UART

• Universal asynchronous receiver-transmitter | Wikipedia
• Pins, both maintains a FIFO, interrupts can be raised with change in buffers
  • Tx, transmitting
  • Rx, receiving
  • Cross wiring, Tx <--> Rx
• Frame components
  • Start: force transition, clock synchronized
  • Data
  • Parity: (optional) data with parity should have even number of 1 bits.
  • Stop: to distinguish from the start
• Typical Baud Rate: hundreds of kb/s
• Protocols
  • RS-232, 3 Volts to 15 Volts
  • RS-485, 36 Volts, medical/laboratory equipments
• Flow Control
  • Driven by receiver, “I have space for you” or “I’m full, slow down”
  • Software: using XON (Ctrl-Q) and XOFF (Ctrl-S) as control bits.
  • Hardware
    • RTS (ready to send) and CTS (ok to receive) pins are added
    • DSR and DST (dataset read/terminated)
• Handshaking: confirm whether the frame is received correctly, using ACK and NAK

Drawbacks

• Clock needs to be embedded on both sides
• Point to point protocol. (Bus is introduced in SPI.)