• The 8251A is a programmable serial communication interface chip designed for synchronous and asynchronous serial data communication.

  • It supports the serial transmission of data.

  • It is packed in a 28 pin DIP.

Read/Write control logic:

  • The Read/Write Control logic interfaces the 8251A with CPU, determines the functions of the 8251A according to the control word written into its control register.

  • It monitors the data flow.

  • This section has three registers and they are control register, status register and data buffer.

  • The active low signals RD, WR, CS and C/D(Low) are used for read/write operations with these three registers.

  • When C/D(low) is high, the control register is selected for writing control word or reading status word.

  • When C/D(low) is low, the data buffer is selected for read/write operation.

  • When the reset is high, it forces 8251A into the idle mode.

  • The clock input is necessary for 8251A for communication with CPU and this clock does not control either the serial transmission or the reception rate.

Transmitter section:

  • The transmitter section accepts parallel data from CPU and converts them into serial data.

  • The transmitter section is double buffered, i.e., it has a buffer register to hold an 8-bit parallel data and another register called output register to convert the parallel data into serial bits.

  • 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.

  • If output register is empty then TxEMPTY goes to high.

  • The clock signal, TxC (low) controls the rate at which the bits are transmitted by the USART.

  • The clock frequency can be 1,16 or 64 times the baud rate.

Receiver Section:

  • The receiver section accepts serial data and convert them into parallel data

  • The receiver section is double buffered, i.e., it has an input register to receive serial data and convert to parallel, and a buffer register to hold the parallel data.

  • When the RxD line goes low, the control logic assumes it as a START bit, waits for half a bit time and samples the line again.

  • 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 CPU reads the parallel data from the buffer register.

  • When the input register loads a parallel data to buffer register, the RxRDY line goes high.

  • The clock signal RxC (low) controls the rate at which bits are received by the USART.

  • During asynchronous mode, the signal SYNDET/BRKDET will indicate the break in the data transmission.

  • During synchronous mode, the signal SYNDET/BRKDET will indicate the reception of synchronous character.

MODEM Control:

  • The MODEM control unit allows to interface a MODEM to 8251A and to establish data communication through MODEM over telephone lines.

  • This unit takes care of handshake signals for MODEM interface.
Block Diagram:

The functional block diagram of 825 1A consists five sections. They are:

  • Read/Write control logic
  • Transmitter
  • Receiver
  • Data bus buffer
  • Modem control.

The functional block diagram is shown in fig:
Functional block diagram of 8251A-USART
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