a car. If a byte of data such as 0x55 is to be transmitted, if the low position is in the front and the high position is in the back, then the transmission method is: 0-1-0. -1-0-1-0-1, one bit for transmission, to send 8 times to send a byte
STC89C52 has two pins which are specially used for serial communication. One is P3.0 (RXD) and the other is P3.1 (TXD). The communication interface they form is serial interface, referred to as serial port. Used for two UARTs for UART communication. Two MCU communication interface connection: RXD - TXD, TXD - RXD.
The TXD transmission channel of the single chip microcomputer 1 is connected to the RXD receiving channel of the single chip microcomputer 2, and the RXD receiving channel of the single chip microcomputer 1 is connected to the TXD transmission channel of the single chip microcomputer 2, thereby realizing mutual communication.
When the MCU 1 wants to send data to the MCU 2, for example, it sends 0xCE, and the binary representation is 11001110. In the serial communication process, it is the principle of low-order first, high post-issue, then let TXD pull low first for a period of time. Time, send a bit 0, then pull high, for a period of time, send a bit, continue to pull high, for a period of time, send a bit, always send the 8-bit binary number 11001110 all over, here is involved One problem is the duration of a period of time. Therefore, a concept baud rate, also called bit rate, which is very important in communication is introduced.
Baud rate
The baud rate is the rate at which binary data bits are transmitted. It is customary to use baud, which is the time we send a binary data for 1/baud. Before communication, MCU 1 and MCU 2 must first clearly agree on the communication baud rate between them, and must be consistent, and both parties can communicate normally.
After agreeing on the speed, we have to consider the second question. When is the data started and when is the end? Both the advance and the end of the delay will receive the error. In uart communication, a byte is 8 bits, which stipulates that when no communication signal occurs, the communication line remains high. Before the data is transmitted, a 0 is sent to indicate the start bit, and then 8 bits of data are transmitted. The data bit is low first and then high. After the digital bit is sent, it will send a bit to indicate the stop bit. Then we want to send the 8-bit data. In fact, we sent 10 bits. One of the two is Start bit, one is the stop bit. The receiver always maintains a high level. Once a low level is detected, it is ready to accept data, accept 8 bits of data, then detect the stop bit, and then prepare for the next data reception.
The schematic diagram of serial port data transmission is actually a time domain diagram, which is the correspondence of signals with time. For example, on the transmit pin of the MCU, the left side occurs first, and the right side is followed. The data bit switching time is one second of the baud rate. If you can understand the concept of time domain, the timing of many communication behind. The diagram is easy to understand.
RS232
On our computer, there is usually a 9-pin serial interface. This serial interface is called RS232 interface. It is related to UART communication. However, since the notebook computer does not have a 9-pin serial port, communication with the MCU is becoming more and more popular. Use a USB virtual serial port.
Nine-pin serial port division and female
5 on the male 4, upper 5 from left to right is 1.2.3.4.5; bottom 4 is from left to right is 6.7.8.9;
5 on the female head 4, upper 5 from left to right is 5.4.3.2.1; lower 4 from left to right is 9.8.7.6;
RS232 interface has a total of 9 pins, respectively defined as: 1, carrier detection DCD; 2, receiving data RXD; 3, sending data TXD; 4, data terminal ready DTR; 5, signal ground SG; 6, data preparation Good DSR; 7, request to send RTS; 8, clear to send CTS; 9, ringing prompt RI. We want to let this serial port communicate with our microcontroller. We only need to care about the 2 pin RXD, 3 pin TXD and 5 pin GND.
Although the names of these three pins are the same as those of the serial port on our MCU, they cannot communicate directly with the MCU. Why is this? As we learn more and more, we have to slowly know that it is not All circuits are 5V for high level and 0V for low level. For the RS232 standard, it is an inverse logic, also called negative logic. Why is it negative logic? Its TXD and RXD voltages, -3V ~ -15V voltage represents 1, and +3 ~ +15V voltage represents 0. Low level represents 1 and high level represents 0, so it is called negative logic. Therefore, the computer's 9-pin RS232 serial port can not be directly connected to the microcontroller, it needs to be completed with a level conversion chip MAX232.
This chip can convert the standard RS232 serial port level to the UART 0V/5V level that our microcontroller can recognize and withstand. From here, everyone seems to understand a little bit, in fact, RS232 serial port and UART serial port, their protocol type is the same, but the level standard is different, and the MAX232 chip plays the role of a middleman, it converts the UART level Into the RS232 level, the RS232 level is also converted to the UART level, thereby achieving a communication connection between the standard RS232 interface and the microcontroller UART.
USB to serial communication
With the development of technology, there is still a large amount of RS232 serial communication used in the industry. However, the application of commercial technology has gradually replaced the RS232 serial port with USB to UART technology. Most laptops have no serial port. So what do we do to achieve communication between the microcontroller and the computer?
We only need to add a USB to serial port chip on the circuit to successfully convert the USB communication protocol and the standard UART serial communication protocol. On our development board, we use the CH340T chip.
We need to use a jumper cap to short the middle and bottom pins together. The circuit on the right side of the CH340T is very simple. After the power supply and the crystal oscillator are connected, the DP and DM of the 6-pin and 7-pin are respectively connected to the 2 data pins of the USB port, and the 3rd and 4th pins are connected to our MCU through the jumper. The TXD and RXD go up.
It is a trick to add a 4148 diode to the 3-pin position of the CH340T circuit. Because the STC89C52 this MCU downloads the program when it needs a cold start, it is first to download and then power on. At the moment of power-on, the MCU will detect the need to download the program first. Although the VCC of the MCU is controlled by the switch, since the 3 pin of the CH340T is the output pin, if there is no such diode, the MCU of the switch after the power is turned off, the pin 3 of the CH340T and the P3.0 of the MCU (ie RXD) The pins are connected together, and a current will flow into the subsequent stage circuit through this pin and charge the capacitor of the latter stage, causing a certain magnitude of voltage in the latter stage. Although this voltage value is only about two or three volts, it may affect the normal. Cold start. After adding a diode, on the one hand, it does not affect communication, and another aspect can eliminate this adverse effect. This place can be used for temporary understanding. If you make such a circuit yourself, you can refer to it.
IO port analog UART serial communication
UART serial port baud rate, commonly used values ​​are 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 38400, 57600, 115200 and other rates. The IO port analog UART serial communication program is a simple demonstration program. We use the serial port debugging assistant to send a data. After the data is incremented by 1, it will automatically return.
Serial debugging assistant, here we directly use the serial port debugging assistant that comes with the STC-ISP software. Let me talk about the use of the serial debugging assistant, as shown in Figure 11-6. The first step is to select the serial assistant menu, the second step is to select the hexadecimal display, the third step is to select the hexadecimal transmission, and the fourth step is to select the COM port. This COM port should be the same as the COM in the computer device manager. The port is consistent, the baud rate is set according to our program. In our program, the data bit duration is 1/9600 seconds. Then the baud rate is 9600, the check bit is N, and the data bit is 8 , stop bit 1.
The essence of the serial port debugging assistant is to use the UART communication interface on the computer to send data to our MCU, and also to receive the data sent by our MCU to the debugging assistant interface.
Because I first came into contact with the communication technology, I explained the following IO analog serial communication program. You can watch the program while watching my explanation and thoroughly understand the underlying principle.
Needless to say, the variable definition part, look directly at the main main function. The first is the setting of the baud rate of the communication. Here we configure the baud rate to be 9600, then the serial debugging assistant has to be 9600. When configuring the baud rate, we use mode 2 of timer T0. In mode 2, TH0 is no longer the upper 8 bits, TL0 is the lower 8 bits, and only TL0 is counting. When TL0 overflows, not only will TF0 be changed to 1, but the content in TH0 will be re-automatically Loaded into TL0. This has the advantage that we can pre-exist the desired initial value of the timer in TH0. When TL0 overflows, TH0 automatically re-enters the initial value into TL0. It is fully automatic and does not need to be re-introduced to TL0 in the program. Assigned, the configuration is very simple, you can look at the program yourself and calculate the initial value.
After the baud rate is set, open the interrupt and wait for the data sent by the serial debugging assistant. When receiving data, first check the low level while (PIN_RXD). If there is no low level, there is no data. Once the low level is detected, the start receiving function StartRXD() is entered. The receiving function starts with a half-baud cycle, and beginners may not understand it here. Let's take a look back at the serial port data diagram in Figure 11-2. If the data bit level is changed, it is easy to read the wrong data because of timing error and signal stability. So we hope that Read the data when the signal is most stable. Except for the position of the edge of the signal change, the other positions are very stable, then we now agree to read the level state in the middle of the signal, which ensures that we must read correctly.
Once the start signal is read, we set the current state to the receive state and turn on the timer interrupt. The first time is half a cycle after entering the interrupt, the start bit is judged twice to confirm the start bit. It is low level, not an interference signal. Each time an interrupt is entered every 1/9600 seconds, and the state of this pin is read into RxdBuf. After waiting for the reception, we will add 1 to this RxdBuf and send it through the TXD pin. It also needs to send a start bit first, then send 8 data bits, and then send the end bit. After the transmission is completed, the program runs to While (PIN_RXD), waiting for the start of the second round of signal reception.
Serial communication basic application
Three basic types of communication
Common communication transmission methods can be divided into simplex communication, half-duplex communication, and full-duplex communication.
Simplex communication means that only one direction is allowed to transmit information in the other direction, while the other party cannot return the message. For example: TV remote control, radio base, etc.
Half-duplex communication means that data can be transmitted between two parties, but only one of them can be sent to the other party at the same time, such as: walkie-talkie
Full-duplex communication means that data can be sent at the same time, and the two can be synchronized. Just like our phone, we can hear the other party's voice at the same time.
Uart module introduction
IO port analog serial port communication, let everyone understand the nature of serial communication, but our MCU program needs to continuously detect the data received by scanning the IO port of the MCU, which occupies a large amount of running time of the MCU. At this time, there will be smart people thinking about it. In fact, we are not very concerned about the process of communication. We only need the result of a communication, and finally get the data received. So we can make a hardware module inside the MCU, let it automatically receive the data, after receiving it, let us know it. We have such a UART module inside our 51 MCU. To use it correctly, of course, we must first respond accordingly. The special function registers are configured.
The structure of the UART serial port of the 51 MCU consists of the serial port control register SCON, the transmitting and receiving circuits. Let's first understand the serial port control register SCON.
Bit assignment of SCON serial controller (address: 0x98)
Bit: Symbol: Reset value: 0: RI: 0; 1: TI: 0; 2: RB8: 0; 3: TB8: 0; 4: REN: 0; 5: SM2: 0; 6: SM1: 0; :SM0:0;
0-bit RI: Receive interrupt flag bit. When the receiving circuit receives the intermediate position of the stop bit, RI is set by hardware and must be cleared by software.
1-bit TI: Transmit interrupt flag bit. When the transmit circuit sends to the middle of the stop bit, TI is set by hardware and must be cleared by software.
2-bit RB8: The 9th bit of data received in modes 2 and 3 (rarely used), mode 1 is used to receive the stop bit.
3-bit TB8: 9th bit of data to be transmitted in modes 2 and 3 (rarely used).
4-bit REN: Enable serial reception. Set by software to enable reception, and software clear to disable reception.
5-bit SM2: Multi-machine communication control bit (rarely used), mode 1 is directly cleared.
6-bit SM1 and 7-bit SM0:
These two together determine the total of four modes of mode 0 to mode 3 of serial communication. The most commonly used one is mode 1, which is SM0=0, SM1=1. Below we focus on mode 1, and other modes are omitted.
For the four modes of the serial port, mode 1 is the most commonly used, which is the 1-bit start bit, the 8-bit data bit and the 1-bit stop bit mentioned earlier. Below we will introduce the details of the work of Mode 1 and how to use it. As for the other three modes, it is similar to this. When you really need to use it, you can check the relevant information.
When we use the IO port to simulate serial communication, the baud rate of the serial port is reflected by the interrupt of timer T0. In the hardware serial port module, there is a dedicated baud rate generator to control the speed at which data is sent and received. For the STC89C52 microcontroller, this baud rate generator can only be generated by timer T1 or timer T2, but not by timer T0, which is a completely different concept from our analog communication.
If timer 2 is used, additional registers need to be configured. By default, timer 1 is used. In this chapter, we mainly use timer T1 as the baud rate generator. The baud rate generator in mode 1 must use timing. Mode 2 of the T1, that is, the automatic reload mode, the formula for calculating the reload value of the timer is:
TH1 = TL1 = 256 - crystal value /12 /2/16 / baud rate
Also related to the baud rate is a register, which is a power management register PCON. Its highest bit can double the baud rate, that is, if PCON |= 0x80 is written, the calculation formula becomes:
TH1 = TL1 = 256 - crystal value / 12 /16 / baud rate
The meaning of the numbers in the formula is explained here. 256 is the overflow value of the 8-bit timer, which is the overflow value of TL1. The crystal value is 11059200 on our development board. 12 means that one machine cycle is equal to 12 clock cycles. It is worth paying attention to this 16, we will focus on the explanation. When the IO port analog serial communication receives data, the middle position of this bit is collected. In fact, the serial port module is more complicated and accurate than our simulation. The way he took it was to collect a signal 16 times, of which 7th, 8th, and 9th were taken out. If two of the three times are high, then the data is considered to be 1, if twice Low level, then it is assumed that this bit is 0, so that once the data is accidentally disturbed, the correct data can still be guaranteed.
The serial communication transmission and reception circuit physically has two SBUF registers with the same name, and their addresses are also 0x99, but one is used for sending buffer and one for receiving buffer. That is to say, there are 2 rooms, the house numbers of the two rooms are the same, one of them only does not enter the person, the other one only enters the person, so that we can realize the full-duplex communication of the UART. There will be no interference between each other. But logically, each time we only operate SBUF, the microcontroller will automatically choose whether to receive SBUF or send SBUF according to whether it is performing a "read" or "write" operation. After passing the program, we will thoroughly understand the problem. .
##UARTVLAN program
In general, the basic steps for writing a serial communication program are as follows:
1. Configure the serial port to mode 1.
2. Configure timer T1 to be mode 2, that is, automatic reload mode.
3. Calculate the initial values ​​of TH1 and TL1 according to the baud rate. If necessary, you can use PCON to double the baud rate.
4. Turn on the timer control register TR1 to let the timer run.
It is also important to note here that when using T1 as the baud rate generator, do not enable the T1 interrupt again.
Let's first look at the program code when the IO port analog serial communication is directly changed to use the hardware UART module, to see if the program is much simpler, because most of the working hardware modules are done for us. The program function is exactly the same as the IO port simulation.
Communication example and ASCLL code
Let's not let go of the Chinese characters we use, then our commonly used characters include 0~9 numbers, A~Z/a~z letters, and various punctuation marks. So how do we express them in the MCU system? ASCII (American Standard Code for Information Interchange) can accomplish this mission: we know that one byte of data in a microcontroller can have 0~ 255 a total of 256 values, we take a total of 128 values ​​from 0 to 127 to give it another layer of meaning
We use a character format to send a lowercase a, return a hexadecimal 0x61, the display on the digital tube is also 61, the character a in the ASCII code table corresponds to decimal 97, equal to hexadecimal 0x61; we use characters again The format sends a number 1, returns a hexadecimal 0x31, and the digital tube displays 31. The decimal value corresponding to the character 1 in the ASCII table is 49, which is equal to 0x31 in hexadecimal. Everyone should be clear about this: the so-called hexadecimal transmission and hexadecimal reception are performed according to the true value of the byte data; while the character format transmission and the character format reception are based on the characters in the ASCII code table. The form is carried out, but it is actually a byte of data that is ultimately transmitted. This form, of course, does not require everyone to remember, understand it, when you use it, check it out.
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