PIC18F87J11_12 Datasheet, PDF (200/466 Pages) Microchip Technology – 64/80-Pin, High-Performance, 1-Mbit Flash Microcontrollers

PIC18F87J11 FAMILY
If a high-speed circuit must be located near the oscilla-
tor (such as the ECCP1 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 14-4, may be helpful when used on a
single-sided PCB or in addition to a ground plane.
FIGURE 14-4:
OSCILLATOR CIRCUIT
WITH GROUNDED
GUARD RING
VDD
VSS
OSC1
OSC2
RC0
RC1
Note: Not drawn to scale.
RC2
14.4 Timer1 Interrupt
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>).
14.5 Resetting Timer1 Using the
ECCPx Special Event Trigger
If ECCP1 or ECCP2 is configured to use Timer1 and to
generate a Special Event Trigger in Compare mode
(CCPxM<3:0> = 1011), this signal will reset Timer3.
The trigger from ECCP2 will also start an A/D conver-
sion if the A/D module is enabled (see Section 19.2.1
“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 CCPRxH:CCPRxL 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.
Note:
The Special Event Triggers from the
ECCPx module will not set the TMR1IF
interrupt flag bit (PIR1<0>).
14.6 Using Timer1 as a Real-Time Clock
Adding an external LP oscillator to Timer1 (such as the
one described in Section 14.3 “Timer1 Oscillator”)
gives users the option to include RTC functionality 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 14-1, demonstrates a simple method to
increment a counter at one-second intervals using an
Interrupt Service Routine. Incrementing the TMR1
register pair to overflow triggers the interrupt and calls
the routine. which increments the seconds counter by
one. Additional counters for minutes and hours are
incremented as the previous counter overflows.
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.
DS39778E-page 200
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