di2546.txt
;**************************************************************************************************
; LISTING 1 - AUTOMATIC BIT-RATE-DETECTION ROUTINE
; 
; "Scheme autodetects baud rate," EDN, July 20, 2000, pg 140
;
; http://www.ednmag.com/ednmag/reg/2000/072000/designideas.htm#15di1
;***************************************************************************************************

;Bit rate automatic detection routine
;***********************************************
ser_out   =     ra.3         ;transmit pin
ser_in    =     ra.2         ;receive pin
;
     org  8
;
k               ds    1      ;derived time constant
clk_cnt         ds    1      ;counter for serial timing
bit_num         ds    1      ;number of received/transmitted bits
xmt_b           ds    1      ;byte to transmit
;
     device pic16c54, xt_osc,wdt_off,protect_off
     reset start
;
     org  0
;
start     mov   !ra,#4       ;ra.2 - input, ra.3 - output
          mov   option,#3    ;set prescaler to divide by 16
;
;******************* serial in ***************************************
;
st_by     snb   ser_in       ;skip next instruction if Rx goes low
          jmp   st_b         ;go back and wait till start bit
          clr   rtcc         ;reset timer, start to count ticks
          jb    ser_in,st_by ;go back if it was only a glitch
;
;******************* start is good ***********************************
;
roll      snb   ser_in,roll  ;wait till line goes high
          jmp   k, rtcc      ;record timer’s value
          clr   k            ;make k = N-2
          jb    k            ;decrement
;
;******************* success *****************************************
;
          mov   xmt_b,#'O'   ;output "OK" using calculated
          call  out          ;Bit Rate
          mov   xmt_b,#'K'
          call  out
          mov   xmt_b,#13    ;carriage return
          call  out
          mov   xmt_b,#10    ;line feed
          call  out
          goto  do_something ;continue with your application
          nop
;
;****************** serial out ****************************************
;
out       mov   bit_num,#8   ;number bits to transmit
          clrb  ser_out      ;ready to send start bit
          call  bit_clk      ;send start bit
xmit      rr    xmt_b        ;move LSB to carry
          movb  ser_out,c    ;move carry to Tx
          call  bit_clk      ;end
          djnz  bit num,xmit ;decrement and go on if not done yet
          setb  ser_out      ;load stop bit
          call  bit_clk      ;and send
          ret                ;done
          nop
;
bit_clk   mov   clk_cnt,k
loop      nop
          djnz  clk_cnt,loop
          ret
          nop