;******************************************************************* ; ; Altair Cassette Interface ; ; Data 1 is represented by a 2400hz carrier. Data 0 is represented ; by an 1850hz carrier. Earliest boards used 2225hz and 2025hz. This ; is still a 2125hz center frequency. The Kansas City Standard (KCS) ; used 2400hz and 1200hz. This program reads any of these standards ; but always writes using the 2400/1850hz frequencies. ; ; Author: Mike Douglas ; ; Revision: 1.2 (10-1-23) ; - Allow use of the RS-232 port for data in/out from/to the ; cassette in addition to the normal connection to the Altair ; Clone (through the six pin header J1). This allows use of ; the board as a modem for an RS-232 connection. The RS-232 ; option is the default. For use in an Altair Clone, install ; a jumper between pins 2&4 on the programming header. ; ; - Eliminated the two cycle filter for reading data and went ; with an input capture ignore period of 50us to allow any ; oscillations of the comparator to settle out. ; ; Checksum 8407 ; ; Revision: 1.1 (12-30-13), Update input frequencies to better match ; the actual 2125hz center frequency used for both the ; early 2225/2025hz boards and the later 2400/1850hz boards. ; Drop the lowest frequency accepted from 1562hz to 1000hz ; so KCS tapes can be read (2400/1200hz). ; Checksum: D833 ; ; Revision: 1.0 (9-20-13), Initial release ; Checksum: D771 ; ;******************************************************************* ; ; 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 104 ;208us at 500khz (1/2 cycle of 2400hz) DATA0_PER equ 135 ;270us at 500khz (1/2 cycle of 1850hz) MIN_PERIOD equ 632 ;minimum valid input period in us * 2 (3165hz) MID_PERIOD equ 941 ;midpoint input period in us * 2 (2125hz) MAX_PERIOD equ 2000 ;maximum valid input period in us * 2 (1000hz) GOOD_DATA equ 20 ;twenty good cassette in cycles in a row ; 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 RS232_OUT equ 3 ;serial data out to the RS-232 port RS232_IN equ 1 ;seria data in from the RS-232 port ; Data reception variables cblock 0x70 ;common RAM all pages captLo ;this capture low byte captHi ;this capture high byte prevLo ;previous counter low byte prevHi ;previous counter high byte deltaLo ;difference low byte deltaHi ;difference high byte goodCnt ;count of good pulses in a row flags ;flags byte endc ; Flags variable equates F_CASS_IN equ 0 ;1 = cassette input is active F_CLONE equ 1 ;1 = running inside an Altair Clone ;************************************************************************ ; ; Program Space ; ;************************************************************************ ; Reset and interrupt vectors org 0 ;page zero for jump sbrs goto reset_pwr ;reset vector location zero goto $ ;loop here and die goto $ goto $ goto tmr2_int ;interrupt vector location 0x04 ; Start of code. Reset entry point reset_pwr equ $ banksel INTCON clrf INTCON ;disable all interrupts just in case banksel OSCCON movlw b'01110000' ;8 Mhz internal clock movwf OSCCON banksel OPTION_REG movlw b'01010110' ;weak pullups enabled, /128 on TMR0 movwf OPTION_REG ; Initialize PORT A banksel PORTA movlw b'00100000' ;serial output to Altair idle at 1 movwf PORTA banksel LATA movwf LATA ;init Port A latch = Port A banksel ANSELA clrf ANSELA ;all digital - no analog banksel TRISA movlw b'00011001' ;Pgmming bits input, bit 4 in from Altair movwf TRISA ; (bit 1 temporarily output for now) banksel WPUA movlw b'00011011' ;pullup on all input bits movwf WPUA ; Initialize PORT C banksel PORTC movlw b'00001000' ;TX out to DB25 idle at 1 movwf PORTC ;no output bits really matter banksel LATC movwf LATC ;init Port C latch = Port C banksel ANSELC movlw b'00000100' ;RC2 used as analog comparator input movwf ANSELC banksel TRISC movlw b'00100110' ;RC5 IC, RC2 audio in, RC1 RX DB25 in movwf TRISC banksel WPUC movlw b'00000000' ;no pullups used movwf WPUC ; If jumper installed across programming header pins 2&4, then set flag for CLONE, ; else, we're running with RS-232 host input clrf flags ;all flags false banksel PORTA btfss PORTA,0 ;header pin 2 shorted to pin 4? bsf flags,F_CLONE ;yes, running inside the Altair Clone banksel TRISA movlw b'00011011' ;idle header pin 2 as input movwf TRISA clrf goodCnt ;no good cassette input cycles yet ; Initialize voltage reference, DAC and comparator for processing the audio input. ; The audio input comes in on C2- where it is compared to 0.25v on the + input. ; The output of the comparator goes is connected externally to CCP1 (the input ; capture pin). The comparator output is inverted so it goes high when audio ; in is > 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 8 ;DAC 8/32 = .25v 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. Input is captured on rising edges. 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 compare every 1/2 cycle for 2400hz (one) or ; 1850hz (zero). The timer is run at 500khz. Set up to interrupt ; on each compare match. banksel TMR2 ;timer 2 for audio out generation movlw b'00000101' ;timer 2 on, F-Inst/4 = 500Khz movwf T2CON clrf TMR2 ;start timer at zero movlw DATA1_PER ;assume data out is 1 movwf PR2 clrf PIR1 ;clear any pending interrupts banksel PIE1 ;enable TMR2 interrupts movlw b'00000010' ;enable TMR2 interrupts movwf PIE1 clrf PIE2 banksel INTCON ;back to page zero registers movlw b'11000000' ;global and peripheral interrupts enabled movwf INTCON ;let things run ;--------------------------------------------------------------------------- ; Main loop ; 1) Copy data line from Altair directly to RS-232 output. The audio ; output of the Altair data is done by the tmr2_int interrupt. ; ; 2) While cassette audio input is not present, copy data from the ; RS-232 input directly to the Altair data input. ; ; 3) While cassette audio is being received, copy data from the ; cassette input directly to the Altair data input. ;---------------------------------------------------------------------------- main btfss flags,F_CLONE ;running inside an Altair Clone? goto chk_cass ;no btfss PORTA,FROM_ALTAIR ;receiving one or zero? goto set_rs232L ;data is zero bsf PORTC,RS232_OUT ;set data out to one goto chk_data_in set_rs232L bcf PORTC,RS232_OUT ;set data out to zero ; chk_data_in - If not receiving from cassette, then copy RS-232 in to the Altair chk_data_in btfsc flags,F_CASS_IN ;presently receiving audio? goto chk_cass ;yes btfss PORTC,RS232_IN ;copy RS-232 in to the Altair goto set_altairL ;data is zero bsf PORTA,TO_ALTAIR ;set data to Altair high goto chk_cass ;try to detect audio set_altairL bcf PORTA,TO_ALTAIR ;set data to Altair low ;fall through to chk_cass ; chk_cass - process cassette audio input. ; If not presently receiving cassette data (F_CASS_IN false), then GOOD_DATA ; pulses in a row from the cassette will set the F_CASS_IN flag true and data to ; the Altair will then come from the cassette. ; ; If presently receiving cassette data (F_CASS_IN true), then data to the Altair ; comes from the cassette input. If TMR0 bit 7 becomes set, (8.2ms without a ; valid audio pulse), then F_CASS_IN is set back to false. chk_cass btfsc PIR1,CCP1IF ;capture occur? goto have_pulse ;yes ; No pulse occurred. If TMR0 bit 7 is set, then we've had 8.2ms of idle time. ; Set F_CASS_IN false. btfss TMR0,7 goto main ;not 8.2ms yet bcf flags,F_CASS_IN ;no longer getting cassette data goto main ; Capture occured, save the value capture in captHi/captLo have_pulse equ $ 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 ; Save current capture time as the previous capture time movfw captLo ;save current timer as previous movwf prevLo movfw captHi movwf prevHi ; See if period is shorter than MIN_PERIOD. If so, treat as invalid pulse ; but a data one. movlw low(MIN_PERIOD) subwf deltaLo,W movlw high(MIN_PERIOD) subwfb deltaHi,W btfsc STATUS,C ;carry clear if < MIN_PERIOD goto chk_mid clrf goodCnt ;invalid pulse goto cass_in1 ;treat as data 1 ; See if period is shorter than MID_PERIOD. If so, treat as a valid pulse ; and a data one (2400hz) chk_mid movlw low(MID_PERIOD) subwf deltaLo,W movlw high(MID_PERIOD) subwfb deltaHi,W btfsc STATUS,C ;carry clear if < MID_PERIOD goto chk_max incf goodCnt,F ;have a good pulse clrf TMR0 ;reset bad data timeout goto cass_in1 ;cassette data is a one ; See if period is shorter than MAX_PERIOD. If so, treat as a valid pulse ; and a data zero (1850hz) chk_max movlw low(MAX_PERIOD) subwf deltaLo,W movlw high(MAX_PERIOD) subwfb deltaHi,W btfsc STATUS,C ;carry clear if < MAX_PERIOD goto bad_zero incf goodCnt,F ;have a good pulse clrf TMR0 ;reset bad data timeout goto cass_in0 ;cassette data is a zero ; Period is longer than MAX_PERIOD. Treat as an invalid pulse and data zero bad_zero clrf goodCnt ;invalid pulse ;fall through to data zero ; Received a 0 from the cassette cass_in0 btfss flags,F_CASS_IN ;are we accepting cassette input data? goto chk_goodCnt ;no btfss flags,F_CLONE ;skip RS-232 out if in an Altair Clone bcf PORTC,RS232_OUT ;output data via RS-232 bcf PORTA,TO_ALTAIR ;output to Altair now zero goto main ; Received a 1 from the cassette cass_in1 btfss flags,F_CASS_IN ;are we accepting cassette input data? goto chk_goodCnt ;no btfss flags,F_CLONE ;skip RS-232 out if in an Altair Clone bsf PORTC,RS232_OUT ;output data via RS-232 bsf PORTA,TO_ALTAIR ;output to Altair now 1 goto main ; chk_goodCnt - if goodCnt reaches GOOD_DATA then set F_CASS_IN true; chk_goodCnt movlw GOOD_DATA subwf goodCnt,W ;reach GOOD_DATA in a row? btfss STATUS,C ;carry set if goodCnt >= GOOD_DATA goto main bsf flags,F_CASS_IN ;now accepting cassette data goto main ;---------------------------------------------------------------------------- ; ; tmr2_int ; Timer 2 interrupt occurs every 1/2 cycle of the audio output waveform. ; This is 208us for 2400hz (one) or 270us for 1850 hz (zero). Clear ; the interrupt, toggle the output state and set the period register ; based on the current data input from the Altair. ; ;---------------------------------------------------------------------------- tmr2_int equ $ banksel PORTA ;select bank 0 bcf PIR1,TMR2IF ;clear the interrupt btfss PORTC,AUDIO_OUT ;presently high or low? goto set_audioH ;is low, go set high bcf PORTC,AUDIO_OUT ;was high, set low goto tmr2_next ;set next wakeup set_audioH bsf PORTC,AUDIO_OUT ;was low, set high tmr2_next movlw DATA1_PER ;assume data is one btfss flags,F_CLONE ;inside the Altair Clone? goto rs232_host ;no btfss PORTA,FROM_ALTAIR ;receiving one or zero from Altair? movlw DATA0_PER ;receiving zero movwf PR2 ;set next wakeup retfie rs232_host btfss PORTC,RS232_IN ;receiving one or zero from host? movlw DATA0_PER ;receiving zero movwf PR2 ;set next wakeup retfie end