PIC18CXX8
DS30475A-page 116
Advanced Information
2000 Microchip Technology Inc.
10.3
Timer0 Interrupt
The TMR0 interrupt is generated when the TMR0 reg-
ister 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 soft-
ware 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.
10.4
16-Bit Mode Timer Reads and Writes
Timer0 can be set in 16-bit mode by clearing T0CON
T08BIT. Registers TMR0H and TMR0L are used to
access 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 10-1). 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 buffered value of TMR0H,
when a write occurs to TMR0L. This allows all 16 bits
of Timer0 to be updated at once.
TABLE 10-1:
REGISTERS ASSOCIATED WITH TIMER0
Note 1: Bit 6 of PORTA, LATA and TRISA are enabled in ECIO and RCIO oscillator modes only. In all other oscillator
modes, they are disabled and read as ‘0’.
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’s Low Byte Register
xxxx xxxx
uuuu uuuu
TMR0H
Timer0 Module’s High Byte Register
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
TRISA
PORTA Data Direction Register(1)
--11 1111
Legend: x = unknown, u = unchanged, - = unimplemented locations read as '0'. Shaded cells are not used by Timer0.
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