
2004 Microchip Technology Inc.
Preliminary
DS39637A-page 149
PIC18F2480/2580/4480/4580
11.3
Prescaler
An 8-bit counter is available as a prescaler for the Timer0
module. The prescaler is not directly readable or writable;
its value is set by the PSA and T0PS2:T0PS0 bits
(T0CON<3:0>)
which
determine
assignment and prescale ratio.
Clearing the PSA bit assigns the prescaler to the
Timer0 module. When it is assigned, prescale values
from 1:2 through 1:256 in power-of-2 increments are
selectable.
When assigned to the Timer0 module, all instructions
writing to the TMR0 register (e.g.,
CLRF TMR0, MOVWF
TMR0, BSF TMR0
, etc.) clear the prescaler count.
the
prescaler
11.3.1
SWITCHING PRESCALER
ASSIGNMENT
The prescaler assignment is fully under software
control and can be changed “on-the-fly” during program
execution.
11.4
Timer0 Interrupt
The TMR0 interrupt is generated when the TMR0
register overflows from FFh to 00h in 8-bit mode, or
from FFFFh to 0000h in 16-bit mode. This overflow sets
the TMR0IF flag bit. The interrupt can be masked by
clearing the TMR0IE bit (INTCON<5>). Before re-
enabling the interrupt, the TMR0IF bit must be cleared
in software by the Interrupt Service Routine.
Since Timer0 is shut down in Sleep mode, the TMR0
interrupt cannot awaken the processor from Sleep.
TABLE 11-1:
REGISTERS ASSOCIATED WITH TIMER0
Note:
Writing to TMR0 when the prescaler is
assigned to Timer0 will clear the prescaler
count but will not change the prescaler
assignment.
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Reset
Values
on page
TMR0L
TMR0H
INTCON
T0CON
TRISA
Legend:
Timer0 Module Low Byte Register
Timer0 Module High Byte Register
GIE/GIEH PEIE/GIEL
TMR0ON
T08BIT
—
PORTA Data Direction Register
— = unimplemented locations, read as ‘
0
’. Shaded cells are not used by Timer0.
50
50
49
50
52
TMR0IE
T0CS
INT0IE
T0SE
RBIE
PSA
TMR0IF
T0PS2
INT0IF
T0PS1
RBIF
T0PS0