16F84A Microchip Technology, 16F84A Datasheet - Page 17

• Manufacturer Part Number: 16F84A
• Description: 18-pin Enhanced Flash/EEPROM 8-Bit Microcontroller
• Manufacturer: Microchip Technology

4.0
The Timer0 module timer/counter has the following fea-
tures:
• 8-bit timer/counter
• Readable and writable
• Internal or external clock select
• Edge select for external clock
• 8-bit software programmable prescaler
• Interrupt on overflow from FFh to 00h
Figure 4-1 is a simplified block diagram of the Timer0
module.
Additional information on timer modules is available in
the
(DS33023).
4.1
Timer0 can operate as a timer or as a counter.
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.
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
(OPTION_REG<4>). Clearing bit T0SE selects the ris-
ing edge. Restrictions on the external clock input are
discussed below.
When an external clock input is used for Timer0, it must
meet certain requirements. The requirements ensure
the external clock can be synchronized with the internal
phase clock (T
incrementing of Timer0 after synchronization.
FIGURE 4-1:
1998 Microchip Technology Inc.
RA4/T0CKI
pin
Note 1: T0CS, T0SE, PSA, PS2:PS0 (OPTION_REG<5:0>).
PICmicro™
Timer0
TIMER0 MODULE
Timer0 Operation
2: The prescaler is shared with Watchdog Timer (refer to Figure 4-2 for detailed block diagram).
OSC
T0SE
Source
F
TIMER0 BLOCK DIAGRAM
). Also, there is a delay in the actual
OSC
Mid-Range
/4
Edge
T0CS
Reference
0
1
Select
PS2, PS1, PS0
Programmable
bit
Prescaler
Manual,
T0SE
3
Preliminary
PSA
1
0
Additional information on external clock requirements
is available in the PICmicro™ Mid-Range Reference
Manual, (DS33023).
4.2
An 8-bit counter is available as a prescaler for the
Timer0 module, or as a postscaler for the Watchdog
Timer, respectively (Figure 4-2). For simplicity, this
counter is being referred to as “prescaler” throughout
this data sheet. Note that there is only one prescaler
available which is mutually exclusively shared between
the Timer0 module and the Watchdog Timer. Thus, a
prescaler assignment for the Timer0 module means
that there is no prescaler for the Watchdog Timer, and
vice-versa.
The prescaler is not readable or writable.
The PSA and PS2:PS0 bits (OPTION_REG<3:0>)
determine the prescaler assignment and prescale ratio.
Clearing bit PSA will assign the prescaler to the Timer0
module. When the prescaler is assigned to the Timer0
module, prescale values of 1:2, 1:4, ..., 1:256 are
selectable.
Setting bit PSA will assign the prescaler to the Watch-
dog Timer (WDT). When the prescaler is assigned to
the WDT, prescale values of 1:1, 1:2, ..., 1:128 are
selectable.
When assigned to the Timer0 module, all instructions
writing to the TMR0 register (e.g. CLRF 1, MOVWF 1,
BSF
assigned to WDT, a CLRWDT instruction will clear the
prescaler along with the WDT.
PSout
Note:
(2 cycle delay)
1,x....etc.) will clear the prescaler. When
Sync with
Prescaler
Internal
clocks
Writing to TMR0 when the prescaler is
assigned to Timer0 will clear the prescaler
count, but will not change the prescaler
assignment.
PSout
PIC16F84A
Data bus
TMR0
8
DS35007A-page 17
Set interrupt
flag bit T0IF
on overflow