PIC12F519-ESLASHMS datasheet(42/96 Pages) MICROCHIP

42 / 96 page

PIC12F519

DS41319B-page 40

8.2.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator or a simple oscillator

circuit with TTL gates can be used as an external

crystal oscillator circuit. Prepackaged oscillators provide

a wide operating range and better stability. A

well-designed crystal oscillator will provide good

performance with TTL gates. Two types of crystal

oscillator circuits can be used: one with parallel

resonance, or one with series resonance.

Figure 8-3 shows implementation of a parallel resonant

oscillator circuit. The circuit is designed to use the

fundamental frequency of the crystal. The 74AS04

inverter performs the 180-degree phase shift that a

parallel oscillator requires. The 4.7 k

Ω resistor provides

the negative feedback for stability. The 10 k

potentiometers bias the 74AS04 in the linear region.

This circuit could be used for external oscillator designs.

FIGURE 8-3:

EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 8-4 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental

frequency of the crystal. The inverter performs a

180-degree phase shift in a series resonant oscillator

circuit. The 330

Ω resistors provide the negative

feedback to bias the inverters in their linear region.

FIGURE 8-4:

EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

8.2.4

EXTERNAL RC OSCILLATOR

For timing insensitive applications, the RC circuit option

offers additional cost savings. The RC oscillator

frequency is a function of the supply voltage, the

resistor (REXT) and 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 will also affect the oscillation frequency, especially

for low CEXT values. The user also needs to take into

account variation due to tolerance of external R and C

components used.

Figure 8-5 shows how the R/C combination is

connected to the PIC12F519 device. For REXT values

below 3.0 k

Ω, the oscillator operation may become

unstable, or stop completely. For very high REXT values

(e.g., 1 M

Ω), the oscillator becomes sensitive to noise,

humidity and leakage. It is recommended keeping REXT

between 5.0 k

Ω and 100 kΩ.

Although the oscillator will operate with no external

capacitor (CEXT = 0 pF), it is recommended using

values above 20 pF for noise and stability reasons. With

no or small 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 Figure 11-1

and Figure 11-2.

FIGURE 8-5:

EXTERNAL RC

OSCILLATOR MODE

8.2.5

INTERNAL 4/8 MHz RC

OSCILLATOR

The internal RC oscillator provides a fixed 4/8 MHz

(nominal) system clock at VDD = 3.5V and 25°C, (see

Section 11.0

“Electrical

Characteristics”

for

information on variation over voltage and temperature).

In addition, a calibration instruction is programmed into

the last address of memory, which contains the

calibration value for the internal RC oscillator. This

location is always non-code protected, regardless of the

code-protect settings. This value is programmed as a

MOVLW XX

instruction where XX is the calibration value,

and is placed at the Reset vector. This will load the W

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC12F519

CLKIN

To Other

Devices

330

74AS04

CLKIN

To Other

Devices

XTAL

330

74AS04

0.1 mF

PIC12F519

VDD

REXT

CEXT

VSS

OSC1

Internal

clock

PIC16F519

PIC12F519-E/MS Datasheet(PDF) 42 Page - Microchip Technology

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