PIC18F2480 MICROCHIP [Microchip Technology], PIC18F2480 Datasheet - Page 156

• Manufacturer Part Number: PIC18F2480
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F2480/2580/4480/4580
12.3.3
The Timer1 oscillator circuit draws very little power
during operation. Due to the low-power nature of the
oscillator, it may also be sensitive to rapidly changing
signals in close proximity.
The oscillator circuit, shown in Figure 12-3, should be
located as close as possible to the microcontroller.
There should be no circuits passing within the oscillator
circuit boundaries other than V
If a high-speed circuit must be located near the oscilla-
tor (such as the CCP1 pin in Output Compare or PWM
mode, or the primary oscillator using the OSC2 pin), a
grounded guard ring around the oscillator circuit, as
shown in Figure 12-4, may be helpful when used on a
single-sided PCB or in addition to a ground plane.
FIGURE 12-4:
12.4
The TMR1 register pair (TMR1H:TMR1L) increments
from 0000h to FFFFh and rolls over to 0000h. The
Timer1 interrupt, if enabled, is generated on overflow,
which is latched in interrupt flag bit, TMR1IF
(PIR1<0>). This interrupt can be enabled or disabled
by setting or clearing the Timer1 Interrupt Enable bit,
TMR1IE (PIE1<0>).
DS39637A-page 154
Note: Not drawn to scale.
Timer1 Interrupt
TIMER1 OSCILLATOR LAYOUT
CONSIDERATIONS
OSCILLATOR CIRCUIT
WITH GROUNDED
GUARD RING
SS
V
OSC1
OSC2
RC0
RC1
V
RC2
or V
DD
SS
DD
.
Preliminary
12.5
If either of the CCP modules is configured in Compare
mode
(CCP1M3:CCP1M0 or CCP2M3:CCP2M0 = 1011),
this signal will reset Timer1. The trigger from ECCP1
will also start an A/D conversion if the A/D module is
enabled (see Section 15.3.4 “Special Event Trigger”
for more information.).
The module must be configured as either a timer or a
synchronous counter to take advantage of this feature.
When used this way, the CCPRH:CCPRL register pair
effectively becomes a period register for Timer1.
If Timer1 is running in Asynchronous Counter mode,
this Reset operation may not work.
In the event that a write to Timer1 coincides with a
special event trigger, the write operation will take
precedence.
12.6
Adding an external LP oscillator to Timer1 (such as the
one described in Section 12.3 “Timer1 Oscillator”
above) gives users the option to include RTC function-
ality to their applications. This is accomplished with an
inexpensive watch crystal to provide an accurate time
base and several lines of application code to calculate
the time. When operating in Sleep mode and using a
battery or supercapacitor as a power source, it can
completely eliminate the need for a separate RTC
device and battery backup.
The application code routine, RTCisr, shown in
Example 12-1, demonstrates a simple method to incre-
ment a counter at one-second intervals using an Inter-
rupt Service Routine. Incrementing the TMR1 register
pair to overflow triggers the interrupt and calls the rou-
tine, which increments the seconds counter by one;
additional counters for minutes and hours are
incremented as the previous counter overflow.
Since the register pair is 16 bits wide, counting up to
overflow the register directly from a 32.768 kHz clock
would take 2 seconds. To force the overflow at the
required one-second intervals, it is necessary to pre-
load it. The simplest method is to set the MSb of
TMR1H with a BSF instruction. Note that the TMR1L
register is never preloaded or altered; doing so may
introduce cumulative error over many cycles.
For this method to be accurate, Timer1 must operate in
Asynchronous mode and the Timer1 overflow interrupt
must be enabled (PIE1<0> = 1) as shown in the
routine, RTCinit. The Timer1 oscillator must also be
enabled and running at all times.
Note:
to
Resetting Timer1 Using the CCP
Special Event Trigger
Using Timer1 as a Real-Time
Clock
The special event triggers from the
ECCP1 module will not set the TMR1IF
interrupt flag bit (PIR1<0>).
generate
 2004 Microchip Technology Inc.
a
special
event
trigger