PIC18F2480/2580/4480/4580
15.2 Timer3 16-Bit Read/Write Mode
15.4 Timer3 Interrupt
Timer3 can be configured for 16-bit reads and writes
(see Figure 15-2). When the RD16 control bit
(T3CON<7>) 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.
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 the interrupt flag bit, TMR3IF
(PIR2<1>). This interrupt can be enabled or disabled
by setting or clearing the Timer3 Interrupt Enable bit,
TMR3IE (PIE2<1>).
15.5 Resetting Timer3 Using the CCP
Special Event Trigger
If the ECCP1 module is configured to generate a
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.
special
event
trigger
in
Compare
mode
(ECCP1M<3:0> = 1011), this signal will reset Timer3.
It will also start an A/D conversion if the A/D module is
enabled (see Section 16.3.4 “Special Event Trigger”
for more information.).
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.
The module must be configured as either a timer or
synchronous counter to take advantage of this feature.
When used this way, the ECCPR2H:ECCPR2L register
pair effectively becomes a period register for Timer3.
Writes to TMR3H do not clear the Timer3 prescaler.
The prescaler is only cleared on writes to TMR3L.
If Timer3 is running in Asynchronous Counter mode,
the Reset operation may not work.
15.3 Using the Timer1 Oscillator as the
Timer3 Clock Source
In the event that a write to Timer3 coincides with a
Special Event Trigger from a CCP module, the write will
take precedence.
The Timer1 internal oscillator may be used as the clock
source for Timer3. The Timer1 oscillator is enabled by
setting the T1OSCEN (T1CON<3>) bit. 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.
Note:
The special event triggers from the
ECCP1 module will not set the TMR3IF
interrupt flag bit (PIR1<0>).
The Timer1 oscillator is described in Section 13.0
“Timer1 Module”.
TABLE 15-1: REGISTERS ASSOCIATED WITH TIMER3 AS A TIMER/COUNTER
Reset
Values
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
on Page:
INTCON GIE/GIEH PEIE/GIEL TMR0IE
INT0IE
EEIF
RBIE
BCLIF
BCLIE
BCLIP
TMR0IF
HLVDIF
HLVDIE
HLVDIP
INT0IF
RBIF
55
58
58
57
57
57
56
57
PIR2
OSCFIF
OSCFIE
OSCFIP
CMIF(2)
CMIE(2)
CMIP(2)
—
—
—
TMR3IF ECCP1IF(2)
TMR3IE ECCP1IE(2)
TMR3IP ECCP1IP(2)
PIE2
EEIE
EEIP
IPR2
TMR3L
Timer3 Register Low Byte
TMR3H Timer3 Register High Byte
T1CON
T3CON
RD16
RD16
T1RUN
T1CKPS1 T1CKPS0 T1OSCEN T1SYNC TMR1CS TMR1ON
T3ECCP1(1) T3CKPS1 T3CKPS0 T3CCP1(1) T3SYNC TMR3CS TMR3ON
Legend: — = unimplemented, read as ‘0’. Shaded cells are not used by the Timer3 module.
Note 1: These bits are available in PIC18F4X80 devices only.
2: These bits are available in PIC18F4X80 devices and reserved in PIC18F2X80 devices.
© 2009 Microchip Technology Inc.
DS39637D-page 165
MICROCHIP [ MICROCHIP ]
