?SU?PUTC SEGMENT CODE
?SU?GETC SEGMENT CODE

PUBLIC _putc
PUBLIC getc

txd_pin EQU     P3.1            ;Transmit on this pin
rxd_pin EQU     P3.0            ;Receive on this pin

;Formula to calculate the bit time delay constant
;This constant is calculated as: (((crystal/baud)/12) - 5) / 2
;crystal is the frequency of crystal in Hz
;baud is required baudrate
;Please try to keep baudrate below 9600
;to get best results :-)

BITTIM  EQU     45;             (((11059200/9600)/12) - 5) / 2

;--------------------------------------------
;To send data serially
;For C programs
;Protype definition:
;               void putc(unsigned char);
;Usage:
;               putc(data);
;Return:
;               This function returns nothing
;
;For Assembly Programs:
;
;Usage:
;       data to be send has to be moved to R7
;       for example:
;               mov R7,#'a'
;               lcall _putc
;--------------------------------------------
RSEG ?SU?PUTC
_putc:
        push ACC
        Push PSW
        mov a,r7
        CLR txd_pin                     ;Drop line for start bit
        MOV R0,#BITTIM          ;Wait full bit-time
        DJNZ R0,$                       ;For START bit
        MOV R1,#8                       ;Send 8 bits
putc1:
        RRC A                           ;Move next bit into carry
        MOV txd_pin,C           ;Write next bit
        MOV R0,#BITTIM          ;Wait full bit-time
        DJNZ R0,$                       ;For DATA bit
        DJNZ R1,putc1           ;write 8 bits
        SETB txd_pin            ;Set line high
        RRC A                           ;Restore ACC contents
        MOV R0,#BITTIM          ;Wait full bit-time
        DJNZ R0,$                       ;For STOP bit
        POP PSW
        pop ACC
        RET

;--------------------------------------------
;To receive data Serially
;If you want to use this routine in your
;C program then define function prototype
; as:
;       unsigned char getc(void);
;
;       Usage:
;               data = getc();
;       Return value:
;               Returns data received
;
;
;If you are using it in assembly program
;       Usage:
;               lcall getc
;       Return:
;               data received is stored in R7
;--------------------------------------------

RSEG ?SU?GETC
getc:   
        Push ACC
        Push PSW
        JB rxd_pin,$            ;Wait for start bit
        MOV R0,#BITTIM/2        ;Wait 1/2 bit-time
        DJNZ R0,$                       ;To sample in middle
        JB rxd_pin,getc         ;Insure valid
        MOV R1,#8                       ;Read 8 bits
getc1:
        MOV R0,#BITTIM          ;Wait full bit-time
        DJNZ R0,$                       ;For DATA bit
        MOV C,rxd_pin           ;Read bit
        RRC A                           ;Shift it into ACC
        DJNZ R1,getc1           ;read 8 bits
        mov r7,a
        POP PSW
        pop ACC
        RET                                     ;go home