PIC16F631/677/685/687/689/690
5.1
Timer0 Operation
5.0
TIMER0 MODULE
When used as a timer, the Timer0 module can be used
as either an 8-bit timer or an 8-bit counter.
The Timer0 module is an 8-bit timer/counter with the
following features:
• 8-bit timer/counter register (TMR0)
5.1.1
8-BIT TIMER MODE
• 8-bit prescaler (shared with Watchdog Timer)
• Programmable internal or external clock source
• Programmable external clock edge selection
• Interrupt on overflow
When used as a timer, the Timer0 module will
increment every instruction cycle (without prescaler).
Timer mode is selected by clearing the T0CS bit of the
OPTION register to ‘0’.
Figure 5-1 is a block diagram of the Timer0 module.
When TMR0 is written, the increment is inhibited for
two instruction cycles immediately following the write.
Note:
The value written to the TMR0 register can
be adjusted, in order to account for the two
instruction cycle delay when TMR0 is
written.
5.1.2
8-BIT COUNTER MODE
When used as a counter, the Timer0 module will
increment on every rising or falling edge of the T0CKI
pin. The incrementing edge is determined by the T0SE
bit of the OPTION register. Counter mode is selected by
setting the T0CS bit of the OPTION register to ‘1’.
FIGURE 5-1:
BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER
FOSC/4
Data Bus
0
1
8
1
Sync 2
TMR0
cycles
T0CKI
pin
0
0
1
Set Flag bit T0IF
on Overflow
T0CS
T0SE
8-bit
Prescaler
PSA
8
PSA
WDTE
SWDTEN
1
PS<2:0>
WDT
Time-out
16-bit
Prescaler
0
16
31 kHz
INTOSC
Watchdog
Timer
PSA
WDTPS<3:0>
Note 1: T0SE, T0CS, PSA, PS<2:0> are bits in the OPTION register.
2: SWDTEN and WDTPS<3:0> are bits in the WDTCON register.
3: WDTE bit is in the Configuration Word register.
© 2007 Microchip Technology Inc.
DS41262D-page 81
MICROCHIP [ MICROCHIP ]
