need help...

[anshi83]

i tried writing some code

;expected result is a high on portC,0 when
;start bit is detected.
;**********************************************************

__config _LVP_OFF & _XT_OSC & _WDT_OFF & _PWRTE_ON & _CP_OFF & _BODEN_OFF & _DEBUG_OFF


include "p16f877.inc"
STORAGE1 set 0x20
INNERWHEEL set 0x21
OUTERWHEEL set 0x22
COUNTER set 0x23

; Start at the reset vector
org 0x000
goto Start
org 0x004
Interrupt
retfie
Start
bsf STATUS,RP0 ;bank 1
bcf STATUS,RP1
movlw H'FF'
movwf TRISA ;porta [0] input
movlw H'00'
movwf TRISC ;portc [7-0] outputs
movlw H'00'
movwf TRISB ;portc [7-0] outputs
movlw H'07'
movwf OPTION_REG ;set prescaler
movlw H'06'
movwf ADCON1 ;set input bits to be digital

bcf STATUS,RP0 ;bank 0
clrf PORTC

Main

WaitS btfss PORTA,0 ;if high, then skip (break out
;of loop)
goto WaitS
clrf TMR0

WaitT btfsc PORTA,0 ;wait until a low pulse is
;obtained.
goto WaitT

movf TMR0,W ;TMR0 value is in W
sublw .35 ;W = .35 - W
btfss STATUS,CARRY ;if W> .35 then carry bit will
;be 0
bsf PORTC,0
goto Main
end

it should detect a .35 TMR0 cycle long high pulse (or longer) from portA, bit 0,... but it doesn't work.... i don't know why, but nothing appears in PORTC bit 0..... can anyone tell me why?

 

[anshi83]

isn't there anyone who can help me?

 

[Eclipsed]

I'm not too good at assembly, but it looks like you need a pulse LESS than 35 cycles to set PORTC bit 0. if W> .35 then carry bit will be 1 not 0.So change your btfss to btfsc.Also I am not familar with the "." in .35

 

[Exo]

You need to set Timer0's clock source mux and prescaler before it starts running

Read the datasheet... prescaler and clock selection mux for TMR0 is done with OPTION_REG

 

[Exo]

I'm not too good at assembly, but it looks like you need a pulse LESS than 35 cycles to set PORTC bit 0. if W> .35 then carry bit will be 1 not 0.So change you btfss to btfsc.Also I am not familar with the "." in .35

.35 means 35 decimal