UART on the 8051 [Jaeyoung Wiki]

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====== UART on the 8051 ======

===== Configuration =====

In order to configure the following registers must be initialized:

==== SCON (Serial port control register) ====

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

==== TMOD (Timer/counter mode control register) ====

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

==== TCON ====

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 ====

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

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

===== Receiving =====

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

<code asm>
mov A, SBUF ; Store the received byte in the accumulator.
</code>

===== Transmitting =====

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

<code asm>
mov SBUF, A ; Transmit the byte stored in the accmulator.
</code>