PIC16LF18854 Datasheet, PDF (412/668 Pages) Microchip Technology – C Compiler Optimized RISC Architecture

PIC16(L)F18856/76
28.3 Timer Prescaler
Timer1 has four prescaler options allowing 1, 2, 4 or 8
divisions of the clock input. The CKPS 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.
28.4 Secondary Oscillator
A dedicated low-power 32.768 kHz oscillator circuit is
built-in between pins SOSCI (input) and SOSCO
(amplifier output). This internal circuit is designed to be
used in conjunction with an external 32.768 kHz
crystal.
The oscillator circuit is enabled by setting the SOSCEN
bit of the OSCEN register. The oscillator will continue to
run during Sleep.
Note:
The oscillator requires a start-up and
stabilization time before use. Thus,
SOSCEN should be set and a suitable
delay observed prior to using Timer1 with
the SOSC source. A suitable delay similar
to the OST delay can be implemented in
software by clearing the TMR1IF bit then
presetting the TMR1H:TMR1L register
pair to FC00h. The TMR1IF flag will be set
when 1024 clock cycles have elapsed,
thereby indicating that the oscillator is
running and reasonably stable.
28.5 Timer Operation in Asynchronous
Counter Mode
If the control bit SYNC of the T1CON register is set, the
external clock input is not synchronized. The timer
increments asynchronously to the internal phase
clocks. If the 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 28.5.1 “Reading and Writing Timer1 in
Asynchronous Counter Mode”).
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.
28.5.1
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.
28.6 Timer Gate
Timer1 can be configured to count freely or the count
can be enabled and disabled using the time gate
circuitry. This is also referred to as Timer Gate Enable.
The timer gate can also be driven by multiple select-
able sources.
28.6.1 TIMER GATE ENABLE
The Timer Gate Enable mode is enabled by setting the
GE bit of the T1GCON register. The polarity of the
Timer Gate Enable mode is configured using the GPOL
bit of the T1GCON register.
When Timer Gate Enable mode is enabled, the timer
will increment on the rising edge of the Timer1 clock
source. When Timer Gate Enable mode is disabled, no
incrementing will occur and the timer will hold the
current count. See Figure 28-3 for timing details.
TABLE 28-2: TIMER GATE ENABLE
SELECTIONS
T1CLK T1GPOL T1G Timer Operation

1
1 Counts

1
0 Holds Count

0
1 Holds Count

0
0 Counts
DS40001824A-page 412
Preliminary
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