PIC模擬異步串行通訊UART

用TMR0實現定時查詢。任何帶中斷的PIC上都可以實現。可用此法擴展多個串口。

;|--------------------------------------------------------------|
;|  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                                         |
;|--------------------------------------------------------------|

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    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 0x0c
w_temp                ;W context saving during interrupt
status_temp            ;STATUS context saving during interrupt
pclath_temp            ;PCLATH context saving during interrupt

USART_F                ;containing flags for USART
RX_BUFF                ;USART received data buffer
TX_BUFF                ;USART transmitting data buffer
RX_SLICE            ;RX bit-timing control
TX_SLICE            ;TX bit-timing control
RX_bcnt                ;RX received bit counting
TX_bcnt                ;TX transmitting bit counting
RX_STA                ;RX STATE-MACHINE controller
TX_STA                ;TX STATE-MACHINE controller
    endc

;===============================
;pre-definition for readability
#define    RX_PIN    PORTA2        ;assign RX pin
#define    TX_PIN    PORTA3        ;assign TX pin
#define    TXEN    USART_F0    ;USART transmit enable
#define    TXBUSY    USART_F1    ;USRAT transmit is in progress
#define    RXBF    USART_F2    ;USART receive buff full
#define    RXBUSY    USART_F3    ;USART receive is in progress
#define    RX_ERR    USART_F4    ;USART receive error
#define    TX_ERR    USART_F5    ;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    STATUSw    ; move status register into W register
        banksel    status_temp
        movwf    status_temp    ; save off contents of STATUS register
        movf    PCLATHw
        movwf    pclath_temp    ; save off contents of PCLATH

        banksel    INTCON        ;select bank
        skp0    INTCONT0IF    ;test for TMR0 interrupt
        goto    tmr0IntStart    ;do TMR0 ISR
        ;here test for any other interrupt source
        goto    int_end

tmr0IntStart                ;TIMER0 interrupt service
        bcf    INTCONT0IF    ;clear T0IF
        
        ;====== start of RX =======
        movlw    high($)
        movwf    PCLATH        ;set PCLATH before PCL change
        movf    RX_STAw    ;get the state value for RX
        andlw    0x03        ;for safeguard purpose
        addwf    PCLf        ;switch to STATE
        goto    rxStartChk    ;check for START bit
        goto    rxReceiveBit    ;receive DATA bit
        goto    rxIdle        ;wait for idle
        goto    rxEnd        ;do nothing
rxStartChk    ;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    rxEnd
rxReceiveBit    ;receive DATA bit
        decfsz    RX_SLICEf    ;wait of bit timing
        goto    rxEnd
        ;time to sample incoming data bit
        rrf    RX_BUFFf    ;right shift for new bit space
        bcf    RX_BUFF7    ;pre-set to 0
        skp0    RX_PIN        ;incoming data bit test
        bsf    RX_BUFF7    ;set if data bit = 1
        movlw    .3        ;3 slice for data bit timing
        movwf    RX_SLICE    ;bit timing for next data bit
        decfsz    RX_bcntf    ;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_BUFFw    ;display data on PORTB
        movwf    PORTB
        goto    rxEnd
rxIdle        ;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_SLICEf    ;see if in bit-timing delay
        skpnz            ;
        goto    txDo        ;bit-timing completed
        decfsz    TX_SLICEf    ;keep bit-timing delay
        goto    txEnd
txDo
        ;Transmit STATE-MACHINE control
        movlw    high($)
        movwf    PCLATH        ;set PCLATH before PCL change
        movf    TX_STAw    ;get current state
        andlw    0x03        ;make sure in range
        addwf    PCLf        ;switch to TX STATE
        goto    txStartBit    ;send START bit
        goto    txDatBit    ;send DATA bit
        goto    txStop        ;send STOP bit
        goto    txIdle        ;set transtim IDLE
txStartBit    ;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    txEnd
txDatBit    ;TX_STA=1 send DATA bit here
        ;time for next bit sending
        rrf    TX_BUFFf    ;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_bcntf
        goto    txEnd        ;8 bit serial not end
        movlw    .2
        movwf    TX_STA        ;set transmit STATE-MACHINE
        goto    txEnd
txStop        ;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    txEnd
txIdle        ;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 here
tmr0IntEnd
        movlw    TMR0CONST
        addwf    TMR0f
        goto    int_end

int_end
        banksel    pclath_temp
        movf    pclath_tempw    ; retieve copy of PCLATH register
        movwf    PCLATH
        movf    status_tempw    ; retrieve copy of STATUS register
        movwf    STATUS        ; restore pre-isr STATUS register contents
        swapf   w_tempf
        swapf   w_tempw    ; restore pre-isr W register contents
        retfie            ; return from interrupt


;=======================================
;Code wriiten for test purpose
MAIN
        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    INTCONT0IE
        bsf    INTCONGIE
        
LOOP                    ;test code
        skp1    RXBF        ;wait for data received
        goto    $-1
        bcf    RXBF        ;clear data flag
        movf    RX_BUFFw
        movwf    TX_BUFF        ;send back received data
        bsf    TXEN
        skp0    TXEN        ;wait for transmit completion
        goto    $-1
        goto    LOOP        ;



;=======================================
;Initializtion of software USART
USART_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


        END                     ;


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