Embedded basic interface UART

UART

Universal asynchronous transmitter receiver transmitter ,UART It's a universal serial data bus , For asynchronous communication . The bus communicates in both directions , Can achieve full duplex transmission and reception .

UART Pin

1、VCC： Power supply pin , In general 3.3V, In many embedded systems MCU On UART All controllers are internally powered .

2、GND： Ground pin , Both devices pass through UART When communicating, it is better to share , Otherwise, there may be problems with data reception .

3、RX： Data receiving pin .（ From the other party TX）

4、TX： Data transmission pin .（ From the other party RX）

5、RTS： The output signal , Used to indicate that the device is ready to receive data , Low level active , Low level indicates that the device can receive data .（ May not be used ）

6、CTS： Input signal , Used to determine whether data can be sent to the other party , Low level active , Low level indicates that the device can send data to the other party .（ May not be used ）

UART It's usually used 4 individual pin（VCC, GND, RX, TX）, With TTL level , The low level is 0(0V), The high level is 1（3.3V Or more ）.

Communication protocol

UART Its working principle is to transmit each character of the transmitted data bit by bit , To ensure the correct sending and receiving of data , Both sides of communication need unified configuration , This usually includes configuring the following 4 bits .
1、 Baud rate ： A measure of the speed of data transmission . For example, the baud rate is 115200,8 Digit bit ,1 Stop bit , No parity bit , Then the data transmission speed is ：115200/10*1024=11.25（KB/ second ）

2、 Check bit ： Used to verify the correctness of data transmission . Usually include 4 Verification methods , Odd check （ODD）、 Even check （EVEN）、 Check bit is 0（SPACE） The sum check digit is 1（MARK）.

3、 Data bits ： The number of data bits per transmission , Make a character , Usually use ASCII code .

4、 Stop bit ： The end flag of a character data . Because the data is timed on the transmission line , And every device has its own clock , It's likely that there is a small out of sync between the two devices in the communication . So the stop bit is not just the end of the transmission , And provide the opportunity for the computer to correct the clock synchronization . The more bits apply to stop bits , The greater the tolerance of different clock synchronization , But the data transfer rate is also slower .

The basic structure

1、 Output buffer register ： It receives CPU Parallel data sent from the data bus , And keep it .

2、 Output shift register ： It receives parallel data from the output buffer , Move data bit by bit out at the rate of the transmission clock , Convert parallel data to serial data output .

3、 Input shift register ： It moves the data appearing on the serial data input line bit by bit at the rate of the receiving clock , When the data is full , Sent in parallel to the input buffer register , Convert serial data into parallel data .

4、 Input buffer register ： Receive parallel data from the input shift register , Then from CPU Take away .

5、 Control register ： It receives CPU The control word sent , By the content of the control word , Determine the transmission mode and data format during communication . For example, asynchronous mode or synchronous mode , Number of digits of data characters , Whether there is parity , Odd check or even check , Parameters such as the number of stop bits .

6、 Status register ： The status register stores various status information of the interface , For example, whether the output buffer is empty , Whether the input characters are ready, etc . In the process of communication , When in a certain state , The state detection logic in the interface will the corresponding position of the state register “1”, In order to make CPU Inquire about .

Reference link

https://baike.baidu.com/item/UART/4429746?fr=aladdin