Design Idea of ​​Software UART Based on Single Chip Microcomputer

At present, there are mainly the following methods to expand the serial port: ①, using serial port expansion chips, such as ST16C550, ST16C554, SP2538, MAX3110, etc. Although the cost is high, the reliability of the system is guaranteed, which is suitable for systems with large data volume and large serial port requirements; ②, using the time-sharing switching method to expand a serial port to communicate with multiple serial port devices. The time-sharing multiplexing method has low cost, but is only suitable for occasions with small data volume, and can only be actively communicated with multiple devices by this single-chip microcomputer, and the real-time performance is poor; ③, using the software simulation method to expand the serial port, its advantages are also low cost and good real-time performance, but it takes up some CPU time.

The general software simulation method of expanding the serial port uses 1 I/O port, 1 INT external interrupt and timer. The serial port expanded by this method has two disadvantages: ①, because the INT external interrupt is used, only 2 INT external interrupts can be used to expand 2 serial ports. ②, the efficiency of sending and receiving data in the article is relatively low, which occupies a lot of CPU time and cannot be performed simultaneously with other tasks, so the scope of use is limited.

The method of simulating serial ports proposed in this paper uses only 2 common I/Os and 1 timer. Since there is no need for INT restrictions, multiple serial ports can be expanded, and the function of FIFO is included. This method expands the sending and receiving data of the simulated serial port in the interrupt service, so it is very efficient. Generally, the single-chip microcomputer supports timer interrupts, so this method can be applied to most single-chip microcomputers.

For low-speed single-chip microcomputers (such as 89S51), low-speed serial ports (9600, 4800, etc.) can be expanded, and for high-speed single-chip microcomputers (such as AVR, PIC, C8051, STC12), high-speed serial ports (such as 19200, 28800, 38400, 57600, etc.) can be expanded. At present, the processing speed of single-chip microcomputers is getting higher and higher, and the price is getting cheaper and cheaper. The STC12C1052 chip used in this paper has high speed and low price, and the price is only RMB 3.8 per piece. When developing and designing electronic products, it is required to reduce hardware costs while ensuring performance. Software simulation and expansion of serial ports provides a good way to reduce costs.

1. Principle of serial communication

In asynchronous communication of serial port, data is transmitted in byte frames with bytes as units. The sender and receiver must communicate in the same byte frame format and baud rate. The byte frame format specifies the start bit, data bit, parity bit, and stop bit. The start bit is the beginning of the byte frame, which makes the data line in the logic 0 state, used to indicate to the receiver that the data frame has started to be sent, so as to synchronize the sending and receiving devices. The stop bit is the end of the byte frame, which makes the data line in the logic 1 state, used to indicate to the receiver that the data frame has been sent. The baud rate adopts standard speed, such as 4800, 9600, 19200, 28800, 38400, 57600, etc.

2. Design idea of ​​software UART

In terms of hardware requirements for this design, only any two I/O ports and one timer of the microcontroller are occupied, and the timer interrupt function of the timer is used to achieve accurate baud rate timing. Both sending and receiving are carried out under the control of the timer interrupt.

The idea of ​​data transmission is that when starting byte transmission, the start bit is first sent through TxD, then the data bit and the parity check bit are sent, and finally the stop bit is sent. The transmission process is controlled by the transmission state machine. Each interruption only sends 1 bit, and after several timed interruptions, the transmission of 1 byte frame is completed. The

idea of ​​data reception is that when it is not in the byte frame reception process, each timed interrupt monitors the state of RxD at 3 times the baud rate. When its sampling level is 1, 0, and 0 for 3 consecutive times, it is considered that the start bit is detected, and then a byte frame reception is started. The byte frame reception process is controlled by the receiving state machine. Each interruption only receives 1 bit, and after several timed interruptions, the reception of 1 byte frame is completed.

In order to improve the performance of the serial port, the FIFO function is implemented in both transmission and reception to improve the real-time performance of the communication. The length of the FIFO can be freely defined to meet the different needs of users.

The baud rate is calculated according to the calculation formula. When setting the highest baud rate, the execution time of the simulated serial port program code must be considered. The timing time must be greater than the specified time of the simulated serial port program. The faster the execution speed of the microcontroller, the higher the serial port communication speed can be achieved.

3. Implementation of software UART design

This program is run and tested on the STC12C1052 high-speed microcontroller produced by Hongjing Technology (Shenzhen). The STC12C1052 microcontroller is a single-clock/machine cycle MCS51 core microcontroller, which is fully compatible with the 89C2051 pin. Its operating frequency reaches 35MHz, which is equivalent to the 420MHz 89C2051 microcontroller, and each piece costs RMB 3.8. Due to the high speed of the microcontroller, the software expansion serial port method is more convenient to achieve high-speed serial ports.

In the design of this extended serial port, the crystal oscillator frequency used by STC12C1052 is 22.1184Mhz, and the timing time is calculated at 3 times the baud rate. During the receiving process, this timing is used to sample the receiving start bit. During the sending and receiving process, the frequency is divided by 3 to obtain the standard baud rate timing for data sending and receiving.

3.1、Data definition

Define some resources necessary for the simulated serial port program, such as I/O pins, baud rate, data buffer, etc.

#define Fosc 22118400 //Crystal frequency

#define Baud 38400 //Baud rate

#define BaudT (Fosc/Baud/3/12)

#define BufLong 16 //FIFO length

sbit RxD1=P1^7; //Analog receiving RxD

sbit TxD1=P1^6; //Analog sending TxD

bit Brxd1,Srxd1;//RxD detection level

BYTE Rbuf1[BufLong];//FIFO receiving area

BYTE Rptr1,Rnum1;

BYTE Tbuf1[BufLong];//FIFO sending area

BYTE Tptr1,Tnum1;

BYTE TimCnt1A,TimCnt1B;

BYTE Mtbuf1,Mrbuf1,TxdCnt1,RxdCnt1;

3.2、Data receiving subroutine

During data receiving, the logical bits of RxD are stored sequentially to form byte data. When data receiving is completed and the stop bit is 1, it means that valid data has been received, and the result is stored in the receiving FIFO queue.

void Recv()

{

if(RxdCnt1＞0) //Store 8 data bits

{

Mrbuf1＞＞=1;

if(RxD1==1) Mrbuf1=Mrbuf1|0x80;

}

RxdCnt1--;

if(RxdCnt1==0&& RxD1==1) //Data receiving is completed

{

Rbuf1[Rptr1]=Mrbuf1; //Store in FIFO queue

if(++Rptr1＞BufLong-1) Rptr1=0;

if(++Rnum1＞BufLong) Rnum1=BufLong;

}

}

3.3. Data sending subroutine

During this program, when the data sending state ends, it checks whether the sending FIFO queue is empty. If it is not empty, it takes out the sending data and then starts the sending state. When in the sending state, it sends the start bit, data bit and stop bit according to the state of the state machine.

void Send()

{

if(TxdCnt1!=0) //Byte sending state machine

{

if(TxdCnt1==11) TxD1=0;//Send start bit 0

else if(TxdCnt1＞2) //Send data bit

{ Mtbuf1＞＞=1; TxD1=CY;}

else TxD1=1; //Send end bit 1

TxdCnt1--;

}

else if(Tnum1＞0) //Detect FIFO queue

{

Tnum1--;

Mtbuf1=Tbuf1[Tptr1]; //Read FIFO dataif

(++Tptr1＞=BufLong) Tptr1=0;

TxdCnt1=11; //Start sending state machine

}

}

3.4, Interrupt program

The interrupt timing time is 1/3 of the baud rate timing, that is, RxD is sampled at 3 times the baud rate to realize the judgment of the start bit, and the receiving process state machine is started when the start bit arrives. Divide the timer by 3 and call the data sending and receiving process to perform serial communication at an accurate baud rate.

void Uart() interrupt 1 using 1

{

if(RxdCnt1==0 ) //Receiving start identification

{

if(RxD1==0 && Brxd1==0 && Srxd1==1) { RxdCnt1=8; TimCnt1B=0;}

}

Srxd1=Brxd1; Brxd1=RxD1;

if(++TimCnt1B＞=3 && RxdCnt1!=0) { TimCnt1B=0; Recv();}//Data receiving

if(++TimCnt1A＞=3) { TimCnt1A=0; Send();} //Data sending

}

3.5、Serial port initialization

Open the timer interrupt, set the timer to self-loading mode, set the timer interrupt interval according to the baud rate, start the timer, and initialize the UART variables.

void IniUart()

{

IE="0x82"; TMOD="0x22";

TH0=-BaudT; TL0=-BaudT; TR0=1;

Rptr1=0;Rnum1=0;Tptr1=0;Tnum1=0;

}

4. Conclusion

The uniqueness of the analog serial port design method proposed in this paper lies in: only using any two ordinary I/O pins and one timer interrupt to realize full-duplex serial port, less hardware occupation, and multiple serial port expansion capability; the judgment method of three consecutive samplings is used when judging the start bit of serial port reception, which is simple to implement and has high accuracy; using timer interrupt to realize the sending and receiving of serial port data, and realizing FIFO queue, so that the sending and receiving of serial port work is efficient.