PIC16F7X7 datasheet(84/276 Pages) MICROCHIP

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PIC16F7X7

DS30498C-page 82

 2004 Microchip Technology Inc.

7.9

Resetting Timer1 Register Pair

(TMR1H, TMR1L)

TMR1H and TMR1L registers are not reset to 00h on a

POR, or any other Reset, except by the CCP1 special

event triggers.

T1CON register is reset to 00h on a Power-on Reset or

a Brown-out Reset, which shuts off the timer and

leaves a 1:1 prescale. In all other Resets, the register

is unaffected.

7.10

Timer1 Prescaler

The prescaler counter is cleared on writes to the

TMR1H or TMR1L registers.

7.11

Using Timer1 as a Real-Time Clock

Adding an external LP oscillator to Timer1 (such as the

one described in Section 7.6 “Timer1 Oscillator”)

gives users the option to include RTC functionality in

their applications. This is accomplished with an inex-

pensive 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 7-3, demonstrates a simple method to

increment a counter at one-second intervals using an

Interrupt Service Routine. Incrementing the TMR1 reg-

ister 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 as the previous counter overflows.

Since the register pair is 16 bits wide, counting up to

overflow the register directly from a 32.768 kHz clock

would take 2 seconds. 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 instruction. Note that the TMR1L

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 7-3:

IMPLEMENTING A REAL-TIME CLOCK USING A TIMER1 INTERRUPT SERVICE

RTCinit

BANKSEL

TMR1H

MOVLW

0x80

; 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

BANKSEL

PIE1

BSF

PIE1, TMR1IE

; Enable Timer1 interrupt

RETURN

RTCisr

BANKSEL

TMR1H

BSF

TMR1H, 7

; Preload for 1 sec overflow

BCF

PIR1, TMR1IF

; Clear interrupt flag

INCF

secs, F

; Increment seconds

MOVF

secs, w

SUBLW

.60

BTFSS

STATUS, Z

; 60 seconds elapsed?

RETURN

; No, done

CLRF

seconds

; Clear seconds

INCF

mins, f

; Increment minutes

MOVF

mins, w

SUBLW

.60

BTFSS

STATUS, Z

; 60 seconds elapsed?

RETURN

; No, done

CLRF

mins

; Clear minutes

INCF

hours, f

; Increment hours

MOVF

hours, w

SUBLW

.24

BTFSS

STATUS, Z

; 24 hours elapsed?

RETURN

; No, done

CLRF

hours

; Clear hours

RETURN

; Done

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