UART on the 8051

In order to configure the following registers must be initialized:

Configuration: 01010000

• SM0, SM1: 01
  • Mode 1 (8-bit UART)
• SM2: 0
  • Disable multiprocessor communication (modes 2/3 only)
• REN: 1
  • Enable serial reception
• TB8: 0 - 9th data bit to be sent - not used for 8-bit UART
• RB8: 0
  • 9th received data bit
  • Not used for 8-bit UART
• TI: 0
  • Transmit interrupt flag
  • Set by hardware when byte has been transmitted
  • Must be cleared by software
• RI: 0
  • Receive interrupt flag
  • Set by hardware when byte has been received
  • Must be cleared by software

Since UART uses Timer 1, we only need to configure the upper nibble.

Configuration: 0010XXXX

• Timer 1 configuration
  • GATE: 0
    • Disable gating control
  • C/T#: 0
    • Select timer operation
  • M1, M0: 10
    • Select mode 2 (auto reload)
• Timer 0 configuration
  • Don't care

Configuration: 01XX0XXX

• TCON.7: 0
  • Timer 1 overflow flag
  • Set by hardware, should be cleared by software
• TCON.6: 1
  • Timer 1 run control bit
  • Set by software to start timer
• TCON.3: 0
  • Interrupt 1 edge flag
  • Set by hardware, should be cleared by software
• TCON.2: X
  • Interrupt 1 type control bit - we are not using interrupts on timer 1.

$$ TH1 = 256 - \frac{K(f_{OSC})}{384f_{BAUD}} $$

• For $f_{BAUD}=9600$, $K=1$, and $f_{OSC}=11.0592MHz$, $TH1=253$.

• To receive a byte through UART, move the contents of the SBUF register to another register.

mov A, SBUF ; Store the received byte in the accumulator.

• To transmit a byte through UART, move some data to the SBUF register.

mov SBUF, A ; Transmit the byte stored in the accmulator.