Short note: USART

A USART (Universal Synchronous Asynchronous Receiver Transmitter) is a programmable serial communication chip that converts parallel data from the processor into serial form for transmission and serial data received into parallel. The Intel 8251A is the standard example for 8085/8086 systems.

 D0-D7 <-> [Data bus buf] <-> [Transmit buffer] -> TxD
 RD WR CS C/D  [Read/write ]   [Transmit control]  TxRDY TxEMPTY
 RESET CLK     [control    ]   [Receive buffer ] <- RxD
               [Modem ctrl ]   [Receive control]    RxRDY
                DTR DSR RTS CTS                     SYNDET

Features

• Works in asynchronous mode (5–8 data bits, 1/1.5/2 stop bits, parity, baud factor ×1, ×16, ×64) and synchronous mode (internal or external sync detect, 1 or 2 sync characters).
• Full duplex; double-buffered transmitter and receiver.
• Detects parity, overrun and framing errors.
• Modem control signals: DTR, DSR, RTS, CTS.

Programming: after reset, the mode instruction is written first (baud factor, character length, parity, stop bits), followed by the command instruction (enable Tx/Rx, DTR, RTS, error reset, internal reset). The status word is read to check TxRDY, RxRDY, TxEMPTY, PE, OE, FE and others. The C/D pin selects data (0) or control/status (1).

Use: interfacing a microprocessor with RS-232 terminals, modems and other serial devices, usually with a MAX232 level converter.