PIC16F87X
5.0
TIMER0 MODULE
The Timer0 module timer/counter has the following fea-
tures:
•
•
•
•
•
•
8-bit timer/counter
Readable and writable
8-bit software programmable prescaler
Internal or external clock select
Interrupt on overflow from FFh to 00h
Edge select for external clock
Counter mode is selected by setting bit T0CS
(OPTION_REG<5>). In Counter mode, Timer0 will
increment either on every rising, or falling edge of pin
RA4/T0CKI. The incrementing edge is determined by
the Timer0 Source Edge Select bit, T0SE
(OPTION_REG<4>). Clearing bit T0SE selects the ris-
ing edge. Restrictions on the external clock input are
discussed in detail in Section 5.2.
The prescaler is mutually exclusively shared between
the Timer0 module and the Watchdog Timer. The pres-
caler is not readable or writable. Section 5.3 details the
operation of the prescaler.
the prescaler shared with the WDT.
Additional information on the Timer0 module is avail-
able in the PICmicro™ Mid-Range MCU Family Refer-
ence Manual (DS33023).
Timer mode is selected by clearing bit T0CS
(OPTION_REG<5>). In Timer mode, the Timer0 mod-
ule will increment every instruction cycle (without pres-
caler). If the TMR0 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 TMR0 register.
5.1
Timer0 Interrupt
The TMR0 interrupt is generated when the TMR0 reg-
ister overflows from FFh to 00h. This overflow sets bit
T0IF (INTCON<2>). The interrupt can be masked by
clearing bit T0IE (INTCON<5>). Bit T0IF must be
cleared in software by the Timer0 module Interrupt Ser-
vice Routine before re-enabling this interrupt. The
TMR0 interrupt cannot awaken the processor from
SLEEP, since the timer is shut-off during SLEEP.
FIGURE 5-1:
BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER
Data Bus
8
1
0
M
U
X
SYNC
2
Cycles
TMR0 Reg
CLKOUT (= F
OSC
/4)
0
RA4/T0CKI
pin
1
T0SE
M
U
X
T0CS
PSA
PRESCALER
Set Flag Bit T0IF
on Overflow
0
M
U
X
8-bit Prescaler
8
8 - to - 1MUX
PS2:PS0
Watchdog
Timer
1
PSA
0
MUX
1
PSA
WDT Enable bit
WDT
Time-out
Note:
T0CS, T0SE, PSA, PS2:PS0 are (OPTION_REG<5:0>).
2001 Microchip Technology Inc.
DS30292C-page 47
MICROCHIP [ MICROCHIP TECHNOLOGY ]
