8251 USART ARCHITECTURE PDF

-USART. Serial I/O – Programmable Communication Interface. Data Communications. Data communications refers to the ability of one computer to. USART The is a USART (Universal Synchronous Asynchronous Receiver Transmitter) for serial data communication. Interrupt Structure of . The modem control unit handles the modem handshake signals to coordinate the communication between modem and transmit control unit.

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As a peripheral device of a microcomputer system, the receives parallel data from the CPU and transmits serial data after conversion.

This device also receives serial data from the outside and transmits parallel data to the CPU after conversion.

The functional configuration is programed by software. Operation between the and a CPU is executed by program control. Table architeture shows the operation between a CPU and the device. Mode instruction is used for setting the function agchitecture the Mode instruction will be in “wait for write” at either internal reset or external reset.

That is, the writing of a control word after resetting will be recognized as a “mode instruction. The bit configuration of mode instruction is shown in Figures 2 and 3.

In the case of synchronous mode, it architectute necessary to write one-or two byte sync characters. If sync characters were written, a function will be set because the writing of sync characters constitutes part of mode instruction. Command is used for setting the operation of the It is possible to write a command whenever necessary after writing a mode instruction and sync characters.

It is possible to see the internal status of the by reading a status word.

UNIVERSAL SYNCHRONOUS ASYNCHRONOUS RECEIVER TRANSMITTER

The bit configuration of status word is shown in Fig. This is bidirectional data bus which receive control words and transmits data from the CPU and sends status words and received data to CPU.

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A “High” arfhitecture this input forces the into “reset status. CLK signal is used to generate internal device timing. This is the “active low” input terminal which receives a signal for writing transmit data and control words from the CPU into the Architcture is the “active low” input terminal which receives a signal for reading receive data and status words suart the This is an input terminal which receives usaet signal for selecting data or command words and status words when the is accessed by the CPU.

This is the “active low” input terminal which selects the at low level when the CPU accesses. This is an output terminal for transmitting data from which serial-converted data is sent out.

The device is in “mark status” high level after resetting or during a status when transmit is disabled. It is also possible to set the device in “break status” low level by a command.

This is an output terminal which indicates that uzart is ready to accept a transmitted data character. This is an output terminal which indicates that the has transmitted all the characters and had no data character.

Intel 8251

In “synchronous mode,” the terminal is at high level, if transmit data characters are no longer remaining and sync characters are automatically transmitted.

As the transmitter is disabled by setting CTS “High” or command, data written before disable will be sent out. Even if a data is written after disable, that data is not sent out and TXE will be “High”.

After the transmitter is enabled, it sent out. This is a clock input signal which determines the transfer speed of transmitted data. In “synchronous mode,” the baud rate will be the same as the frequency of TXC. In “asynchronous mode”, it is possible to select the baud rate factor by mode instruction. The falling edge of TXC sifts the serial data out of the This is a terminal which indicates that the contains a character that is ready to READ. Unless the CPU reads a data character before the next one is received completely, the preceding data will be lost.

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In such a case, an overrun error flag status word will be set. This is a clock input signal which determines the transfer speed of received data. In “synchronous mode,” the baud rate is the same as the frequency of RXC. In “asynchronous mode,” it is possible to select the baud rate factor by mode instruction. This is a terminal whose function changes according to mode.

In “internal synchronous mode. If a status word is read, the terminal will be reset. In “external synchronous mode, “this is an input terminal. A “High” on this input forces the to start receiving data characters. In “asynchronous mode,” this is an output terminal which generates “high level”output upon the detection of a “break” character if receiver data contains a “low-level” space between the stop bits of two continuous characters.

The terminal will be reset, if RXD is at high level. After Reset is active, the terminal will be output at low level.

The input status of usqrt terminal can be recognized by the CPU reading status words. It is possible to set the status of DTR by a command. The terminal controls data transmission if the device is set in “TX Enable” status by a command.

Data is transmitable if the terminal is at low level. It is possible to set the status RTS by a command.