Datasheet

PIC16F87/88
DS30487D-page 68  2002-2013 Microchip Technology Inc.
6.3 Using Timer0 with an External
Clock
When no prescaler is used, the external clock input is
the same as the prescaler output. The synchronization
of T0CKI, with the internal phase clocks, is accom-
plished by sampling the prescaler output on the Q2 and
Q4 cycles of the internal phase clocks. Therefore, it is
necessary for T0CKI to be high for at least 2 T
OSC (and
a small RC delay of 20 ns) and low for at least 2 T
OSC
(and a small RC delay of 20 ns). Refer to the electrical
specification of the desired device.
6.4 Prescaler
There is only one prescaler available, which is mutually
exclusively shared between the Timer0 module and the
Watchdog Timer. A prescaler assignment for the
Timer0 module means that the prescaler cannot be
used by the Watchdog Timer and vice versa. This
prescaler is not readable or writable (see Figure 6-1).
The PSA and PS2:PS0 bits (OPTION_REG<3:0>)
determine the prescaler assignment and prescale ratio.
When assigned to the Timer0 module, all instructions
writing to the TMR0 register (e.g., CLRF 1, MOVWF 1,
BSF 1, x....etc.) will clear the prescaler. When assigned
to WDT, a CLRWDT instruction will clear the prescaler
along with the Watchdog Timer. The prescaler is not
readable or writable.
REGISTER 6-1: OPTION_REG: OPTION CONTROL REGISTER (ADDRESS 81h, 181h)
Note: Although the prescaler can be assigned to
either the WDT or Timer0, but not both, a
new divide counter is implemented in the
WDT circuit to give multiple WDT time-out
selections. This allows TMR0 and WDT to
each have their own scaler. Refer to
Section 15.12 “Watchdog Timer (WDT)”
for further details.
Note: Writing to TMR0, when the prescaler is
assigned to Timer0, will clear the
prescaler count but will not change the
prescaler assignment.
R/W-1 R/W-1 R/W-1 R/W-1 R/W-1 R/W-1 R/W-1 R/W-1
RBPU
INTEDG T0CS T0SE PSA PS2 PS1 PS0
bit 7 bit 0
bit 7 RBPU: PORTB Pull-up Enable bit
bit 6 INTEDG: Interrupt Edge Select bit
bit 5 T0CS: TMR0 Clock Source Select bit
1 = Transition on T0CKI pin
0 = Internal instruction cycle clock (CLKO)
bit 4 T0SE: TMR0 Source Edge Select bit
1 = Increment on high-to-low transition on T0CKI pin
0 = Increment on low-to-high transition on T0CKI pin
bit 3 PSA: Prescaler Assignment bit
1 = Prescaler is assigned to the WDT
0 = Prescaler is assigned to the Timer0 module
bit 2-0 PS<2:0>: Prescaler Rate Select bits
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
Note: To avoid an unintended device Reset, the instruction sequence shown in the ”PIC
®
Mid-Range MCU Family Reference Manual” (DS33023) must be executed when
changing the prescaler assignment from Timer0 to the WDT. This sequence must
be followed even if the WDT is disabled.
000
001
010
011
100
101
110
111
1 : 2
1 : 4
1 : 8
1 : 16
1 : 32
1 : 64
1 : 128
1 : 256
1 : 1
1 : 2
1 : 4
1 : 8
1 : 16
1 : 32
1 : 64
1 : 128
Bit Value TMR0 Rate WDT Rate