PIC16C620 Microchip Technology, PIC16C620 Datasheet - Page 31

• Manufacturer Part Number: PIC16C620
• Description: EPROM-Based 8-Bit CMOS Microcontroller
• Manufacturer: Microchip Technology

6.0
The Timer0 module timer/counter has the following
features:
• 8-bit timer/counter
• Readable and writable
• 8-bit software programmable prescaler
• Internal or external clock select
FIGURE 6-1:
FIGURE 6-2:
• Interrupt on overflow from FFh to 00h
• Edge select for external clock
Figure 6-1 is a simplified block diagram of the Timer0
module.
Timer mode is selected by clearing the T0CS bit
(OPTION<5>). In timer mode, the TMR0 will increment
every instruction cycle (without prescaler). If Timer0 is
written, the increment is inhibited for the following two
cycles (Figure 6-2 and Figure 6-3). The user can work
around this by writing an adjusted value to TMR0.
Counter mode is selected by setting the T0CS bit. In
this mode Timer0 will increment either on every rising
or falling edge of pin RA4/T0CKI. The incrementing
edge is determined by the source edge (T0SE) control
1998 Microchip Technology Inc.
RA4/T0CKI
Instruction
PC
(Program
Counter)
Fetch
TMR0
Instruction
Executed
Note 1:
pin
TIMER0 MODULE
2:
T0SE
The prescaler is shared with Watchdog Timer (Figure 6-6)
Bits T0SE, T0CS, PS2, PS1, PS0 and PSA are located in the OPTION register.
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
T0
F
TIMER0 BLOCK DIAGRAM
TIMER0 (TMR0) TIMING: INTERNAL CLOCK/NO PRESCALER
OSC
PC-1
/4
MOVWF TMR0
T0+1
T0CS
PC
0
1
MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W
T0+2
Write TMR0
executed
Programmable
PS2:PS0
PC+1
Prescaler
Preliminary
Read TMR0
reads NT0
PC+2
PSA
1
0
bit (OPTION<4>). Clearing the T0SE bit selects the
rising edge. Restrictions on the external clock input are
discussed in detail in Section 6.2.
The prescaler is shared between the Timer0 module
and the Watchdog Timer. The prescaler assignment is
controlled in software by the control bit PSA
(OPTION<3>). Clearing the PSA bit will assign the
prescaler to Timer0. The prescaler is not readable or
writable. When the prescaler is assigned to the Timer0
module, prescale value of 1:2, 1:4, ..., 1:256 are
selectable. Section 6.3 details the operation of the
prescaler.
6.1
Timer0 interrupt is generated when the TMR0 register
timer/counter overflows from FFh to 00h. This overflow
sets the T0IF bit. The interrupt can be masked by
clearing the T0IE bit (INTCON<5>). The T0IF bit
(INTCON<2>) must be cleared in software by the
Timer0 module interrupt service routine before
re-enabling this interrupt. The Timer0 interrupt cannot
wake the processor from SLEEP since the timer is shut
off during SLEEP. See Figure 6-4 for Timer0 interrupt
timing.
PSout
NT0
Read TMR0
reads NT0
PC+3
(2 T
Sync with
TIMER0 Interrupt
Internal
clocks
CY
MOVF TMR0,W
delay)
Read TMR0
reads NT0
PC+4
PSout
PIC16C62X
MOVF TMR0,W
NT0+1
Read TMR0
reads NT0 + 1
Data bus
TMR0
PC+5
8
DS30235G-page 31
Set Flag bit T0IF
NT0+2
Read TMR0
reads NT0 + 2
on Overflow
PC+6
T0