Serial Communication 8051

Question

I am studying serial communication in 8051 using UART and interrupts. Today I came across this code in which author says he is constantly transfering data coming on Port 0. The way transfer is occuring, I think is voilating the rules of serial communication in 8051.

org 00h
ljmp main

org 23h
ljmp serial_ISR


org 30h

main:

   mov TMOD,#20h
   mov TH1,#-03h
   mov SCON,#50h

   setb IE.7
   setb IE.4

   setb TR1

back:

    mov A,P0
    mov SBUF,A
    sjmp back

serial_ISR:

    jb TI,trans

    mov R0,SBUF

    clr RI

    RETI

trans:

   clr TI

   RETI

The thing that is confusing me is, in back label we constantly writing on SBUF register which is voilating the rule that we should not write on SBUF until the previous data has been sent (which is notified by the TI flag).

Is constantly writing data on SBUF register in above code valid? will UART send correct data?

Regards

Answer 1

You are definitely right, the code inside back label should be rewritten like this:

back:
    jb TI,$
    mov A,P0
    mov SBUF,A
    sjmp back

Coding back label like I did before guarantee you that you are not going to move any data to SBUF until it finishes sending the last data.

There is one detail here to take into account, remember that serial port interrrupts (by receiving or transmitting) are not cleared automatically, so in the code before I am assuming that you cleared the TI interrupt flag manually.