Datasheet

© 2006 Microchip Technology Inc. DS41159E-page 111
PIC18FXX8
11.1 Timer0 Operation
Timer0 can operate as a timer or as a counter.
Timer mode is selected by clearing the T0CS bit. In
Timer mode, the Timer0 module will increment every
instruction cycle (without prescaler). If the TMR0L
register is written, the increment is inhibited for the
following two instruction cycles. The user can work
around this by writing an adjusted value to the TMR0L
register.
Counter mode is selected by setting the T0CS bit. In
Counter mode, Timer0 will increment either on every
rising or falling edge of pin RA4/T0CKI. The increment-
ing edge is determined by the Timer0 Source Edge
Select bit (T0SE). Clearing the T0SE bit selects the
rising edge. Restrictions on the external clock input are
discussed below.
When an external clock input is used for Timer0, it must
meet certain requirements. The requirements ensure
the external clock can be synchronized with the internal
phase clock (T
OSC). Also, there is a delay in the actual
incrementing of Timer0 after synchronization.
11.2 Prescaler
An 8-bit counter is available as a prescaler for the
Timer0 module. The prescaler is not readable or
writable.
The PSA and T0PS2:T0PS0 bits determine the
prescaler assignment and prescale ratio.
Clearing bit PSA will assign the prescaler to the Timer0
module. When the prescaler is assigned to the Timer0
module, prescale values of 1:2, 1:4, ..., 1:256 are
selectable.
When assigned to the Timer0 module, all instructions
writing to the TMR0 register (e.g., CLRF TMR0, MOVWF
TMR0, BSF TMR0, x.... etc.) will clear the prescaler
count.
11.2.1 SWITCHING PRESCALER
ASSIGNMENT
The prescaler assignment is fully under software
control (i.e., it can be changed “on-the-fly” during
program execution).
11.3 Timer0 Interrupt
The TMR0 interrupt is generated when the TMR0
register overflows from FFh to 00h in 8-bit mode or
FFFFh to 0000h in 16-bit mode. This overflow sets the
TMR0IF bit. The interrupt can be masked by clearing
the TMR0IE bit. The TMR0IF bit must be cleared in
software by the Timer0 module Interrupt Service
Routine before re-enabling this interrupt. The TMR0
interrupt cannot awaken the processor from Sleep
since the timer is shut-off during Sleep.
11.4 16-Bit Mode Timer Reads
and Writes
Timer0 can be set in 16-bit mode by clearing the
T08BIT in T0CON. Registers TMR0H and TMR0L are
used to access the 16-bit timer value.
TMR0H is not the high byte of the timer/counter in
16-bit mode, but is actually a buffered version of the
high byte of Timer0 (refer to Figure 11-1). The high byte
of the Timer0 timer/counter is not directly readable nor
writable. TMR0H is updated with the contents of the
high byte of Timer0 during a read of TMR0L. This
provides the ability to read all 16 bits of Timer0 without
having to verify that the read of the high and low byte
were valid, due to a rollover between successive reads
of the high and low byte.
A write to the high byte of Timer0 must also take place
through the TMR0H Buffer register. Timer0 high byte is
updated with the contents of the buffered value of
TMR0H when a write occurs to TMR0L. This allows all
16 bits of Timer0 to be updated at once.
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
Value on
POR, BOR
Value on
all other
Resets
TMR0L Timer0 Module Low Byte Register xxxx xxxx uuuu uuuu
TMR0H Timer0 Module High Byte Register 0000 0000 0000 0000
INTCON GIE/GIEH PEIE/GIEL TMR0IE
INT0IE RBIE TMR0IF INT0IF RBIF 0000 000x 0000 000u
T0CON TMR0ON T08BIT T0CS T0SE PSA T0PS2 T0PS1 T0PS0 1111 1111 1111 1111
TRISA
— PORTA Data Direction Register
(1)
-111 1111 -111 1111
Legend: x = unknown, u = unchanged, - = unimplemented locations read as ‘0’. Shaded cells are not used by Timer0.
Note 1: Bit 6 of PORTA, LATA and TRISA is enabled in ECIO and RCIO Oscillator modes only. In all other oscillator modes, it is
disabled and reads as ‘0’.