Datasheet

 2011 Microchip Technology Inc. Preliminary DS41569A-page 145
PIC16LF1904/6/7
17.3 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.
17.4 Timer1 Oscillator
A dedicated low-power 32.768 kHz oscillator circuit is
built-in between pins T1OSI (input) and T1OSO. 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.
17.5 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 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 17.5.1 “Reading and Writing Timer1 in
Asynchronous Counter Mode”).
17.5.1 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.
17.6 Timer1 Gate
Timer1 can be configured to count freely or the count
can be enabled and disabled using Timer1 gate
circuitry. This is also referred to as Timer1 Gate Enable.
Timer1 gate can also be driven by multiple selectable
sources.
17.6.1 TIMER1 GATE ENABLE
The Timer1 Gate Enable mode is enabled by setting
the TMR1GE bit of the T1GCON register. The polarity
of the Timer1 Gate Enable mode is configured using
the T1GPOL bit of the T1GCON register.
When Timer1 Gate Enable mode is enabled, Timer1
will increment on the rising edge of the Timer1 clock
source. When Timer1 Gate Enable mode is disabled,
no incrementing will occur and Timer1 will hold the
current count. See Figure 17-3 for timing details.
Note: The oscillator requires a start-up and
stabilization time before use. Thus,
T1OSCEN should be set and a suitable
delay observed prior to using Timer1. 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.
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.
TABLE 17-3: TIMER1 GATE ENABLE
SELECTIONS
T1CLK T1GPOL T1G Timer1 Operation
 00Counts
 01Holds Count
 10Holds Count
 11Counts