PIC16C71X
bit T0SE selects the rising edge. Restrictions on the
external clock input are discussed in detail in
Section 6.2.
6.0
TIMER0 MODULE
Applicable Devices 710 71 711 715
The Timer0 module timer/counter has the following fea-
tures:
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 6.3 details the operation
of the prescaler.
• 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 6-1 is a simplified block diagram of the Timer0
module.
6.1
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 6-4 for Timer0 interrupt timing.
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 6-2 and
Figure 6-3). 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 Timer0
Source Edge Select bit T0SE (OPTION<4>). Clearing
FIGURE 6-1: TIMER0 BLOCK DIAGRAM
Data bus
FOSC/4
0
1
PSout
8
1
0
Sync with
Internal
clocks
TMR0
Programmable
Prescaler
RA4/T0CKI
pin
PSout
(2 cycle delay)
T0SE
3
Set interrupt
flag bit T0IF
on overflow
PS2, PS1, PS0
PSA
T0CS
Note 1: T0CS, T0SE, PSA, PS2:PS0 (OPTION<5:0>).
2: The prescaler is shared with Watchdog Timer (refer to Figure 6-6 for detailed block diagram).
FIGURE 6-2: TIMER0 TIMING: INTERNAL CLOCK/NO PRESCALE
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)
PC-1
PC
PC+1
PC+2
PC+3
PC+4
PC+5
PC+6
Instruction
Fetch
MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W
MOVWF TMR0
T0
T0+1
T0+2
NT0
NT0
NT0
NT0+1
NT0+2
T0
TMR0
Instruction
Executed
Read TMR0
reads NT0 + 1
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0 + 2
Write TMR0
executed
1997 Microchip Technology Inc.
DS30272A-page 31
MICROCHIP [ MICROCHIP ]
