PIC单片机模拟异步串行通讯UART源程序

;|--------------------------------------------------------------|;|  Implement duplex USART base on normal I/O pin              |;|  Using TIMER0 interrupt for bit timing                      |;|  Tested on PIC16F83 running at 4MHz                          |;|  Written by Paul Zhang, Microchip Tech Inc                  |;|  6 Aug, 2000                                                |;|  All rights reserved                                        |;|--------------------------------------------------------------|    errorlevel    -302    ;no bank warning    errorlevel    -301    ;no default file warning       list      p=16F83    ;define processor    #include <p16F83.inc>    ;    __CONFIG  _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC    ;code protect        =    OFF    ;watchdog        =    OFF    ;power-up delay timer    =    ON    ;oscillator mode    =    XT;===============================;define RAM variables    cblock    0x0c        ;GPR start from 0x0cw_temp                ;W context saving during interruptstatus_temp            ;STATUS context saving during interruptpclath_temp            ;PCLATH context saving during interruptUSART_F                ;containing flags for USARTRX_BUFF                ;USART received data bufferTX_BUFF                ;USART transmitting data bufferRX_SLICE            ;RX bit-timing controlTX_SLICE            ;TX bit-timing controlRX_bcnt                ;RX received bit countingTX_bcnt                ;TX transmitting bit countingRX_STA                ;RX STATE-MACHINE controllerTX_STA                ;TX STATE-MACHINE controller    endc;===============================;pre-definition for readability#define    RX_PIN    PORTA,2        ;assign RX pin#define    TX_PIN    PORTA,3        ;assign TX pin#define    TXEN    USART_F,0    ;USART transmit enable#define    TXBUSY    USART_F,1    ;USRAT transmit is in progress#define    RXBF    USART_F,2    ;USART receive buff full#define    RXBUSY    USART_F,3    ;USART receive is in progress#define    RX_ERR    USART_F,4    ;USART receive error#define    TX_ERR    USART_F,5    ;USART transmit error;===============================;define constant#define    OSC_FREQ    .4000    ;oscillator frequency in KHz#define    BAUDRATE    .2400#define TMR0CONST    .118    ;256-OSC_FREQ*1000/4/(BAUDRATE*3) + 2;===============================;for my personal style#define    skp0    btfsc#define    skp1    btfss;**********************************************************************        ORG    0x000        clrwdt          goto    MAIN        ; go to beginning of program;=======================================;Interrupt service routine        ORG    0x004        ; interrupt vector location        movwf  w_temp        ; save off current W register contents        movf    STATUS,w    ; move status register into W register        banksel    status_temp        movwf    status_temp    ; save off contents of STATUS register        movf    PCLATH,w        movwf    pclath_temp    ; save off contents of PCLATH        banksel    INTCON        ;select bank        skp0    INTCON,T0IF    ;test for TMR0 interrupt        goto    tmr0IntStart    ;do TMR0 ISR        ;here test for any other interrupt source        goto    int_endtmr0IntStart                ;TIMER0 interrupt service        bcf    INTCON,T0IF    ;clear T0IF               ;====== start of RX =======        movlw    high($)        movwf    PCLATH        ;set PCLATH before PCL change        movf    RX_STA,w    ;get the state value for RX        andlw    0x03        ;for safeguard purpose        addwf    PCL,f        ;switch to STATE        goto    rxStartChk    ;check for START bit        goto    rxReceiveBit    ;receive DATA bit        goto    rxIdle        ;wait for idle        goto    rxEnd        ;do nothingrxStartChk    ;check for START bit        skp0    RX_PIN        ;test RX pin for START bit        goto    rxEnd        ;not found        ;start bit found. do following        movlw    .8        movwf    RX_bcnt        ;count for 8 bits incoming data        movlw    .4        movwf    RX_SLICE    ;wait 4 time-slice for 1st data bit        movlw    .1        movwf    RX_STA        ;switch to STATE 1 for 1st data bit sampling        goto    rxEndrxReceiveBit    ;receive DATA bit        decfsz    RX_SLICE,f    ;wait of bit timing        goto    rxEnd        ;time to sample incoming data bit        rrf    RX_BUFF,f    ;right shift for new bit space        bcf    RX_BUFF,7    ;pre-set to 0        skp0    RX_PIN        ;incoming data bit test        bsf    RX_BUFF,7    ;set if data bit = 1        movlw    .3        ;3 slice for data bit timing        movwf    RX_SLICE    ;bit timing for next data bit        decfsz    RX_bcnt,f    ;see if 8-bit completed        goto    rxEnd        ;bit receive completed, do follwoing        movlw    .2        movwf    RX_STA        ;set to STATE 2 for idle waiting        bsf    RXBF        ;set receive buffer full        movf    RX_BUFF,w    ;display data on PORTB        movwf    PORTB        goto    rxEndrxIdle        ;wait for idle        skp0    RX_PIN        ;try to find STOP bit        clrf    RX_STA        ;back to STATE 0 for next byte        goto    rxEnd        ;====== End of RX =========rxEnd        ;====== start of TX =======        ;do TX, if transmit is engaged        skp1    TXEN        ;skip if TXEN set, do TX        goto    tmr0IntEnd    ;not in transmit mode        movf    TX_SLICE,f    ;see if in bit-timing delay        skpnz            ;        goto    txDo        ;bit-timing completed        decfsz    TX_SLICE,f    ;keep bit-timing delay        goto    txEndtxDo        ;Transmit STATE-MACHINE control        movlw    high($)        movwf    PCLATH        ;set PCLATH before PCL change        movf    TX_STA,w    ;get current state        andlw    0x03        ;make sure in range        addwf    PCL,f        ;switch to TX STATE        goto    txStartBit    ;send START bit        goto    txDatBit    ;send DATA bit        goto    txStop        ;send STOP bit        goto    txIdle        ;set transtim IDLEtxStartBit    ;TX_STA=0, send START bit here        bsf    TXBUSY        ;set TX busy flag        movlw    .8        movwf    TX_bcnt        ;count for 8 bit transmitting        bcf    TX_PIN        ;start bit        movlw    .3        movwf    TX_SLICE    ;set bit timing        movlw    .1        movwf    TX_STA        ;set transmit STATE-MACHINE        goto    txEndtxDatBit    ;TX_STA=1, send DATA bit here        ;time for next bit sending        rrf    TX_BUFF,f    ;rotate bit to C        skpnc            ;test C        goto    $+3        bcf    TX_PIN        ;0 out        goto    $+2        bsf    TX_PIN        ;1 out        movlw    .3        movwf    TX_SLICE    ;wait 3 time-slices        decfsz    TX_bcnt,f        goto    txEnd        ;8 bit serial not end        movlw    .2        movwf    TX_STA        ;set transmit STATE-MACHINE        goto    txEndtxStop        ;TX_STA=2, send STOP bit here        bsf    TX_PIN        ;send STOP bit        movlw    .3        movwf    TX_SLICE    ;set bit timing        movlw    .3        movwf    TX_STA        ;set transmit STATE-MACHINE        goto    txEndtxIdle        ;TX_STA=3, reset transmission to IDLE        bcf    TXBUSY        ;not busy        bcf    TXEN        ;not in transmission        clrf    TX_STA        ;reset transmit STATE-MACHINE        goto    txEnd        ;====== End of TX =========txEnd        ;add more TMR0 related code heretmr0IntEnd        movlw    TMR0CONST        addwf    TMR0,f        goto    int_endint_end        banksel    pclath_temp        movf    pclath_temp,w    ; retieve copy of PCLATH register        movwf    PCLATH        movf    status_temp,w    ; retrieve copy of STATUS register        movwf    STATUS        ; restore pre-isr STATUS register contents        swapf  w_temp,f        swapf  w_temp,w    ; restore pre-isr W register contents        retfie            ; return from interrupt;=======================================;Code wriiten for test purposeMAIN        banksel    TRISA        ;select respective bank        movlw    b'00000100'    ;RA2-input, RA3-output        movwf    TRISA        clrf    TRISB        movlw    b'10001000'    ;TMR0 in timer mode        movwf    OPTION_REG        clrf    STATUS        ;make sure in bank 0        call    USART_INIT        movlw    TMR0CONST        movwf    TMR0        movlw    0xff        movwf    PORTB        bsf    INTCON,T0IE        bsf    INTCON,GIE       LOOP                    ;test code        skp1    RXBF        ;wait for data received        goto    $-1        bcf    RXBF        ;clear data flag        movf    RX_BUFF,w        movwf    TX_BUFF        ;send back received data        bsf    TXEN        skp0    TXEN        ;wait for transmit completion        goto    $-1        goto    LOOP        ;;=======================================;Initializtion of software USARTUSART_INIT        clrf    USART_F        ;clear all flag bit        clrf    RX_STA        ;reset STATE MACHINE        clrf    TX_STA        bsf    TX_PIN        ;TX is in Idle        return