Microchip Technology DM183021 Data Sheet
2010 Microchip Technology Inc.
DS39616D-page 129
PIC18F2331/2431/4331/4431
12.1
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 TMR0 regis-
ter 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 TMR0 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, RC3/T0CKI/T5CKI/INT0.
The incrementing edge is determined by the Timer0
Source Edge Select bit (T0SE). Clearing the T0SE bit
selects the rising edge.
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
Timer mode is selected by clearing the T0CS bit. In
Timer mode, the Timer0 module will increment every
instruction cycle (without prescaler). If the TMR0 regis-
ter 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 TMR0 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, RC3/T0CKI/T5CKI/INT0.
The incrementing edge is determined by the Timer0
Source Edge Select bit (T0SE). Clearing the T0SE bit
selects the rising edge.
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.
12.2
Prescaler
An 8-bit counter is available as a prescaler for the Timer0
module. The prescaler is not readable or writable.
The PSA and T0PS<2:0> 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
module. The prescaler is not readable or writable.
The PSA and T0PS<2:0> 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.
12.2.1
SWITCHING PRESCALER
ASSIGNMENT
ASSIGNMENT
The prescaler assignment is fully under software con-
trol (i.e., it can be changed “on-the-fly” during program
execution).
trol (i.e., it can be changed “on-the-fly” during program
execution).
12.3
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
mode, since the timer requires clock cycles, even when
T0CS is set.
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
mode, since the timer requires clock cycles, even when
T0CS is set.
12.4
16-Bit Mode Timer Reads and
Writes
Writes
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
16-bit mode, but is actually a buffered version of the
high byte of Timer0 (refer to
). The high byte
of the Timer0 counter/timer is not directly readable nor
writable. TMR0H is updated with the contents of the
high byte of Timer0 during a read of TMR0L. This pro-
vides 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 TMR0H when a write
occurs to TMR0L. This allows all 16 bits of Timer0 to be
updated at once.
writable. TMR0H is updated with the contents of the
high byte of Timer0 during a read of TMR0L. This pro-
vides 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 TMR0H when a write
occurs to TMR0L. This allows all 16 bits of Timer0 to be
updated at once.
TABLE 12-1:
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.
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
Timer0 Register Low Byte
TMR0H
Timer0 Register High Byte
INTCON
GIE/GIEH
PEIE/GIEL
TMR0IE
INT0IE
RBIE
TMR0IF
INT0IF
RBIF
T0CON
TMR0ON
T016BIT
T0CS
T0SE
PSA
T0PS2
T0PS1
T0PS0
TRISA
TRISA7
(1)
TRISA6
(1)
PORTA Data Direction Register
Legend:
Shaded cells are not used by Timer0.
Note 1:
RA6 and RA7 are enabled as I/O pins depending on the oscillator mode selected in Configuration Word 1H.