PIC18F2420/2520/4420/4520
cycle (FOSC/4). When the bit is set, Timer1 increments
on every rising edge of the Timer1 external clock input
or the Timer1 oscillator, if enabled.
12.1 Timer1 Operation
Timer1 can operate in one of these modes:
• Timer
When Timer1 is enabled, the RC1/T1OSI and RC0/
T1OSO/T13CKI pins become inputs. This means the
values of TRISC<1:0> are ignored and the pins are
read as ‘0’.
• Synchronous Counter
• Asynchronous Counter
The operating mode is determined by the clock select
bit, TMR1CS (T1CON<1>). When TMR1CS is cleared
(= 0), Timer1 increments on every internal instruction
FIGURE 12-1:
TIMER1 BLOCK DIAGRAM
Timer1 Oscillator
Timer1 Clock Input
1
0
On/Off
T1OSO/T13CKI
T1OSI
1
Synchronize
Detect
Prescaler
1, 2, 4, 8
FOSC/4
Internal
Clock
0
2
Sleep Input
T1OSCEN(1)
TMR1CS
Timer1
On/Off
T1CKPS<1:0>
T1SYNC
TMR1ON
Set
TMR1
High Byte
Clear TMR1
(CCP Special Event Trigger)
TMR1L
TMR1IF
on Overflow
Note 1: When enable bit, T1OSCEN, is cleared, the inverter and feedback resistor are turned off to eliminate power drain.
FIGURE 12-2:
TIMER1 BLOCK DIAGRAM (16-BIT READ/WRITE MODE)
Timer1 Oscillator
Timer1 Clock Input
1
0
T1OSO/T13CKI
T1OSI
1
0
Synchronize
Detect
Prescaler
1, 2, 4, 8
FOSC/4
Internal
Clock
2
Sleep Input
T1OSCEN(1)
T1CKPS<1:0>
T1SYNC
Timer1
On/Off
TMR1CS
TMR1ON
Set
TMR1
High Byte
Clear TMR1
(CCP Special Event Trigger)
TMR1L
TMR1IF
on Overflow
8
Read TMR1L
Write TMR1L
8
8
TMR1H
8
8
Internal Data Bus
Note 1: When enable bit, T1OSCEN, is cleared, the inverter and feedback resistor are turned off to eliminate power drain.
DS39631E-page 128
© 2008 Microchip Technology Inc.
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