Datasheet

 1997-2013 Microchip Technology Inc. DS30561C-page 39
PIC12C67X
7.0 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
Figure 7-1 is a simplified block diagram of the Timer0
module.
Timer mode is selected by clearing bit T0CS
(OPTION<5>). In timer mode, the Timer0 module will
increment every instruction cycle (without prescaler). If
the TMR0 register is written, the increment is inhibited
for the following two instruction cycles (Figure 7-2 and
Figure 7-3). The user can work around this by writing
an adjusted value to the TMR0 register.
Counter mode is selected by setting bit T0CS
(OPTION<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 bit T0SE
(OPTION<4>). Clearing bit T0SE selects the rising
edge. Restrictions on the external clock input are dis-
cussed in detail in Section 7.2.
The prescaler is mutually exclusively shared between
the Timer0 module and the Watchdog Timer. The pres-
caler assignment is controlled in software by control bit
PSA (OPTION<3>). Clearing bit PSA will assign the
prescaler to the Timer0 module. The prescaler is not
readable or writable. When the prescaler is assigned to
the Timer0 module, prescale values of 1:2, 1:4, ...,
1:256 are selectable. Section 7.3 details the operation
of the prescaler.
7.1 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. See
Figure 7-4 for Timer0 interrupt timing.
FIGURE 7-1: TIMER0 BLOCK DIAGRAM
FIGURE 7-2: TIMER0 TIMING: INTERNAL CLOCK/NO PRESCALE
Note 1: TOCS, TOSE, PSA, PS<2:0> (OPTION<5:0>).
2: The prescaler is shared with Watchdog Timer (refer to Figure 7-6 for detailed block diagram).
GP2/TOCKI/
TOSE
0
1
1
0
AN2/INT
TOCS
FOSC/4
Programmable
Prescaler
Sync with
Internal
clocks
TMR0
(2 TCY delay)
Data Bus
8
PSA
PS<2:0>
Set interrupt
flag bit T0IF
on overflow
3
PC-1
Q1 Q2 Q3 Q4
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
PC
(Program
Counter)
Instruction
Fetch
TMR0
PC PC+1 PC+2 PC+3 PC+4 PC+5 PC+6
T0
T0+1 T0+2 NT0 NT0 NT0 NT0+1 NT0+2 T0
MOVWF TMR0
MOVF TMR0,WMOVF TMR0,WMOVF TMR0,WMOVF TMR0,WMOVF TMR0,W
Write TMR0
executed
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0 + 1
Read TMR0
reads NT0 + 2
Instruction
Executed