PIC16F84 datasheet(41/128 Pages) MICROCHIP

41 / 128 page

PIC16F8X

 1996-2013 Microchip Technology Inc.

DS30430D-page 41

FIGURE 8-6:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

8.2.4

RC OSCILLATOR

For timing insensitive applications the RC device option

offers additional cost savings. The RC oscillator

frequency is a function of the supply voltage, the

resistor (Rext) values, capacitor (Cext) values, and the

operating temperature. In addition to this, the oscillator

frequency will vary from unit to unit due to normal

process

parameter

variation.

Furthermore,

the

difference in lead frame capacitance between package

types also affects the oscillation frequency, especially

for low Cext values. The user needs to take into

account variation due to tolerance of the external

R and C components. Figure 8-7 shows how an R/C

combination is connected to the PIC16F8X. For Rext

values below 4 k

, the oscillator operation may

become unstable, or stop completely. For very high

Rext values (e.g., 1 M

), the oscillator becomes

sensitive to noise, humidity and leakage. Thus, we

recommend keeping Rext between 5 k

 and 100 k.

Although the oscillator will operate with no external

capacitor (Cext = 0 pF), we recommend using values

above 20 pF for noise and stability reasons. With little

or no external capacitance, the oscillation frequency

can vary dramatically due to changes in external

capacitances, such as PCB trace capacitance or

package lead frame capacitance.

See

the

electrical

specification

section

for

frequency variation from part to part due to normal

process variation. The variation is larger for larger R

(since

leakage

current

variation

will

affect

frequency more for large R) and for smaller C (since

variation of input capacitance has a greater affect on

RC frequency).

See the electrical specification section for variation of

oscillator frequency due to VDD for given Rext/Cext

values

well

frequency

variation

due

operating temperature.

The oscillator frequency, divided by 4, is available on

the OSC2/CLKOUT pin, and can be used for test

purposes or to synchronize other logic (see Figure 3-2

for waveform).

FIGURE 8-7:

RC OSCILLATOR MODE

8.3

Reset

The PIC16F8X differentiates between various kinds

of reset:

• Power-on Reset (POR)

•MCLR reset during normal operation

•MCLR reset during SLEEP

• WDT Reset (during normal operation)

• WDT Wake-up (during SLEEP)

Figure 8-8 shows a simplified block diagram of the

on-chip reset circuit. The MCLR reset path has a noise

filter to ignore small pulses. The electrical specifica-

tions state the pulse width requirements for the MCLR

pin.

Some registers are not affected in any reset condition;

their status is unknown on a POR reset and unchanged

in any other reset. Most other registers are reset to a

“reset state” on POR, MCLR or WDT reset during

normal operation and on MCLR reset during SLEEP.

They are not affected by a WDT reset during SLEEP,

since this reset is viewed as the resumption of normal

operation.

Table 8-3 gives a description of reset conditions for the

program counter (PC) and the STATUS register.

Table 8-4 gives a full description of reset states for all

registers.

The TO and PD bits are set or cleared differently in dif-

ferent reset situations (Section 8.7). These bits are

used in software to determine the nature of the reset.

330 k



74AS04

PIC16FXX

CLKIN

To Other

Devices

XTAL

330 k



74AS04

0.1

F

Note:

When the device oscillator is in RC mode,

do not drive the OSC1 pin with an external

clock or you may damage the device.

OSC2/CLKOUT

Cext

Rext

PIC16FXX

OSC1

Fosc/4

Internal

clock

VDD

VSS

Recommended values:

5 k

  Rext  100 k

Cext > 20pF

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