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external interrupt

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neelam29

New Member
hi
i am trying to make a system including frequency meter. for frequency

measurement method:- measuring time between two consequtive pulses. i'm

feeding the frequency at external interrupt pin. i'm facing a small

problem. when i start the system lcd displays the correct frequency

value. (i've added 5 seconds delay for refreshing the data) but second

time the frequency is refreshed the value comes wrong and after that

everytime absart values are displayed. i've tried everything. i've

tried re-intializing everything but nothing is helping out. here's the

code (the main part which measures freq. and displays it. Please help

me.

Kind regards.




Code:
	LIST	p=16F877A, W=2, X=ON, R=DEC	;tell assembler what chip we are using
	include "P16F877A.inc"			;include the defaults for the chip
	ERRORLEVEL	0,	-302		;suppress bank selection messages
	__CONFIG    0x393A			;sets the configuration settings (oscillator type etc.)


		cblock	0x20			;start of general purpose registers
			count
			count1	
			counta	
			countb	
			COUNT_SEC
			LoX		
			Bit_Cntr	
			Timer_H		
			Flags		
			Flags2		
			tmp1		
			tmp2		
			tmp3			
			NumL	
			NumH

			
			
			AARGB0
			AARGB1
			AARGB2
			AARGB3
			BARGB0
			BARGB1
			LOOPCOUNT
			REMB0
			REMB1
	


			TenK		
			Thou	
			Hund	
			Tens			
			Ones		
			templcd	
			templcd2	
			Acc1L			;16 bit maths register 1
			Acc1H
			Point			;position of decimal point

;FREQUENCY VARIABLES			
			FreqL
			FreqH
			TIMER1_LOW
			TIMER1_HIGH

			w_temp 				; To save off current W register contents
			status_temp 
			
			freq_over
			
			
			
			
		endc

LCD_PORT	Equ	PORTD
LCD_TRIS	Equ	TRISD
LCD_RS		Equ	0x05			;LCD handshake lines
LCD_RW		Equ	0x06
LCD_E		Equ	0x07


		
		ORG	0x0000
		NOP
		BCF	PCLATH,	3
		BCF	PCLATH,	4
		GOTO	start

		
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>frequency call>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

		ORG	0x0004
		
		MOVWF 	w_temp 		; save off current W register contents
		MOVF 	STATUS,w 		; move status register into W register
		MOVWF 	status_temp 	; save off contents of STATUS register
		
		BTFSC	T1CON, TMR1ON
		goto	Stop_timer1
		
		CLRF	TMR1H
		CLRF	TMR1L
		BSF		T1CON, TMR1ON
		GOTO	END_ANS_INT


Stop_timer1
		BCF		T1CON, TMR1ON
		MOVF	TMR1H, W
		MOVWF	TIMER1_HIGH
		MOVF	TMR1L, W
		MOVWF	TIMER1_LOW
		BSF		freq_over, 0X00
		
		;BCF		INTCON, INTE
END_ANS_INT		
		MOVF 	status_temp,w 	; retrieve copy of STATUS register
		MOVWF 	STATUS 		; restore pre-isr STATUS register contents
		SWAPF 	w_temp,f
		SWAPF 	w_temp,w 		; restore pre-isr W register contents              	
		BCF		INTCON, INTF
		
		retfie
;=====================================================================

		




start
		errorlevel	-302
		bsf	INTCON,INTE		; Enable external interrupt
		bsf	INTCON,GIE


		

Initialise	clrf	count
		clrf	PORTA
		clrf	PORTD
		clrf	PORTC
		clrf	Flags
		
		clrf	REMB0
		clrf	REMB1
		movlw	b'00110000'
		movwf	T1CON



SetPorts	bsf 	STATUS,		RP0	;select bank 1
		movlw	0x00			;make all LCD pins outputs
		movwf	LCD_TRIS
		movlw	B'00000001'			;make all pins outputs
		movwf	TRISB
		bsf     OPTION_REG, NOT_RBPU
		bcf 	STATUS,		RP0	;select bank 0
		call	LCD_Init		;setup LCD module
		call	LCD_CurOff		;turn cursor off
		call 	Delay5
    	

Main


;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;>>>>>>>>>>>>>>>>>>>>>>> READING VALUES FROM EXT. INT. FOR FREQUENCY >>>>>>>>>>>>>>>>>>>>>>
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

		movlw	b'00110000'
		movwf	T1CON
		BCF	freq_over, 0X00
		
		bsf	INTCON,INTE		; Enable external interrupt
		
CHECK_INT		
		BTFSS	freq_over, 0X00
		GOTO	CHECK_INT
		
		BCF	INTCON,INTE
		BCF	freq_over, 0X00
		
		CALL	CALCULATE_FREQ
		

;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> DISPLAY FREQ >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
		
				
		movlw	0xC0			;move to 1st row, first column
		call	LCD_Cmd
		movf	FreqH,w
		movwf	NumH
		movf	FreqL,w
		movwf	NumL
		call	LCD_Decimal		;and display on LCD (in decimal)
		movlw	' '
		call	LCD_Char		;display a space
		nop
		nop
		
		
		CALL	Delay5sec
		goto	Main			;loop for ever



;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> CALCULATE_FREQ >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
CALCULATE_FREQ
		movlw	0x5F
		movwf	AARGB0
		movlw	0x5E
		movwf	AARGB1
		movlw	0x10
		movwf	AARGB2
		movf	TIMER1_HIGH,W
		movwf	BARGB0
		movf	TIMER1_LOW,W
		movwf	BARGB1
		
		call	FXD2416U		;DIVIDE 6250000/TIMER VALUE TO GET FREQUENCY WITH .1 RESOLUTION
		movf	AARGB1,w
		movwf	FreqH
		movf	AARGB2,w
		movwf	FreqL
		RETLW	0X00
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

;LCD ROUTINES.........
;DIVIDE ROUTINES.......


		end
 

motion

New Member
First of all, there are safer and more accurate ways to measuring the time duration between edges than using the external interrupt. Using the capture module is an example.

Anyway, my guess is that although you have disabled external interrupts while calculating and displaying the frequency, the clock continues to be applied to the external interrupt pin. While the PIC is busy displaying the calculated result, the "INTCON, INTF" flag will be set but this will not cause an interrupt because it is masked.

After the 5 second delay, the moment you set "INTCON, INTE", with the "INTCON, INTF" flag already set, an interrupt is immediately fired even if the edge occured much earlier. The second clock edge will be encountered soon after that and the frequency read will be much higher.

The ff. code should correct this problem.

Code:
Main 
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 
;>>>>>>>>>>>>>>>>>>>>>>> READING VALUES FROM EXT. INT. FOR FREQUENCY >>>>>>>>>>>>>>>>>>>>>> 
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

      movlw   b'00110000' 
      movwf   T1CON 
      BCF   freq_over, 0X00 
       
      BCF      INTCON, INTF ; **** Clear this first *****
      bsf   INTCON,INTE      ; Enable external interrupt 
       
CHECK_INT       
      BTFSS   freq_over, 0X00 
      GOTO   CHECK_INT 
       
      BCF   INTCON,INTE 
      BCF   freq_over, 0X00 
       
      CALL   CALCULATE_FREQ
 

neelam29

New Member
hi thanks motion, your solution worked out very well.

i wanted to ask u abt the capture mode. wht is it actually m new to pic so i m not familiar with it. i just read in the datasheet abt it. but cant understand how to use it for frequency meter. can u please explain me abt it or give me a small example based on it.

thanks once again
 

motion

New Member
neelam29 said:
hi thanks motion, your solution worked out very well.

i wanted to ask u abt the capture mode. wht is it actually m new to pic so i m not familiar with it. i just read in the datasheet abt it. but cant understand how to use it for frequency meter. can u please explain me abt it or give me a small example based on it.

thanks once again
Before I would get into the capture mode, I would like to mention the problems I see with using the external interrupt. The interrupt service time could severely limit the maximum frequency you can measure, maybe only up to 50-80Khz with increasing inaccuracy as frequency goes up. You didn't mention the frequency range you are trying to measure but since you are getting the reciprocal of the time interval, then I assume it is between 60Hz to 50Khz, not a very big range. You might do better if you simply count the number of pulses over a 1-5 second interval for example.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Why not consult the MicroChip application note for a frequency counter?. It's auto-ranging and goes from low audio to 50MHz, there's also a later update from Weeder Technologies that uses a more modern chip and an LCD disply module.
 
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