PIC16F628-20SLASHSS datasheet(98/160 Pages) MICROCHIP

98 / 160 page

PIC16F62X

DS40300B-page 98

Preliminary

14.2.3

EXTERNAL CRYSTAL OSCILLATOR

CIRCUIT

Either a prepackaged oscillator can be used or a simple

oscillator

circuit

with

TTL

gates

can

built.

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 series resonance, or one with parallel

resonance.

Figure 14-4 shows implementation of a parallel reso-

nant oscillator circuit. The circuit is designed to use the

fundamental frequency of the crystal. The 74AS04

inverter performs the 180

° phase shift that a parallel

oscillator requires. The 4.7 k

Ω resistor provides the

negative

feedback

for

stability.

The

10 k

Ω

potentiometers bias the 74AS04 in the linear region.

This could be used for external oscillator designs.

FIGURE 14-4: EXTERNAL PARALLEL

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

Figure 14-5 shows a series resonant oscillator circuit.

This circuit is also designed to use the fundamental

frequency of the crystal. The inverter performs a 180

phase shift in a series resonant oscillator circuit. The

330 k

Ω resistors provide the negative feedback to bias

the inverters in their linear region.

FIGURE 14-5: EXTERNAL SERIES

RESONANT CRYSTAL

OSCILLATOR CIRCUIT

20 pF

+5V

20 pF

10k

4.7k

10k

74AS04

XTAL

10k

74AS04

PIC16F62X

CLKIN

To other

Devices

330 k

Ω

74AS04

PIC16F62X

CLKIN

To other

Devices

XTAL

330 k

Ω

74AS04

0.1

µF

14.2.4

EXTERNAL CLOCK IN

For applications where a clock is already available else-

where, users may directly drive the PIC16F62X pro-

vided that this external clock source meets the AC/DC

timing requirements listed in Section 17.4. Figure 14-6

below shows how an external clock circuit should be

configured.

FIGURE 14-6: EXTERNAL CLOCK INPUT

OPERATION (HS, XT OR LP

OSC CONFIGURATION)

14.2.5

ER OSCILLATOR

For timing insensitive applications, the ER (External

Resistor) clock mode offers additional cost savings.

Only one external component, a resistor to VSS, is

needed to set the operating frequency of the internal

oscillator. The resistor draws a DC bias current which

controls the oscillation frequency. In addition to the

resistance value, the oscillator frequency will vary from

unit to unit, and as a function of supply voltage and tem-

perature. Since the controlling parameter is a DC cur-

rent and not a capacitance, the particular package type

and lead frame will not have a significant effect on the

resultant frequency.

Figure 14-7 shows how the controlling resistor is con-

nected to the PIC16F62X. For Rext values below 38k,

the oscillator operation may become unstable, or stop

completely. For very high Rext values (e.g. 1M), the

oscillator becomes sensitive to noise, humidity and

leakage. Thus, we recommend keeping Rext between

38k and 1M.

FIGURE 14-7: EXTERNAL RESISTOR

The Electrical Specification section shows the relation-

ship between the resistance value and the operating

frequency as well as frequency variations due to oper-

ating temperature for given R and VDD values.

The ER oscillator mode has two options that control the

unused OSC2 pin. The first allows it to be used as a

general purpose I/O port. The other configures the pin

as an output providing the Fosc signal (internal clock

divided by 4) for test or external synchronization pur-

poses.

Clock from

ext. system

PIC16F62X

OSC1/RA7

OSC2/RA6

RA6

RA7/OSC1/CLKIN

RA6/OSC2/CLKOUT

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