PIC17C752-08SLASHP datasheet(15/320 Pages) MICROCHIP

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Preliminary

DS30264A-page 15

PIC17C75X

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. These are selected by

the device conﬁguration bits during device program-

ming. 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 power-up only, 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 ﬁt the application. The RC oscillator option

saves system cost while the LF crystal option saves

power. Conﬁguration bits are used to select various

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 overtone

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 number D033 and D043 in the elec-

trical speciﬁcation section).

FIGURE 4-1: OSCILLATOR / RESONATOR

START-UP

CHARACTERISTICS

Crystal Start-up Time

Time

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