It takes data serially from peripheral outside devices and converts into parallel data. After converting the data into parallel form, it transmits it to the CPU. Similarly, it receives parallel data from microprocessor and converts it into serial form. After converting data into serial form, it transmits it to outside device peripheral. The data transmission is possible between and CPU by the data bus buffer block. It controls the overall working by selecting the operation to be done.

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When information is to be sent by over long distances, it is economical to send it on a single line. The has to convert parallel data to serial data and then output it. Thus lot of microprocessor time is required for such a conversion. Similarly, if receives serial data over long distances, the has to internally convert this into parallel data before processing it. Again, lot of time is required for such a conversion.

The can delegate the job of conversion from serial to parallel and vice versa to the A USART used in the system. The A converts the parallel data received from the processor on the D data pins into serial data, and transmits it on TxD transmit data output pin of A. Similarly, it converts the serial data received on RxD receive data input into parallel data, and the processor reads it using the data pins D Features Compatible with extended range of Intel microprocessors. It provides both synchronous and asynchronous data transmission.

Synchronous bit characters. It has full duplex, double buffered transmitter and receiver. Detects the errors-parity, overrun and framing errors. All inputs and outputs are TTL compatible. Available in pin DIP package. 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 internal block diagram of A is shown in fig below. Data Bus Buffer: This bidirectional, 8-bit buffer used to interface the A to the system data bus and also used to read or write status, command word or data from or to the A. This section has three registers and they are control register, status register and data buffer.

When the reset is high, it forces A into the idle mode. Transmitter section: The transmitter section accepts parallel data from microprocessor and converts them into serial data. The transmitter section is double buffered, i. When output register is empty, the data is transferred from buffer to output register.

Now the processor can again load another data in buffer register. If buffer register is empty, then TxRDY is goes to high. The clock frequency can be 1,16 or 64 times the baud rate. The receiver section is double buffered, i. If the line is still low, then the input register accepts the following bits, forms a character and loads it into the buffer register. The microprocessor reads the parallel data from the buffer register.

When the input register loads a parallel data to buffer register, the RxRDY line goes high. Input terminal : This is the "active low" input terminal which receives a signal for writing transmit data and control words from the CPU into the Input terminal : This is the "active low" input terminal which receives a signal for reading receive data and status words from the Input terminal : This is an input terminal which receives a signal for selecting data or command words and status words when the is accessed by the CPU.

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.

TXRDY output terminal : This is an output terminal which indicates that the is ready to accept a transmitted data character. In "synchronous mode," the terminal is at high level, if transmit data characters are no longer remaining and sync characters are automatically transmitted. Note : As the transmitter is disabled by setting CTS "High" or command, data written before disable will be sent out.

After the transmitter is enabled, it sent out. 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 Unless the CPU reads a data character before the next one is received completely, the preceding data will be lost.

In such a case, an overrun error flag status word will be set. RXC Input terminal : 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.

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


Microprocessor | 8251 USART

Data Bus Buffer D0-D7 : 8-bit data bus used to read or write status, command word or data from or to the A 2. RD : When signal goes low, the MPU either reads a status from the status register or accepts data from data buffer. CLK : Clock input, usually connected to the system clock for communication with the microprocessor. Mode word : Specifies the general characteristics of operation such as baud, parity, number of bits etc. Command word : Enables the data transmission and reception. Status register : Status register Checks the ready status of the peripheral.


8251A programmable communication interface block diagram


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