Microchip Technology MA160014 Data Sheet
PIC18(L)F2X/4XK22
DS41412F-page 160
2010-2012 Microchip Technology Inc.
11.2
Timer0 Operation
Timer0 can operate as either a timer or a counter; the
mode is selected with the T0CS bit of the T0CON
register. In Timer mode (T0CS = 0), the module
increments on every clock by default unless a different
prescaler value is selected (see
mode is selected with the T0CS bit of the T0CON
register. In Timer mode (T0CS = 0), the module
increments on every clock by default unless a different
prescaler value is selected (see
). Timer0 incrementing is inhibited for two
instruction cycles following a TMR0 register write. The
user can work around this by adjusting the value written
to the TMR0 register to compensate for the anticipated
missing increments.
user can work around this by adjusting the value written
to the TMR0 register to compensate for the anticipated
missing increments.
The Counter mode is selected by setting the T0CS bit
(= 1). In this mode, Timer0 increments 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 of the T0CON register; clearing this bit
selects the rising edge. Restrictions on the external
clock input are discussed below.
(= 1). In this mode, Timer0 increments 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 of the T0CON register; clearing this bit
selects the rising edge. Restrictions on the external
clock input are discussed below.
An external clock source can be used to drive Timer0;
however, it must meet certain requirements (see
however, it must meet certain requirements (see
) to ensure that the external clock can be
synchronized with the internal phase clock (T
OSC
).
There is a delay between synchronization and the
onset of incrementing the timer/counter.
onset of incrementing the timer/counter.
11.3
Timer0 Reads and Writes in
16-Bit Mode
16-Bit Mode
TMR0H is not the actual high byte of Timer0 in 16-bit
mode; it is actually a buffered version of the real high
byte of Timer0 which is neither directly readable nor
writable (refer to
mode; it is actually a buffered version of the real high
byte of Timer0 which is neither directly readable nor
writable (refer to
). 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 the need to verify that the read of the
high and low byte were valid. Invalid reads could
otherwise occur due to a rollover between successive
reads of the high and low byte.
TMR0L. This provides the ability to read all 16 bits of
Timer0 without the need to verify that the read of the
high and low byte were valid. Invalid reads could
otherwise occur due to a rollover between successive
reads of the high and low byte.
Similarly, a write to the high byte of Timer0 must also
take place through the TMR0H Buffer register. Writing
to TMR0H does not directly affect Timer0. Instead, the
high byte of Timer0 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.
take place through the TMR0H Buffer register. Writing
to TMR0H does not directly affect Timer0. Instead, the
high byte of Timer0 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.
FIGURE 11-1:
TIMER0 BLOCK DIAGRAM (8-BIT MODE)
Note:
Upon Reset, Timer0 is enabled in 8-bit mode with clock input from T0CKI max. prescale.
T0CKI pin
T0SE
0
1
1
0
T0CS
F
OSC
/4
Programmable
Prescaler
Sync with
Internal
Clocks
TMR0L
(2 T
CY
Delay)
Internal Data Bus
PSA
T0PS<2:0>
Set
TMR0IF
on Overflow
TMR0IF
on Overflow
3
8
8