PIC16F870SLASHP datasheet(53/156 Pages) MICROCHIP

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Preliminary

DS30569A-page 53

PIC16F870/871

6.4

Timer1 Operation in Asynchronous

Counter Mode

If control bit T1SYNC (T1CON<2>) is set, the external

clock input is not synchronized. The timer continues to

increment asynchronous to the internal phase clocks.

The timer will continue to run during SLEEP and can

generate an interrupt on overflow, which will wake-up

the processor. However, special precautions in soft-

ware are needed to read/write the timer (Section 6.4.1).

In asynchronous counter mode, Timer1 can not be used

as a time-base for capture or compare operations.

6.4.1

READING AND WRITING TIMER1 IN

ASYNCHRONOUS COUNTER MODE

Reading TMR1H or TMR1L while the timer is running

from an external asynchronous clock will guarantee a

valid read (taken care of in hardware). However, the

user should keep in mind that reading the 16-bit timer

in two 8-bit values itself poses certain problems, since

the timer may overflow between the reads.

For writes, it is recommended that the user simply stop

the timer and write the desired values. A write conten-

tion may occur by writing to the timer registers, while

the register is incrementing. This may produce an

unpredictable value in the timer register.

Reading the 16-bit value requires some care. Examples

12-2 and 12-3 in the PICmicro™ Mid-Range MCU Fam-

ily Reference Manual (DS33023) show how to read and

write Timer1 when it is running in asynchronous mode.

6.5

Timer1 Oscillator

A crystal oscillator circuit is built-in between pins T1OSI

(input) and T1OSO (amplifier output). It is enabled by

setting control bit T1OSCEN (T1CON<3>). The oscilla-

tor is a low power oscillator rated up to 200 kHz. It will

continue to run during SLEEP. It is primarily intended

for use with a 32 kHz crystal. Table 6-1 shows the

capacitor selection for the Timer1 oscillator.

The Timer1 oscillator is identical to the LP oscillator.

The user must provide a software time delay to ensure

proper oscillator start-up.

TABLE 6-1:

CAPACITOR SELECTION FOR

THE TIMER1 OSCILLATOR

6.6

Resetting Timer1 using CCP1 Trigger

Output

If the CCP1 module is configured in compare mode to

generate a “special event trigger” (CCP1M<3:0> =

1011

), this signal will reset Timer1.

Timer1 must be configured for either timer or synchro-

nized counter mode to take advantage of this feature. If

Timer1 is running in asynchronous counter mode, this

reset operation may not work.

In the event that a write to Timer1 coincides with a spe-

cial event trigger from CCP1, the write will take prece-

dence.

In this mode of operation, the CCPR1H:CCPR1L regis-

ter pair effectively becomes the period register for

Timer1.

6.7

Resetting of 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 trigger.

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.

6.8

Timer1 Prescaler

The prescaler counter is cleared on writes to the

TMR1H or TMR1L registers.

Osc Type

Freq

32 kHz

33 pF

100 kHz

15 pF

200 kHz

15 pF

These values are for design guidance only.

Crystals Tested:

32.768 kHz

Epson C-001R32.768K-A

± 20 PPM

100 kHz

Epson C-2 100.00 KC-P

± 20 PPM

200 kHz

STD XTL 200.000 kHz

± 20 PPM

Note 1:

Higher capacitance increases the stability of

oscillator, but also increases the start-up time.

2: Since each resonator/crystal has its own charac-

teristics, the user should consult the resonator/

crystal manufacturer for appropriate values of

external components.

Note:

The special event trigger from the CCP1

module will not set interrupt flag bit

TMR1IF (PIR1<0>).

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