External Crystal Uses PIC16F628 TMR1 Generates Interrupt

by Lewis Loflin

A crystal oscillator circuit is built in between pins T1OSI (input) and T1OSO (amplifier output). It is enabled by setting control bit T1OSCEN (T1CON bit 3). The oscillator is a low power oscillator rated up to 200 kHz. It will continue to run during SLEEP. It is primarily intended for a 32 kHz crystal.

As a 16-bit timer with it's own clock source a real time clock base is easy to implement. Note the values in Tables 1 and 2 above. Compare to using TMR0 at PIC16F628 Timer 0 Interrupt Programming Tutorial.

The full ASM can be downloaded: PIC16F628A_TMR1.asm

;*****************************

	ORG     0x000             
	; processor reset vector
	goto    setup              
	; go to beginning of program
	

	ORG     0x004             
	; interrupt vector location begin here
	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

; ISR code can go here or be located 
; as a call subroutine elsewhere

	movlw 0x80 ; reload TMR1
	movwf TMR1H
	bcf PIR1, TMR1IF ; clr TMR1 interrupt flag
	call toggle ; flip state on RA0

; end ISR
	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
	retfie                    
	; return from interrupt

;*******************************************

Above is the interrupt service routine or ISR. Again we save the W and STATUS registers and restore before RETFIE ending the ISR.

	bsf	PIE1, TMR1IE ; enable TMR1 interrupt
	banksel INTCON
	clrf PORTA
	clrf PORTB
	bsf INTCON, GIE 
	; enable global interrupt
	bsf INTCON, PEIE 
	; enable all unmasked interrupts

	bcf PIR1, TMR1IF 
	; clr TMR1 interrupt flag
	clrf T1CON
	bsf T1CON, TMR1ON 
	; Timer1 On bit
	bsf T1CON, TMR1CS 
	; external oscillator crystal
	bsf T1CON, 2 
	; T1SYNC set to asynchronous 
	bsf T1CON, T1OSCEN 
	; Timer1 Oscillator Enable Control bit
	; two lines below same as 5 lines above
	; movlw b'00001111' 
	; movwf T1CON

	clrf TMR1L 
	; set to 0
	movlw 0x80 
	; value for 1 sec. delay
	movwf TMR1H 
	; must be reloaded after every interrupt

Above is the setup code to use the 32 kHz. crystal programmed to generate an interrupt every 1 second. This is regardless of the frequency of the main system clock in my case a 16 mHz. crystal.

See How I got into Electronics