Giúp em đếm từ 30->0 dùng pic16f877a

[phapdp]

PROCESSOR 16F877A
#INCLUDE <P16F877A.INC>
__CONFIG _CP_OFF & _WDT_OFF & _BODEN_OFF & _PWRTE_ON & _XT_OSC & _WRT_OFF & _LVP_OFF & _CPD_OFF
DEM1 EQU 20H
DEM2 EQU 21H
DEM EQU 22H
ORG 0000H
BCF STATUS,6
BCF STATUS,5
CLRF PORTB
CLRF PORTA
BSF STATUS,5
MOVLW B'00000000'
MOVWF TRISB
BCF TRISA,0
BCF TRISA,1
BCF STATUS,5
MAIN
BCF PORTA,0
BSF PORTA,1
MOVLW B'10100100'
MOVWF PORTB
CALL DELAY1
BCF PORTA,1
BSF PORTA,0
MOVLW B'11111001'
MOVWF PORTB
CALL DELAY1


BCF PORTA,0
BSF PORTA,1
MOVLW B'10100100'
MOVWF PORTB
CALL DELAY1
BCF PORTA,1
BSF PORTA,0
MOVLW B'11000000'
MOVWF PORTB
CALL DELAY1


BCF PORTA,0
BSF PORTA,1
MOVLW B'11111001'
MOVWF PORTB
CALL DELAY1
BCF PORTA,1
BSF PORTA,0
MOVLW B'10010000'
MOVWF PORTB
CALL DELAY1


BCF PORTA,0
BSF PORTA,1
MOVLW B'11111001'
MOVWF PORTB
CALL DELAY1
BCF PORTA,1
BSF PORTA,0
MOVLW B'11000000'
MOVWF PORTB
CALL DELAY1
GOTO MAIN

DELAY1
BCF STATUS,6
BSF STATUS,5
BCF OPTION_REG,3
BCF OPTION_REG,2
BSF OPTION_REG,1
BSF OPTION_REG,0
BCF STATUS,5
MOVLW D'250'
MOVWF DEM
LOOP
MOVLW D'5'
MOVWF TMR0
BSF STATUS,5
BCF OPTION_REG,5
BCF STATUS,5
LOOP1
BTFSS INTCON,2
GOTO LOOP1
BCF INTCON,2
DECFSZ DEM,1
GOTO LOOP
RETURN
END


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