
PIC18CXX8
DS30475A-page 124
Advanced Information
2000 Microchip Technology Inc.
13.1
Timer3 Operation
Timer3 can operate in one of these modes:
As a timer
As a synchronous counter
As an asynchronous counter
The operating mode is determined by the clock select
bit, TMR3CS (T3CON register).
When TMR3CS = 0, Timer3 increments every instruc-
tion cycle. When TMR3CS = 1, Timer3 increments on
every rising edge of the Timer1 external clock input or
the Timer1 oscillator, if enabled.
When the Timer1 oscillator is enabled (T1OSCEN is
set), the RC1/T1OSI and RC0/T1OSO/T1CKI pins
become inputs. That is, the TRISC<1:0> value is
ignored.
Timer3 also has an internal “RESET input”. This
RESET can be generated by the CCP module
FIGURE 13-1: TIMER3 BLOCK DIAGRAM
FIGURE 13-2: TIMER3 BLOCK DIAGRAM CONFIGURED IN 16-BIT READ/WRITE MODE
TMR3H
TMR3L
T1OSC
T3SYNC
TMR3CS
T3CKPS1:T3CKPS0
SLEEP Input
T1OSCEN
Enable
Oscillator(1)
TMR3IF
Overflow
Interrupt
Fosc/4
Internal
Clock
TMR3ON
on/off
Prescaler
1, 2, 4, 8
Synchronize
det
1
0
1
Synchronized
Clock Input
2
T1OSO/
T1OSI
Flag bit
Note 1:
When enable bit T1OSCEN is cleared, the inverter and feedback resistor are turned off. This eliminates power drain.
T13CKI
CCP Special Trigger
T3CCPx
CLR
TMR3L
T1OSC
T3SYNC
TMR3CS
T3CKPS1:T3CKPS0
SLEEP Input
T1OSCEN
Enable
Oscillator(1)
FOSC/4
Internal
Clock
TMR3ON
On/Off
Prescaler
1, 2, 4, 8
Synchronize
det
1
0
1
Synchronized
Clock Input
2
T1OSO/
T1OSI
TMR3
T13CKI
CLR
CCP Special Trigger
T3CCPx
To Timer1 Clock Input
Note 1:
When the T1OSCEN bit is cleared, the inverter and feedback resistor are turned off. This eliminates power drain.
TMR3H
Data Bus<7:0>
8
TMR3H
8
Read TMR3L
Write TMR3L
TMR3IF Overflow
Interrupt Flag
bit