PIC16F688
6.2.1
INTERNAL CLOCK SOURCE
6.5
Timer1 Operation in
Asynchronous Counter Mode
When the internal clock source is selected the
TMR1H:TMR1L register pair will increment on multiples
of TCY as determined by the Timer1 prescaler.
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”).
6.2.2
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.
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.
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.
Note:
In Counter mode, a falling edge must be
registered by the counter prior to the first
incrementing rising edge.
6.5.1
READING AND WRITING TIMER1 IN
ASYNCHRONOUS COUNTER
MODE
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.
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.
6.4
Timer1 Oscillator
A low-power 32.768 kHz crystal oscillator is built-in
between pins OSC1 (input) and OSC2 (amplifier
output). The oscillator is enabled by setting the
T1OSCEN control bit of the T1CON register. The
oscillator will continue to run during Sleep.
6.6
Timer1 Gate
The Timer1 oscillator is shared with the system LP
oscillator. Thus, Timer1 can use this mode only when
the primary system clock is derived from the internal
oscillator or when in LP oscillator mode. The user must
provide a software time delay to ensure proper oscilla-
tor start-up.
Timer1 gate source is software configurable to be the
T1G pin or the output of Comparator 2. This allows the
device to directly time external events using T1G or
analog events using Comparator 2. See the CMCON1
register (Register 7-2) 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).
TRISA5 and TRISA4 bits are set when the Timer1
oscillator is enabled. RA5 and RA4 bits read as ‘0’ and
TRISA5 and TRISA4 bits read as ‘1’.
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:
TMR1GE bit of the T1CON register must
be set to use either T1G or C2OUT as the
Timer1 gate source. See Register 7-2 for
more information on selecting the Timer1
gate source.
Timer1 gate can be inverted using the T1GINV bit of
the T1CON register, whether it originates from the T1G
pin or Comparator 2 output. This configures Timer1 to
measure either the active-high or active-low time
between events.
© 2007 Microchip Technology Inc.
DS41203D-page 49
MICROCHIP [ MICROCHIP ]
