PIC16F631/677/685/687/689/690
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’.
6.2.1
INTERNAL CLOCK SOURCE
When the internal clock source is selected the
TMR1H:TMR1L register pair will increment on multiples
of FOSC as determined by the Timer1 prescaler.
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.
6.2.2
EXTERNAL CLOCK SOURCE
When the external clock source is selected, the Timer1
module may work as a timer or a counter.
6.5
Timer1 Operation in
Asynchronous Counter Mode
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 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 6.5.1 “Reading and Writing Timer1 in
Asynchronous Counter Mode”).
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 after any one or
more of the following conditions:
• Timer1 enabled after POR reset
• Write to TMR1H or TMR1L
• Timer1 is disabled
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.
• Timer1 is disabled (TMR1ON 0) when
T1CKI is high then Timer1 is enabled
(TMR1ON=1) when T1CKI is low.
Note:
See Figure 6-2
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:TMR1L 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 the oscillator is in the LP mode. The
user must provide a software time delay to ensure
proper oscillator start-up.
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).
© 2007 Microchip Technology Inc.
DS41262D-page 85
MICROCHIP [ MICROCHIP ]
