autobaud.asm

Automatic Baud Rate Detection for the 80C51

;*****************************************************************************

;                     Automatic Baud Rate Detection Test

;*****************************************************************************


$Title(Automatic Baud Rate Detection Test)
$Date(12-16-91)
$MOD552


;*****************************************************************************
;                                 Definitions
;*****************************************************************************

RX        BIT   P3.0            ;Location of serial receive pin.
CharH     DATA  30h             ;Holds high byte of frame timer result.
CharL     DATA  31h             ;Holds low byte of frame timer result.
BaudRate  DATA  32h             ;Holds final baud rate determination.
Display   EQU   P4              ;Port to display result for debug.


;*****************************************************************************
;                         Reset and Interrupt Vectors
;*****************************************************************************

          ORG   8000h

Start:    ACALL AutoBaud         ;Go try to get a baud rate value.
          MOV   Display,BaudRate ;Display baud rate value for debug.
          SJMP  Start


;*****************************************************************************
;                                 Subroutines
;*****************************************************************************

; AutoBaud Rate Detect Routine.
;   Attempts to detect baud rate from first received character, by measuring
;     the length of the character. Some characters may not work properly,
;     primarily those that end with more than 3 (4?) ones in a row.
;   Returns with ACC = baud rate pointer.

AutoBaud: MOV   TMOD,#01h       ;Initialize timer 0 (UART baud rate timer).
          MOV   TH0,#0          ;Put timer 0 in 16-bit counter mode.
          MOV   TL0,#0
          MOV   TCON,#0

          MOV   CharH,#0        ;Initialize timer result.
          MOV   CharL,#0

AB0:      JB    RX,AB0          ;Wait for serial start bit.
          SETB  TR0             ;Start timer.

AB1:      JB    TF0,AB3         ;Check for timer overflow.
          JNB   RX,AB1          ;Check for a rising edge on serial data.

          MOV   CharH,TH0       ;Capture timer value at this serial edge.
          MOV   CharL,TL0

AB2:      JB    TF0,AB3         ;Check for timer overflow.
          JB    RX,AB2          ;Check for falling edge on serial data.
          SJMP  AB1             ;Go back and repeat sampling.

AB3:      CLR   TR0             ;Maximum sample time has expired, check result.
          CLR   TF0             ;Begin by stopping timer and clearing flag.

          MOV   BaudRate,#19    ;Set up table pointers.
CmpLoop:  MOV   A,BaudRate
          MOV   DPTR,#CmpTable
          MOVC  A,@A+DPTR       ;Get a table entry for comparison.
          DEC   BaudRate
          CJNE  A,CharH,Cmp1    ;Check result range.
          SJMP  CmpLow          ;High byte table = timed value, check low byte.
Cmp1:     JC    CmpMatch        ;A match if table value is < timed value.
          DJNZ  BaudRate,CmpLoop ;Check for end of comparison table.
          SJMP  CmpMatch

CmpLow:   MOV   A,BaudRate
          MOVC  A,@A+DPTR       ;Get a table entry for comparison.
          CJNE  A,CharL,Cmp2    ;Check result range.
          SETB  C               ;Match if equal.
Cmp2:     JC    CmpMatch        ;C set if A < low byte of result.
          DJNZ  BaudRate,CmpLoop ;Check for end of comparison table.

CmpMatch: MOV   A,BaudRate      ;Comparison complete, 
          CLR   C               ;  get final baud rate index,
          RRC   A
          MOV   BaudRate,A      ;  and save.
          RET

; Compare table holds timer values for the transition points of the accepted 
;   baud rates. Entries are LSB, MSB. These values are for 12 MHz operation.

CmpTable:
          DB    40h,0           ;0 - out of range, value too low.
          DB    80h,0           ;1 - 38400 baud.
          DB    0,01h           ;2 - 19200 baud.
          DB    0,02h           ;3 -  9600 baud.
          DB    0,04h           ;4 -  4800 baud.
          DB    0,08h           ;5 -  2400 baud.
          DB    0,10h           ;6 -  1200 baud.
          DB    0,20h           ;7 -   600 baud.
          DB    0,40h           ;8 -   300 baud.
          DB    0,80h           ;9 - out of range, value too high.

          END