PIC18F14K50-I/SS【PDF 12/285 页】IC PIC MCU FLASH 8KX16 20-SSOP

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2010 Microchip Technology Inc.

Preliminary

DS41350E-page 109

PIC18F/LF1XK50

11.6

Using Timer1 as a Real-Time Clock

Adding an external LP oscillator to Timer1 (such as the

one described in Section 11.3 “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 11-1, demonstrates a simple method to

increment a counter at one-second intervals using an

Interrupt Service Routine. Incrementing the TMR1

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 11-1:

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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