Q19Microprocessor and Microcontroller

The Intel 8251 is a Universal Synchronous/Asynchronous Receiver Transmitter (USART). It is a highly specialized programmable communication interface strictly designed for serial data communication. It mathematically converts parallel data from the microprocessor into a continuous serial bit stream for long-distance transmission, and conversely, receives serial data and reconstructs it into parallel bytes. Its internal architecture is violently partitioned into five critical functional blocks.

1. Read/Write Control Logic

This section acts as the primary interface directly to the 8085 microprocessor's system bus. It receives control signals ($\overline{RD}$, $\overline{WR}$, $CS$, $C/\bar{D}$) and physically connects to the 8-bit Data Bus ($D_0-D_7$). The $C/\bar{D}$ (Control/Data) pin is critical: when HIGH, the microprocessor is writing command words or reading status words. When LOW, it is violently transmitting or receiving actual payload data.

2. Transmitter Section

This massive block handles all outgoing serial data. - Transmit Buffer: It receives the parallel byte directly from the microprocessor via the internal bus. - Transmit Shift Register: It mathematically accepts the byte from the buffer, physically appends the necessary Start, Parity, and Stop bits (framing the data), and aggressively shifts the data out, bit by bit, onto the physical $TxD$ (Transmit Data) pin. - $TxC$ (Transmitter Clock): Controls the exact baud rate (shifting speed) of the outgoing data.

3. Receiver Section

This block handles all incoming serial data. - Receive Shift Register: It monitors the $RxD$ (Receive Data) pin. Upon detecting a valid Start bit, it violently clocks in the incoming serial stream, mathematically strips away the Start/Stop framing bits, verifies parity, and assembles a raw 8-bit parallel byte. - Receive Buffer: The completed byte is transferred here, triggering an $RxRDY$ (Receive Ready) interrupt to the microprocessor to fetch the data before it is overwritten.

4. Modem Control

This specialized section handles the complex hardware handshaking protocols required when interfacing specifically with Modems. It utilizes signals like $\overline{DSR}$ (Data Set Ready), $\overline{DTR}$ (Data Terminal Ready), $\overline{CTS}$ (Clear to Send), and $\overline{RTS}$ (Request to Send) to strictly orchestrate the flow of data and prevent catastrophic buffer overflows.

5. Transmit/Receive Control (Baud Rate Generator)

The 8251 can operate in strictly Synchronous mode (relying on an external, shared clock pulse) or Asynchronous mode. The internal control block dynamically adjusts the baud rate divider (1x, 16x, 64x) based on the initial software command word programmed by the 8085.