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2009 Microchip Technology Inc.
DS41341E-page 117
PIC16F72X/PIC16LF72X
12.3
Timer1 Prescaler
Timer1 has four prescaler options allowing 1, 2, 4 or 8
divisions of the clock input. The T1CKPS bits of the
T1CON register control the prescale counter. The
prescale counter is not directly readable or writable;
however, the prescaler counter is cleared upon a write to
TMR1H or TMR1L.
12.4
Timer1 Oscillator
A dedicated low-power 32.768 kHz oscillator circuit is
built-in between pins T1OSI (input) and T1OSO
(amplifier output). This internal circuit is to be used in
conjunction with an external 32.768 kHz crystal.
The oscillator circuit is enabled by setting the
T1OSCEN bit of the T1CON register. The oscillator will
continue to run during Sleep.
12.5
Timer1 Operation in
Asynchronous Counter Mode
If control bit T1SYNC of the T1CON register is set, the
external clock input is not synchronized. The timer
increments asynchronously to the internal phase
clocks. If external clock source is selected then 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
software are needed to read/write the timer (see
Section 12.5.1 “Reading and Writing Timer1 in
Asynchronous Counter Mode”).
12.5.1
READING AND WRITING TIMER1 IN
ASYNCHRONOUS COUNTER
MODE
Reading TMR1H or TMR1L while the timer is running
from an external asynchronous clock will ensure 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
contention may occur by writing to the timer registers,
while the register is incrementing. This may produce an
unpredictable value in the TMR1H:TMR1L register
pair.
Note:
The oscillator requires a start-up and
stabilization
time
before
use.
Thus,
T1OSCEN should be set and a suitable
delay observed prior to enabling Timer1.
Note:
When switching from synchronous to
asynchronous operation, it is possible to
skip an increment. When switching from
asynchronous to synchronous operation,
it is possible to produce an additional
increment.
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