PIC18F2420/2520/4420/4520
internal phase clock (TOSC). There is a delay between
synchronization and the onset of incrementing the
timer/counter.
11.1 Timer0 Operation
Timer0 can operate as either a timer or a counter; the
mode is selected with the T0CS bit (T0CON<5>). In
Timer mode (T0CS = 0), the module increments on
every clock by default unless a different prescaler value
is selected (see Section 11.3 “Prescaler”). If the
TMR0 register is written to, 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.
11.2 Timer0 Reads and Writes in
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 not directly readable nor writ-
able (refer to Figure 11-2). 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.
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 (T0CON<4>); clearing this bit selects
the rising edge. Restrictions on the external clock input
are discussed below.
Similarly, a write to the high byte of Timer0 must also
take place through the TMR0H Buffer register. The 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.
An external clock source can be used to drive Timer0;
however, it must meet certain requirements to ensure
that the external clock can be synchronized with the
FIGURE 11-1:
TIMER0 BLOCK DIAGRAM (8-BIT MODE)
FOSC/4
0
1
0
1
Set
TMR0IF
on Overflow
Sync with
Internal
Clocks
TMR0L
8
Programmable
Prescaler
T0CKI pin
(2 TCY Delay)
T0SE
T0CS
3
T0PS<2:0>
PSA
8
Internal Data Bus
Note: Upon Reset, Timer0 is enabled in 8-bit mode with clock input from T0CKI max. prescale.
FIGURE 11-2:
TIMER0 BLOCK DIAGRAM (16-BIT MODE)
FOSC/4
0
0
Sync with
Internal
Clocks
Set
TMR0
High Byte
1
TMR0L
TMR0IF
Programmable
Prescaler
on Overflow
T0CKI pin
1
8
(2 TCY Delay)
T0SE
T0CS
3
Read TMR0L
Write TMR0L
T0PS<2:0>
PSA
8
8
TMR0H
8
8
Internal Data Bus
Note: Upon Reset, Timer0 is enabled in 8-bit mode with clock input from T0CKI max. prescale.
DS39631E-page 124
© 2008 Microchip Technology Inc.
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