-USART. Serial I/O – Programmable Communication Interface. Data Communications. Data communications refers to the ability of one computer to. The is a Universal Synchronous/Asynchronous Receiver/Transmitter packaged in a pin DIP made by Intel. It is typically used for serial communication. This applet is the first of a series of related applets that demonstrate the USART or universal synchronous and asynchronous receiver and transmitter.

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This is the “active low” input terminal which receives a signal for reading receive data and status words from the Operation between the and a CPU is executed by program control.

This is the “active low” input terminal which receives a signal for writing transmit data and control words from the CPU into the A “High” on this input forces the to start receiving data characters. After the transmitter is enabled, it sent out. The functional configuration is programed by software.

As a peripheral device of a microcomputer system, the receives parallel data from the CPU and transmits serial data after conversion. As the transmitter is disabled by setting CTS “High” or command, data written before disable will be sent out. This is a clock input signal which determines the transfer speed of transmitted data. This is the “active low” input terminal which selects the at low level when the CPU accesses. Mode instruction will be in “wait for write” at either internal reset or external reset.


It is possible to set the status RTS by a command.

In “asynchronous mode”, it is possible to select the baud rate factor by mode instruction. This is a terminal which indicates that the contains a character that is ready to READ. In “synchronous mode,” the terminal is at high level, if transmit data characters are no longer remaining and sync characters are automatically transmitted.

It is also possible to set the device in “break status” low level by a command. The bit configuration of mode instruction is shown in Figures 2 and 3.

This device also receives serial data from the outside and transmits parallel data to the CPU after conversion. CLK signal is used to generate internal device timing. This is a clock input signal which determines the transfer speed of received data. A “High” on this input forces the into “reset status. In “asynchronous mode,” this is an output terminal which generates “high level”output upon the detection of a “break” ueart if receiver data contains a uaart space between the stop bits of two continuous characters.

Intel 8251

Command is used for setting the operation of the After Uwart is active, the terminal will be output at low level. This is an output terminal for transmitting data usat which serial-converted data is sent out. If a status word is read, the terminal will be reset. Table 1 shows the operation between a CPU and the device. The input status of the terminal can be recognized by the CPU reading status words. It is possible to write a command whenever necessary after writing a mode instruction and sync characters.


The terminal will be reset, if RXD is at high level. In “external synchronous mode, “this is an input terminal. The bit configuration of status word is shown in Fig.


In such a case, an overrun error flag status word will be set. Unless the CPU reads a data character before the next one is received completely, the preceding data will be lost.

This is an output terminal which indicates that the is ready to accept a usar data character. The falling edge of TXC sifts the serial data out of the In “synchronous mode,” the baud rate is the same as the frequency of RXC.

This is a terminal whose function changes according to mode. The device is in “mark status” high usarrt after resetting or during a status when transmit is disabled. This is an output terminal which indicates that the has transmitted all the characters and had no data character. If sync characters were written, a function will be set because the writing of sync characters constitutes part of mode instruction.