assembly - PIC interrupt based soft UART timing trouble -


i've tried implement software uart on pic 18f452 using timer interrupts - although i'm begginer microcontrollers - , can't timing working. i'm using mplab asm compilation , pickit 2 programming. 

list p=18f452 include <p18f452.inc> config wdt=off, lvp=off, osc=hs, cp0=off  variable cyclesperbaud=1            ; 1 cycle(s) per baud variable cyclesint=0xffff           ; counter in 16 bit timer variable freq = 20000000            ; clock frequency 20mhz variable baud = 9600                    ; baud rate variable cyclesmain=33              ; cycles in main program branch (intr->checkbitcounter->startstopbit->transfer) variable cyclesidle=16              ; cycles in idle branch (intr->idle) variable offset = (freq/(4*baud))/(cyclesperbaud) variable initoffset = cyclesint - offset variable durroffset = cyclesint - offset + cyclesmain variable hurroffset = cyclesint - offset + cyclesidle  cblock 0x20     char                    ; character send     bitcounter      ; bits left send     startstopbit    ; checks send start of stop bit     buffer              ; buffer char rotated     cycle                   ; current cycle in cyclesperbaud starting high endc  org 0000h     goto main org 0008h     goto intr  main bcf osccon, scs     ; use primary clock bsf rcon, ipen      ; enable priority levels  ; interrupt config: bsf intcon, gieh    ; enable high priority interrupts bcf intcon, giel    ;   disable low priority interrupts bsf intcon, tmr0ie; enable tmr0 interrupts bcf intcon, int0ie; dis. ext. interrupts bcf intcon, rbie    ; dis. rb port change int. bcf intcon, tmr0if; clear tmr0 intr. flag bit bcf intcon, int0if;  bcf intcon, rbif    ;  bsf intcon2, tmr0ip; set tmr0 high priority  ; timer 0 config:    bcf t0con, tmr0on   ; temp. disable timer bcf t0con, t08bit   ; 16 bit counter bcf t0con, t0cs     ; t0cki pin input source bcf t0con, t0se     ; switch on falling edge bsf t0con, psa      ; disable prescaler bcf t0con, t0ps2    ; doesn't matter bcf t0con, t0ps1    ; -||- bcf t0con, t0ps0    ; -||-  ; init output registers , variables clrf trisd clrf latd movlw 0xff movwf portd             ; our output port set high (serial 0 - not sure that) movlw b'01111111' ; symbol send movwf char movlw cyclesperbaud movwf cycle             ; unused clrf bitcounter     ; zeros clrf startstopbit   ; zeros clrf buffer             ; zeros ; eof init  movlw low initoffset    ; dunno why high has reversed low movwf tmr0h              movlw high initoffset    movwf tmr0l              bsf t0con, tmr0on   ; turn on timer  loop  goto loop                   ; waiting     interrupt  intr                ; main branch: 8 cycles btfss intcon, tmr0if     retfie bcf t0con, tmr0on  ; had comment section out - didn't want work - dunno why ;   decf cycle, f ;   btfss status, z ;       goto idle ;   movlw cyclesperbaud ;   movwf cycle  checkbitcounter             ; branches until 'tmr_ret': 16 cycles movf bitcounter, f btfss status, z     goto rotatebuffer  ;startstopbit  movf startstopbit, f btfss status, z     goto stopcopy movlw 0x8 movwf bitcounter movf char, w movwf buffer rlncf buffer, f movlw 0x00  transfer movwf portd decf bitcounter, f btfsc status, z     incf startstopbit, f  tmr_ret         ; 5 cycles (+8 intr = 13 cycles) movlw low durroffset    ; dunno why high has reversed low movwf tmr0h              movlw high durroffset    movwf tmr0l   bsf t0con, tmr0on retfie  stopcopy clrf startstopbit movlw 0xff nop goto transfer  rotatebuffer rrncf buffer, f movf buffer, w nop nop nop nop goto transfer     idle        ; 5 cycles (+7 intr = 12 cycles) movlw low hurroffset    ; dunno why high has reversed low movwf tmr0h              movlw high hurroffset    movwf tmr0l   bsf t0con, tmr0on retfie  end

so i'm wondering doing wrong? mean pic sending stuff on portd, 0 it's supposed it's not data it's supposed sending. when peak transmitted (with realterm), byte being sent 11011111 or 10111111 , 11111111 instead of 01111111.

i'm guessing i'm missing timings not sure what?

also since started using 'variables' in code noticed had copy lower part off timing offsets high , high bytes low tmr0h:l registers - knows why works way? maybe i'm confusing significance of these registers: used them before if tmr0h register hold more significant 8 bits of 2 byte counter - correct?

anyway, i'd appreciate help. thanks, drinker


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