PIC18F2331/2431/4331/4431
A write to the high byte of Timer1 must also take place
13.4 Timer1 Interrupt
through the TMR1H Buffer register. Timer1 high byte is
updated with the contents of TMR1H when a write
occurs to TMR1L. This allows a user to write all 16 bits
to both the high and low bytes of Timer1 at once.
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 Timer1 Interrupt Flag bit, TMR1IF
(PIR1<0>). This interrupt can be enabled/disabled by
setting/clearing Timer1 Interrupt Enable bit, TMR1IE
(PIE1<0>).
The high byte of Timer1 is not directly readable or writ-
able in this mode. All reads and writes must take place
through the Timer1 High Byte Buffer register. Writes to
TMR1H do not clear the Timer1 prescaler. The
prescaler is only cleared on writes to TMR1L.
13.5 Resetting Timer1 Using a CCP
Trigger Output
13.7 Using Timer1 as a Real-Time
Clock (RTC)
If the CCP1 module is configured in Compare mode
to
generate
a
“Special
Event
Trigger”
Adding an external LP oscillator to Timer1 (such as the
one described in Section 13.2 “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.
(CCP1M<3:0> = 1011), this signal will reset Timer1 and
start an A/D conversion if the A/D module is enabled
(see Section 16.4.4 “Special Event Trigger” for more
information).
Note:
The Special Event Triggers from the
CCP1 module will not set interrupt flag bit,
TMR1IF (PIR1<0>).
Timer1 must be configured for either Timer or Synchro-
nized Counter mode to take advantage of this feature.
If Timer1 is running in Asynchronous Counter mode,
this Reset operation may not work.
The application code routine, RTCisr, shown in
Example 13-1, 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 overflow.
In the event that a write to Timer1 coincides with a
Special Event Trigger from CCP1, the write will take
precedence.
In this mode of operation, the CCPR1H:CCPR1L regis-
ter pair effectively becomes the Period register for
Timer1.
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.
13.6 Timer1 16-Bit Read/Write Mode
Timer1 can be configured for 16-bit reads and writes
(see Figure 13-2). When the RD16 control bit
(T1CON<7>) is set, the address for TMR1H is mapped
to a buffer register for the high byte of Timer1. A read
from TMR1L will load the contents of the high byte of
Timer1 into the Timer1 High Byte Buffer register. This
provides the user with the ability to accurately read all
16 bits of Timer1 without having to determine whether
a read of the high byte, followed by a read of the low
byte, is valid due to a rollover between reads.
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.
DS39616D-page 134
2010 Microchip Technology Inc.
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