Datasheet

 2002 Microchip Technology Inc. Preliminary DS39598C-page 61

PIC16F818/819

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, above), gives users the

option to include RTC functionality 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 super capacitor

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 incre-

ment a counter at one-second intervals using an Inter-

rupt Service Routine. Incrementing the TMR1 register

pair to overflow triggers the interrupt and calls the rou-

tine, which increments the seconds counter by one;

additional counters for minutes and hours are

incremented as the previous counter overflow.

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 pre-

load it; the simplest method is to set the MSbit 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 Inter-

rupt must be enabled (PIE1<0> = 1), as shown in the

routine RTCinit. The Timer1 oscillator must also be

enabled and running at all times.