PIC16F872SLASHSP datasheet(38/160 Pages) MICROCHIP

38 / 160 page

PIC16F872

DS30221A-page 38

Preliminary

5.2

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 2TOSC (and

a small RC delay of 20 ns) and low for at least 2TOSC

(and a small RC delay of 20 ns). Refer to the electrical

specification of the desired device.

5.3

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 there is no prescaler for the watch-

dog timer, and vice-versa. This prescaler is not readable

or writable (see Figure 5-1).

The PSA and PS<2:0> bits (OPTION_REG<3:0>) deter-

mine the prescaler assignment and prescale ratio.

When assigned to the Timer0 module, all instructions

writing to the TMR0 register (i.e., 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 pres-

caler is not readable or writable.

OPTION_REG REGISTER

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

RBPU

INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

R = Readable bit

W = Writable bit

U = Unimplemented bit,

read as ‘0’

- n = Value at POR reset

bit 7

bit 0

bit 7:

RBPU

bit 6:

INTEDG

bit 5:

T0CS: TMR0 Clock Source Select bit

= Transition on T0CKI pin

= Internal instruction cycle clock (CLKOUT)

bit 4:

T0SE: TMR0 Source Edge Select bit

= Increment on high-to-low transition on T0CKI pin

= Increment on low-to-high transition on T0CKI pin

bit 3:

PSA: Prescaler Assignment bit

= Prescaler is assigned to the WDT

= Prescaler is assigned to the Timer0 module

bit 2-0:

PS<2:0>: Prescaler Rate Select bits

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

Note:

To avoid an unintended device RESET, the instruction sequence shown in the PICmicro™ 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.

PIC16F872/SP Datasheet(PDF) 38 Page - Microchip Technology

PIC16F872/SP Datasheet(HTML) 38 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL