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PIC18F23K20-E/ML Datasheet(PDF) 164 Page - Microchip Technology |
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PIC18F23K20-E/ML Datasheet(HTML) 164 Page - Microchip Technology |
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164 / 456 page  PIC18F2XK20/4XK20 DS41303G-page 164 2010 Microchip Technology Inc. 13.9 Using Timer1 as a Real-Time Clock Adding an external LP oscillator to Timer1 (such as the one described in Section 13.6 “Timer1 Oscillator” above) gives users the option to include RTC function- ality to their applications. This is accomplished with an inexpensive watch crystal to provide an accurate time base and several lines of application code to calculate the time. When operating in Sleep mode and using a battery or supercapacitor as a power source, it can completely eliminate the need for a separate RTC device and battery backup. The application code routine, RTCisr, shown in Example 13-1, demonstrates a simple method to increment a counter at one-second intervals using an Interrupt Service Routine. Incrementing the TMR1 register pair to overflow triggers the interrupt and calls the routine, which increments the seconds counter by one; additional counters for minutes and hours are incremented on overflows of the less significant counters. Since the register pair is 16 bits wide, a 32.768 kHz clock source will take 2 seconds to count up to over- flow. To force the overflow at the required one-second intervals, it is necessary to preload it; the simplest method is to set the MSb of TMR1H with a BSF instruc- tion. Note that the TMR1L register is never preloaded or altered; doing so may introduce cumulative error over many cycles. For this method to be accurate, Timer1 must operate in Asynchronous mode and the Timer1 overflow interrupt must be enabled (PIE1<0> = 1), as shown in the routine, RTCinit. The Timer1 oscillator must also be enabled and running at all times. EXAMPLE 13-1: IMPLEMENTING A REAL-TIME CLOCK USING A TIMER1 INTERRUPT SERVICE RTCinit MOVLW 80h ; Preload TMR1 register pair MOVWF TMR1H ; for 1 second overflow CLRF TMR1L MOVLW b’00001111’ ; Configure for external clock, MOVWF T1CON ; Asynchronous operation, external oscillator CLRF secs ; Initialize timekeeping registers CLRF mins ; MOVLW .12 MOVWF hours BSF PIE1, TMR1IE ; Enable Timer1 interrupt RETURN RTCisr BSF TMR1H, 7 ; Preload for 1 sec overflow BCF PIR1, TMR1IF ; Clear interrupt flag INCF secs, F ; Increment seconds MOVLW .59 ; 60 seconds elapsed? CPFSGT secs RETURN ; No, done CLRF secs ; Clear seconds INCF mins, F ; Increment minutes MOVLW .59 ; 60 minutes elapsed? CPFSGT mins RETURN ; No, done CLRF mins ; clear minutes INCF hours, F ; Increment hours MOVLW .23 ; 24 hours elapsed? CPFSGT hours RETURN ; No, done CLRF hours ; Reset hours RETURN ; Done |
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