;******************************************************************* ; ; Altair Cassette Interface (2400 Baud) ; ; MFM encoding is used recording a positive going 1/2 cycle ; of 2400 Hz to generate a pulse and then a negative going ; 1/2 cycle of 2400, 1200, or 800 Hz for the idle period ; between pulses. ; ; Author: Mike Douglas ; ; Revision: 1.0 (11/22/24), Initial release ; Checksum: ; ;******************************************************************* ; ; Processor is a 16F1823, Internal Oscillator w/o PLL at 8 Mhz ; list p=16f1823,b=12,r=dec include _BORV_25 EQU H'FBFF' ;Brown-out Reset Voltage (Vbor), high trip point selected. __config _CONFIG1, _FCMEN_OFF & _IESO_OFF & _CLKOUTEN_OFF & _BOREN_ON & _CPD_OFF & _CP_OFF & _MCLRE_ON & _PWRTE_ON & _WDTE_OFF & _FOSC_INTOSC __config _CONFIG2, _LVP_OFF & _BORV_25 & _STVREN_OFF & _PLLEN_OFF & _WRT_ALL ; Misc Equates DATA1_PER equ 32 ;32*13us=416us (1/2 cycle of 1200h) DATA0_PER equ 16 ;16*13us=208us (1/2 cycle of 2400hz) BIT_TIME equ 416 ;2400 baud bit time in us IC_BITx1p25 equ (BIT_TIME+BIT_TIME/4)*2 ;1.25 bit times IC_BITx0p5 equ (BIT_TIME/2)*2 ;0.5 bit times T2_BITx1p5 equ (BIT_TIME+BIT_TIME/2)/8 ;1.5 bit times (8us clock period) T2_BITx0p5 equ (BIT_TIME/2)/8 ;0.5 bit times ; Port A bit definitions FROM_ALTAIR equ 4 ;serial data from Altair board TO_ALTAIR equ 5 ;serial data to Altair board ; Port C bit definitions AUDIO_OUT equ 0 ;audio waveform out to cassette AUDIO_MASK equ (1< 0.25 volts and goes low when audio input is < 0.25 volts. banksel FVRCON ;fixed voltage reference movlw b'10000100' ;FVR on, 1.024v output movwf FVRCON banksel DACCON0 ;DAC control register movlw 16 ;DAC 16/32 = .5v movwf DACCON1 movlw b'10001000' ;DAC on, FVR provides 1.024v movwf DACCON0 banksel CM2CON0 ;comparator 2 control register movlw b'00010010' ;IN+ is DAC, IN- is C12IN2 movwf CM2CON1 movlw b'10110110' ;cmp on, output enabled, hysteresis on, inverted movwf CM2CON0 ; Setup Timer 1 and input capture to measure the cycle length of the audio input. ; The timer free-runs at 2 MHz. banksel T1CON ;timer 1 for input capture movlw b'00000101' ;FOSC/4, no prescale, timer on movwf T1CON banksel CCP1CON ;capture control register movlw b'00000101' ;capture mode, every rising edge movwf CCP1CON ; Setup Timer 2 to generate a 13us tic used for the audio tone generator loop banksel TMR2 ;timer 2 for audio out generation movlw b'00000100' ;timer 2 on, 2MHz movwf T2CON clrf TMR2 ;start timer at zero movlw 13*2-1 ;13us wakeup movwf PR2 clrf PIR1 ;clear any pending interrupt flags clrf flags ;our flags false ;fall into audio output loop ;--------------------------------------------------------------------------- ; Audio output loop ;---------------------------------------------------------------------------- audioOut movlw b'00000110' ;timer 2 on, 125KHz movwf T2CON clrf TMR2 ;init TMR2 to time 0.5 bit periods movlw T2_BITx0p5 movwf PR2 clrf TMR0 ;8.2ms timer bsf PORTC,RS232_OUT ;set RS232 output to 1 bsf flags,F_PREV_BIT ;idle 1's ; Send idle 1's for 8.2ms idle btfss PIR1,TMR2IF ;sync to 1/2 bit cell goto idle bcf PIR1,TMR2IF bcf PORTC,AUDIO_OUT idle2 btfss PIR1,TMR2IF ;sync to 1/2 bit cell goto idle2 bcf PIR1,TMR2IF bsf PORTC,AUDIO_OUT btfss TMR0,7 ;idle 8.2ms when bit 7 becomes 1 goto idle ; Send A-Y movlw 'A' movwf xmitData sendLoop call sendByte btfsc PIR1,CCP1IF ;did an audio input capture occur? goto audioInput ;yes incf xmitData,F movfw xmitData sublw 'Z' bnz sendLoop clrf TMR0 goto idle ;--------------------------------------------------------------------- ; sendByte from outByte ;--------------------------------------------------------------------- sendByte movfw xmitData movwf outByte movlw 10 ;byte plus start & stop bit movwf bitCnt bsf outByteH,0 ;stop bit of 1 rlf outByte,F rlf outByteH,F bcf outByte,0 ;start bit is zero sbLoop btfss PIR1,TMR2IF ;sync to 1/2 bit cell goto sbLoop bcf PIR1,TMR2IF btfsc outByte,0 ;data bit 0 or 1? goto sbOne ;one ; Data is zero btfsc flags,F_PREV_BIT ;prev bit a 1? bcf PORTC,AUDIO_OUT ;yes, go low now btfss flags,F_PREV_BIT bsf PORTC,AUDIO_OUT ;no, set high now bcf flags,F_PREV_BIT wait1 btfss PIR1,TMR2IF ;sync to 1/2 bit cell goto wait1 bcf PIR1,TMR2IF bcf PORTC,AUDIO_OUT goto nextBit ; Data is one sbOne bcf PORTC,AUDIO_OUT ;1 starts with 1/2 cycle low bsf flags,F_PREV_BIT wait2 btfss PIR1,TMR2IF ;sync to 1/2 bit cell goto wait2 bcf PIR1,TMR2IF bsf PORTC,AUDIO_OUT ;1 has pulse in 2nd 1/2 nextBit rrf outByteH,F rrf outByte,F decfsz bitCnt,F goto sbLoop return ;--------------------------------------------------------------------------- ; Audio input loop ; Compute time between input captures (audio input zero crossings) and ; generate the RS-232 data to output. If no input captures occur for ; 8.2ms, then jump back to the audio output loop above. ;---------------------------------------------------------------------------- audioInput clrf TMR0 ;restart the idle timeout movlw b'00000110' ;timer 2 on, 125KHz movwf T2CON clrf TMR2 ;init TMR2 to time 1.5 bit times movlw T2_BITx1p5 ;period for 1.5 bit times movwf PR2 ;--------------------------------------------------------------------------- ; Loop while audio data is one ;--------------------------------------------------------------------------- aiNowOne bsf PORTC,RS232_OUT ;set RS232 output to 1 bcf PIR1,TMR2IF ;clear the timer match flag aiIsOne btfsc TMR0,7 ;idle 8.2ms when bit 7 becomes 1 goto audioOut ;exit back to audio output generation btfsc PIR1,TMR2IF ;gone 1.5 bit times without pulse? goto aiNowZero ;yes, transition to zero btfss PIR1,CCP1IF ;capture occur? goto aiIsOne ;not yet, keep looping ; Capture occured. If less than 1.25 bit times, we are still at one. Otherwise, ; transition to zero. clrf TMR2 ;reset 1.5 bit timer bcf PIR1,TMR2IF ;make sure timer match flag is clear call getIcData ;get time between pulses movlw low(IC_BITx1p25) ;compare to 1.25 bit times subwf deltaLo,W movlw high(IC_BITx1p25) subwfb deltaHi,W btfss STATUS,C ;carry set if >= 1.25 bit times goto aiIsOne ;shorter, so still one ;else, fall into nowZero ;--------------------------------------------------------------------------- ; Loop while audio data is Zero ;--------------------------------------------------------------------------- aiNowZero bcf PORTC,RS232_OUT ;RS-232 out is zero movlw low(IC_BITx0p5) ;move previous capture time to bit boundary addwf prevLo,F ; to make it look like a consecutive zero movlw high(IC_BITx0p5) addwfc prevHi,F aiIsZero btfsc TMR0,7 ;idle 8.2ms when bit 7 becomes 1 goto audioOut ;exit back to audio output generation btfss PIR1,CCP1IF ;capture occur? goto aiIsZero ;not yet, keep looping ; Capture occured. If less than 1.25 bit times, we are still at zero. Otherwise, ; transition to one after a 0.5 bit delay to get back to the bit edge. clrf TMR2 ;sync timer 2 to pulse edge call getIcData ;get time between pulses movlw low(IC_BITx1p25) ;compare to 1.25 bit times subwf deltaLo,W movlw high(IC_BITx1p25) subwfb deltaHi,W btfss STATUS,C ;carry set if >= 1.25 bit times goto aiIsZero ;shorter, so still zero aiWaitBit movfw TMR2 ;delay 1/2 bit time to bit edge sublw T2_BITx0p5 bnz aiWaitBit goto aiNowOne ;now transition to one ;--------------------------------------------------------------------------- ; getIcData - Get Input Capture Data ; Compute time between new capture and previous capture and save in ; deltaHi:deltaLo. Save new capture in prevHi:prevLo ;--------------------------------------------------------------------------- getIcData clrf TMR0 ;reset "no pulse" timer banksel CCPR1L ;save the capture value movfw CCPR1H movwf captHi movfw CCPR1L movwf captLo movlw 33 ;ignore captures for 50us to get past delay50 decfsz WREG,F ; transition crossing noise goto delay50 ; Compute the time since last capture in deltaHi/deltaLo, then save the new ; capture time as the previous capture time banksel PORTA bcf PIR1,CCP1IF ;clear the capture flag movfw prevLo ;subtract previous value subwf captLo,W ;lsb first movwf deltaLo movfw prevHi ;msb second subwfb captHi,W movwf deltaHi movfw captLo ;save current capture as previous movwf prevLo movfw captHi movwf prevHi return end