PIC17C752-08SLASHCL datasheet(15/328 Pages) MICROCHIP

15 / 328 page

DS30289A-page 15

PIC17C7XX

4.0

ON-CHIP OSCILLATOR

CIRCUIT

The internal oscillator circuit is used to generate the

device clock. Four device clock periods generate an

internal instruction clock (TCY).

There are four modes that the oscillator can operate in.

They are selected by the device conﬁguration bits dur-

ing device programming. These modes are:

• LF

Low Frequency (FOSC <= 2 MHz)

• XT

Standard Crystal/Resonator Frequency

(2 MHz <= FOSC <= 33 MHz)

• EC

External Clock Input

(Default oscillator conﬁguration)

• RC

External Resistor/Capacitor

(FOSC <= 4 MHz)

There are two timers that offer necessary delays on

power-up. One is the Oscillator Start-up Timer (OST),

intended to keep the chip in RESET until the crystal

oscillator is stable. The other is the Power-up Timer

(PWRT), which provides a ﬁxed delay of 96 ms (nomi-

nal) on POR and BOR. The PWRT is designed to keep

the part in RESET while the power supply stabilizes.

With these two timers on-chip, most applications need

no external reset circuitry.

SLEEP mode is designed to offer a very low current

power-down mode. The user can wake from SLEEP

through external reset, Watchdog Timer Reset or

through an interrupt.

Several oscillator options are made available to allow

the part to better ﬁt the application. The RC oscillator

option saves system cost while the LF crystal option

saves power. Conﬁguration bits are used to select var-

ious options.

4.1

Oscillator Conﬁgurations

4.1.1

OSCILLATOR TYPES

The PIC17CXXX can be operated in four different oscil-

lator modes. The user can program two conﬁguration

bits (FOSC1:FOSC0) to select one of these four

modes:

• LF

Low Power Crystal

• XT

Crystal/Resonator

• EC

External Clock Input

• RC

Resistor/Capacitor

The main difference between the LF and XT modes is

the gain of the internal inverter of the oscillator circuit

which allows the different frequency ranges.

For more details on the device conﬁguration bits, see

Section 17.0.

4.1.2

CRYSTAL OSCILLATOR / CERAMIC

RESONATORS

In XT or LF modes, a crystal or ceramic resonator is

connected to the OSC1/CLKIN and OSC2/CLKOUT

pins

establish

oscillation

(Figure 4-2).

The

PIC17CXXX oscillator design requires the use of a par-

allel cut crystal. Use of a series cut crystal may give a

frequency out of the crystal manufacturers speciﬁca-

tions.

For frequencies above 20 MHz, it is common for the

crystal to be an overtone mode crystal. Use of over-

tone mode crystals require a tank circuit to attenuate

the gain at the fundamental frequency. Figure 4-3

shows an example circuit.

4.1.2.1

OSCILLATOR / RESONATOR START-UP

As the device voltage increases from Vss, the oscillator

will start its oscillations. The time required for the oscil-

lator to start oscillating depends on many factors.

These include:

• Crystal / resonator frequency

• Capacitor values used (C1 and C2)

• Device VDD rise time.

• System temperature

• Series resistor value (and type) if used

• Oscillator mode selection of device (which selects

the gain of the internal oscillator inverter)

Figure 4-1 shows an example of a typical oscillator/

resonator start-up. The peak-to-peak voltage of the

oscillator waveform can be quite low (less than 50% of

device VDD) when the waveform is centered at VDD/2

(refer to parameter #D033 and parameter #D043 in the

electrical speciﬁcation section).

FIGURE 4-1:

OSCILLATOR / RESONATOR

START-UP

CHARACTERISTICS

Crystal Start-up Time

Time

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