;******************************************************************* ; ; Altair Cassette Interface (1200 Baud) ; ; Data 1 is one cycle of 1200hz. Data 0 is two cycles of 2400hz. ; This differs from the CUTS board (1200hz=1, 1/2 cycle 600hz=0) ; because the CUTS format requires hardware that can process ; the full input audio cycle, and this cassette board chops off ; the bottom half of the audio cycle. 1 and 0 were swapped so ; that idle tone (ones) are always on a full bit boundary and ; the first zero bit following idles ones won't be out of sync. ; ; Author: Mike Douglas ; ; Revision: 1.0 (11/14/24), 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) MIN_PERIOD equ 416 ;minimum valid input period in us * 2 (208us) MID_PERIOD equ 1248 ;midpoint input period in us * 2 (624us) test equ high MID_PERIOD MAX_PERIOD equ 2080 ;maximum valid input period in us * 2 (1040us) ; 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 ; Generate 2400Hz (RS232 in is 1) or 1200Hz (RS232 in is 0) waveforms. ; Sync waveform timing anytime a transition occurs in the RS232 input ; data. If audio is detected coming in from cassette, jump to the ; cassette data input loop. ;---------------------------------------------------------------------------- audioOut bsf PORTC,RS232_OUT ;idle RS-232 output at 1 ; RS-232 input is one nowOne movfw periodCount ;end of old or start of new half cycle? andlw 0xf8 ;not zero if >=8 (1st half of period) bnz set1Period ;already in new half cycle movlw AUDIO_MASK ;else, toggle audio output xorwf PORTC,F ; to start new cycle early set1Period movlw DATA1_PER ;set count of 13us tics per half cycle movwf periodCount isOne btfss PIR1,TMR2IF ;13us tic occur? goto isOne ;no bcf PIR1,TMR2IF ;clear the tic flag btfss PORTC,RS232_IN ;still receiving one? goto nowZero ;no, now receiving zero btfsc PIR1,CCP1IF ;did an audio input capture occur? goto havePulse ;yes decfsz periodCount,F ;time out the half cycle goto isOne ;loop if not done yet movlw AUDIO_MASK ;half cycle done, toggle audio output xorwf PORTC,F goto set1Period ;repeat ; RS-232 input is zero nowZero movfw periodCount ;end of old or start of new half cycle? andlw 0xf0 ;not zero if >= 16 (1st half of period) bnz set0Period ;already in new half cycle movlw AUDIO_MASK ;else, toggle audio output xorwf PORTC,F ; to start new cycle early set0Period movlw DATA0_PER ;set count of 13us tics per half cycle movwf periodCount isZero btfss PIR1,TMR2IF ;13us tic occur goto isZero ;no bcf PIR1,TMR2IF ;clear the tic flag btfsc PORTC,RS232_IN ;still recieving zero? goto nowOne ;no, now receiving one btfsc PIR1,CCP1IF ;did an audio input capture occur? goto havePulse ;yes decfsz periodCount,F ;time out the half cycle goto isZero ;loop if not done yet movlw AUDIO_MASK ;half cycle done, toggle audio output xorwf PORTC,F goto set0Period ;repeat ;--------------------------------------------------------------------------- ; 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. ;---------------------------------------------------------------------------- audioIn btfsc TMR0,7 ;8.2ms when bit 7 becomes 1 goto audioOut ;back to audio output generation btfss PIR1,CCP1IF ;capture occur? goto audioIn ;no, keep looking ; Capture occured, save the value capture in captHi/captLo havePulse clrf TMR0 ;reset "no pulse" timer banksel CCPR1L ;save the capture value movfw CCPR1H movwf captHi movfw CCPR1L movwf captLo ; Ignore captures for 50us to let possible comparator oscillations cease movlw 33 ;33 loops = 50us delay50 decfsz WREG,F goto delay50 ; Compute the time since last capture in deltaHi/deltaLo banksel PORTA bcf PIR1,CCP1IF ;clear the capture flag movfw prevLo ;subtract previous value subwf captLo,W ;lsb first movwf deltaLo movfw prevHi ;ms byte subwfb captHi,W movwf deltaHi ; If period is shorter than MIN_PERIOD, ignore the pulse movlw low(MIN_PERIOD) subwf deltaLo,W movlw high(MIN_PERIOD) subwfb deltaHi,W btfss STATUS,C ;carry set if >= MIN_PERIOD goto audioIn ;too short, ignore the pulse ; See if period is shorter than MID_PERIOD. If so, treat as 2400hz (zero) and ; set the F_IS_ZERO flag. Do NOT save the capture so that the next 2400hz ; capture will be > MID_PERIOD and capture the bit in that routine. movlw low(MID_PERIOD) subwf deltaLo,W movlw high(MID_PERIOD) subwfb deltaHi,W btfsc STATUS,C ;carry clear if < MID_PERIOD goto chkMax bsf flags,F_IS_ZERO ;this bit will be a zero goto audioIn ; See if period is shorter than MAX_PERIOD. If so, treat as a valid pulse ; data 0 if F_IS_ZERO is true, else data 1. chkMax movlw low(MAX_PERIOD) subwf deltaLo,W movlw high(MAX_PERIOD) subwfb deltaHi,W btfsc STATUS,C ;carry clear if < MAX_PERIOD goto savePrev ;too long, ignore the pulse btfsc flags,F_IS_ZERO ;is bit zero or one? goto set0 ;bit is zero bsf PORTC,RS232_OUT ;RS-232 out is one goto savePrev set0 bcf PORTC,RS232_OUT ;RS-232 out is zero ; Save current capture time as the previous capture time savePrev bcf flags,F_IS_ZERO ;start new bit movfw captLo ;save current timer as previous movwf prevLo movfw captHi movwf prevHi goto audioIn ;repeat end