
2010 Microchip Technology Inc.
Preliminary
DS41350E-page 115
PIC18F/LF1XK50
FIGURE 13-2:
TIMER3 BLOCK DIAGRAM (16-BIT READ/WRITE MODE)
13.2
Timer3 16-Bit Read/Write Mode
Timer3 can be configured for 16-bit reads and writes
T3CON register is set, the address for TMR3H is
mapped to a buffer register for the high byte of Timer3.
A read from TMR3L will load the contents of the high
byte of Timer3 into the Timer3 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, has become invalid due to a rollover
between reads.
A write to the high byte of Timer3 must also take place
through the TMR3H Buffer register. The Timer3 high
byte is updated with the contents of TMR3H when a
write occurs to TMR3L. This allows a user to write all
16 bits to both the high and low bytes of Timer3 at once.
The high byte of Timer3 is not directly readable or
writable in this mode. All reads and writes must take
place through the Timer3 High Byte Buffer register.
Writes to TMR3H do not clear the Timer3 prescaler.
The prescaler is only cleared on writes to TMR3L.
13.3
Using the Timer1 Oscillator as the
Timer3 Clock Source
The Timer1 internal oscillator may be used as the clock
source for Timer3. The Timer1 oscillator is enabled by
setting the T1OSCEN bit of the T1CON register. To use
it as the Timer3 clock source, the TMR3CS bit must
also be set. As previously noted, this also configures
Timer3 to increment on every rising edge of the
oscillator source.
13.4
Timer3 Interrupt
The TMR3 register pair (TMR3H:TMR3L) increments
from 0000h to FFFFh and overflows to 0000h. The
Timer3 interrupt, if enabled, is generated on overflow
and is latched in interrupt flag bit, TMR3IF of the PIR2
register. This interrupt can be enabled or disabled by
setting or clearing the Timer3 Interrupt Enable bit,
TMR3IE of the PIE2 register.
T3SYNC
TMR3CS
T3CKPS<1:0>
Sleep Input
T1OSCEN(1)
FOSC/4
Internal
Clock
Prescaler
1, 2, 4, 8
Synchronize
Detect
1
0
2
T13CKI/T1OSI
T1OSO
Note 1: When enable bit, T1OSCEN, is cleared, the inverter and feedback resistor are turned off to eliminate power drain.
1
0
TMR3L
Internal Data Bus
8
Set
TMR3IF
on Overflow
TMR3
TMR3H
High Byte
8
Read TMR1L
Write TMR1L
8
TMR3ON
CCP1 Special Event Trigger
Timer1 Oscillator
On/Off
Timer3
Timer1 Clock Input
CCP1 Select from T3CON<3>
Clear TMR3