Datasheet

PIC18F2220/2320/4220/4320
DS39599G-page 130 © 2007 Microchip Technology Inc.
14.1 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<1>).
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/CCP2 and RC0/T1OSO/T1CKI
pins become inputs. That is, the TRISC1:TRISC0 value
is ignored and the pins are read as ‘0’.
Timer3 also has an internal “Reset input”. This Reset
can be generated by the CCP module (see
Section 15.4.4 “Special Event Trigger”).
FIGURE 14-1: TIMER3 BLOCK DIAGRAM
FIGURE 14-2: TIMER3 BLOCK DIAGRAM CONFIGURED IN 16-BIT READ/WRITE MODE
TMR3H
TMR3L
T1OSC
T3SYNC
TMR3CS
T3CKPS1:T3CKPS0
Peripheral Clocks
T1OSCEN
Enable
Oscillator
(1)
TMR3IF
Overflow
Interrupt
F
OSC/4
Internal
Clock
TMR3ON
On/Off
Prescaler
1, 2, 4, 8
Synchronize
det
1
0
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.
T1CKI
CLR
CCP Special Event Trigger
T3CCPx
Timer3
TMR3L
T1OSC
T3SYNC
TMR3CS
T3CKPS1:T3CKPS0
Peripheral Clocks
T1OSCEN
Enable
Oscillator
(1)
FOSC/4
Internal
Clock
TMR3ON
On/Off
Prescaler
1, 2, 4, 8
Synchronize
det
1
0
0
1
Synchronized
Clock Input
2
T1OSO/
T1OSI
TMR3
T1CKI
CLR
CCP Special Event 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.
High Byte
Data Bus<7:0>
8
TMR3H
8
8
8
Read TMR3L
Write TMR3L
Set TMR3IF Flag bit
on Overflow