Communication can be divided into parallel communication and serial communication according to basic types. When each bit of data is transmitted simultaneously in parallel communication, byte-to-unit communication can be realized, but because the communication line occupies more resources and costs are high. For example, we used P0 = 0xfe in the front; once assigned to the 8 IO ports of P0, and output the signal at the same time, similar to having 8 lanes and 8 cars in the past, this form is parallel, we are used to Also known as P0, P1, P2 and P3 are four sets of parallel buses of 51 single-chip microcomputers.
Serial communication, like a lane, can only be used by one car at a time. If a byte of data such as 0xfe is to be transmitted in the past, if the low position is in the front high position, then the transmission method is 0-1-1- 1-1-1-1-1-1, one-bit transmission, to send 8 times to send a byte.
The UART is a chip for controlling computers and serial devices. One thing to note is that it provides a data terminal device interface so that the computer can communicate with a modem or other serial device that uses an RS-232C interface. As part of the interface, the UART also provides the following features:
Converting parallel data transferred from within the computer into an output serial data stream;
Convert serial data from outside the computer to bytes for use by devices that use parallel data within the computer;
Adding a parity bit to the output serial data stream and performing parity check on the data stream received from the outside;
Adding a start and stop flag to the output data stream, and deleting the start and stop flag from the received data stream;
Handling interrupt signals from the keyboard or mouse (the keyboard and mouse ticket are also serial devices);
Can handle synchronization management issues between computers and external serial devices
Because the computer uses parallel data, it can't directly send the data to the modem. It must be UART-organized for asynchronous transmission. The process is: the CPU first puts the data to be written to the serial device into the UART register (temporary memory block). Then, it is transmitted to the serial device through FIFO (First Input First Output). If there is no FIFO, the information will become messy and cannot be transmitted to the modem.
It is a chip for controlling computers and serial devices. One thing to note is that it provides an RS-232C data terminal device interface so that the computer can communicate with a modem or other serial device that uses the RS-232C interface. As part of the interface, the UART also provides the ability to convert parallel data transferred from within the computer into an output serial data stream. Serial data from outside the computer is converted to bytes for use by devices with parallel data within the computer. A parity bit is added to the output serial data stream, and a parity is received for the data stream received from the outside. A start and stop flag is added to the output data stream, and the start and stop flag is deleted from the received data stream. Handles interrupt signals from the keyboard or mouse (the keyboard and mouse are also serial devices). It can handle the synchronization management of computers and external serial devices. Some of the more upscale UARTs also provide buffers for input and output data. The newer UART is now the 16550, which stores 16 bytes of data in its buffer before the computer needs to process the data. The usual UART is 8250. Now if you buy a built-in modem, there will usually be a 16550 UART inside the modem.
The UART is a key part of the serial communication port in a computer. In a computer, the UART is connected to a circuit that produces a signal that is compatible with the RS232 specification. The RS232 standard defines a logic "1" signal from -3 to -15 volts relative to ground and a logic "0" of +3 to +15 volts relative to ground. Therefore, when a UART in a microcontroller is connected to a PC, it requires an RS232 driver to convert the level.
Uart refers to the TTL level serial port; RS232 refers to the RS232 level serial port.
The TTL level is 5V, while RS232 is a negative logic level, which defines +5~+12V as low level and -12~-5V as high level.
The RXD and TXD of the Uart serial port are generally directly connected to the pins of the processor chip, and the RXD and TXD of the RS232 serial port generally need to be level-converted (usually level-shifted by a chip such as Max232) to be connected to the processor chip. On the pin, otherwise such a high voltage is likely to burn the chip.
The serial port of the computer we usually use is RS232. When we are working on the circuit, we should pay attention to whether the serial port of the peripheral is Uart type or RS232 type. If it does not match, we should find a conversion line (usually this conversion line) There is a chip similar to Max232 for level conversion work), you can not blindly connect the two serial ports.
How the UART communicates:The UART first converts the received parallel data into serial data for transmission. The message frame begins with a low start bit followed by 5~8 data bits, an available parity bit and one or more high stop bits. When the receiver finds the start bit it knows that the data is ready to be sent and tries to synchronize with the transmitter clock frequency. If parity is selected, the UART adds a parity bit after the data bit. The parity bit can be used to help with error checking.
During reception, the UART removes the start and end bits from the message frame, performs parity on the incoming bytes, and converts the data bytes from serial to parallel. The UART also generates additional signals to indicate the status of the transmission and reception. For example, if a parity error is generated, the UART sets the parity flag.
UART communication rate:Data transfer can begin with the least significant bit (LSB). However, some UARTs allow flexible options to send the least significant bit or the most significant bit (MSB) first. The UART in the microcontroller transmits data at speeds ranging from a few hundred bits per second to 1.5 Mb. For example, high-speed UART communications embedded in the ElanSC520 microcontroller can be as fast as 1.152 Mbps. The UART baud rate is also affected by the transmit and receive line pair distance (line length).
Currently, there are two types of hardware on the market that support asynchronous communication and support both asynchronous and synchronous communication. The former is the meaning of the UART name itself, called the Serial Communication Interface (SCI) in Motorola microcontrollers; the Universal Synchronous Asynchronous Receiver Transceiver (USART) in Microchip microcontrollers and the UART in Fujitsu microcontrollers. Two typical examples of people.
UART communication protocol:The UART is a kind of asynchronous serial communication protocol. The working principle is to transmit each character of the transmitted data one by one.
The meaning of each of them is as follows:
Start bit: A signal of logic "0" is sent first to indicate the beginning of the transmitted character.
Data bit: Immediately after the start bit. The number of data bits can be 4, 5, 6, 7, 8, etc., forming one character. Usually ASCII code is used. The transmission starts from the lowest bit and is located by the clock.
Parity bit: When this bit is added to the data bit, the number of bits of "1" should be even (even parity) or odd (odd parity) to verify the correctness of the data transfer.
Stop bit: It is the end of a character data. It can be a high level of 1 bit, 1.5 bits, or 2 bits. Since the data is timed on the transmission line and each device has its own clock, it is likely that there is a small out-of-synchronization between the two devices in the communication. The stop bit therefore not only indicates the end of the transmission, but also provides the opportunity for the computer to correct the clock synchronization. The more bits that apply to the stop bit, the greater the tolerance of different clock synchronizations, but the slower the data transfer rate.
Idle bit: In the logic "1" state, there is no data transmission on the current line.
Baud rate: is a measure of the data transfer rate. Represents the number of binary digits transmitted per second. For example, if the data transfer rate is 120 characters/second and each character is 10 bits, the baud rate is 10&TImes; 120=1200 bits/second=1200 baud.
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