Datasheet

© 2009 Microchip Technology Inc. DS41291F-page 77
PIC16F882/883/884/886/887
6.2.1 INTERNAL CLOCK SOURCE
When the internal clock source is selected the
TMR1H:TMR1L register pair will increment on multiples
of F
OSC as determined by the Timer1 prescaler.
6.2.2 EXTERNAL CLOCK SOURCE
When the external clock source is selected, the Timer1
module may work as a timer or a counter.
When counting, Timer1 is incremented on the rising
edge of the external clock input T1CKI. In addition, the
Counter mode clock can be synchronized to the
microcontroller system clock or run asynchronously.
If an external clock oscillator is needed (and the
microcontroller is using the INTOSC without CLKOUT),
Timer1 can use the LP oscillator as a clock source.
In Counter mode, a falling edge must be registered by
the counter prior to the first incrementing rising edge
after one or more of the following conditions (see
Figure 6-2):
• Timer1 is enabled after POR or BOR Reset
• A write to TMR1H or TMR1L
• T1CKI is high when Timer1 is disabled and when
Timer1 is reenabled T1CKI is low.
6.3 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.
6.4 Timer1 Oscillator
A low-power 32.768 kHz oscillator is built-in between
pins T1OSI (input) and T1OSO (amplifier output). The
oscillator is enabled by setting the T1OSCEN control
bit of the T1CON register. The oscillator will continue to
run during Sleep.
The Timer1 oscillator is identical to the LP oscillator.
The user must provide a software time delay to ensure
proper oscillator start-up.
TRISC0 and TRISC1 bits are set when the Timer1
oscillator is enabled. RC0 and RC1 bits read as ‘0’ and
TRISC0 and TRISC1 bits read as ‘1’.
6.5 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
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 software are needed to read/write the
timer (see Section 6.5.1 “Reading and Writing
Timer1 in Asynchronous Counter Mode”).
6.5.1 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:TTMR1L register
pair.
6.6 Timer1 Gate
Timer1 gate source is software configurable to be the
T1G
pin or the output of Comparator C2. This allows the
device to directly time external events using T1G
or
analog events using Comparator C2. See the
CM2CON1 register (Register 8-3) for selecting the
Timer1 gate source. This feature can simplify the
software for a Delta-Sigma A/D converter and many
other applications. For more information on Delta-Sigma
A/D converters, see the Microchip web site
(www.microchip.com).
Timer1 gate can be inverted using the T1GINV bit of
the T1CON register, whether it originates from the T1G
pin or Comparator C2 output. This configures Timer1 to
measure either the active-high or active-low time
between events.
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 a single spurious
increment.
Note: TMR1GE bit of the T1CON register must be
set to use the Timer1 gate.