PIC17C752 datasheet(17/306 Pages) MICROCHIP

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DS30289C-page 17

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 con-

nected to the OSC1/CLKIN and OSC2/CLKOUT pins to

establish oscillation (Figure 4-2). The PIC17CXXX oscil-

lator design requires the use of a parallel cut crystal. Use

of a series cut crystal may give a frequency out of the

crystal manufacturers specifications.

For frequencies above 24 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.

4.1.3

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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