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PIC16C745/765

DS41124C-page 100

Preliminary

2000 Microchip Technology Inc.

13.2

Oscillator Configurations

13.2.1

OSCILLATOR TYPES

The PIC16C745/765 can be operated in four different

oscillator modes. The user can program a configuration

bit (FOSC0) to select one of these four modes:

EC External Clock

E4 External Clock with internal PLL enabled

HS High Speed Crystal/Resonator

H4 High Speed Crystal/Resonator with

internal PLL enabled

13.2.2

CRYSTAL OSCILLATOR/CERAMIC

RESONATORS

In HS mode, a crystal or ceramic resonator is con-

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

establish oscillation (Figure 13-1). The PIC16C745/

765 oscillator 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. When in

HS mode, the device can have an external clock

source to drive the OSC1/CLKIN pin (Figure 13-2). In

this mode, the oscillator start-up timer is active for a

period of 1024*TOSC. See the PICmicro Mid-Range

MCU Reference Manual (DS33023) for details on

building an external oscillator.

FIGURE 13-1: CRYSTAL/CERAMIC

RESONATOR OPERATION

(HS OSC CONFIGURATION)

TABLE 13-1:

CERAMIC RESONATORS

TABLE 13-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR

13.2.3

H4 MODE

In H4 mode, a PLL module is switched on in-line with

the clock provided across OSC1 and OCS2. The output

of the PLL drives FINT.

13.2.4

PLL

An on-board 4x PLL provides a cheap means of gener-

ating a stable 24 MHz FINT, using an external 6 MHz

resonator. After power-up, a PLL settling time of less

than TPLLRT is required.

XTAL

OSC2

Note1

OSC1

SLEEP

To internal

logic

PIC16C745/765

Note 1:

A series resistor may be required for AT strip cut

crystals.

Ranges Tested:

Mode

Freq

OSC1

OSC2

6.0 MHz

10 - 68 pF

These values are for design guidance only. See notes at

bottom of page.

Osc Type

Crystal

Freq

Cap. Range

6.0 MHz

15 - 33 pF

These values are for design guidance only. See notes at

bottom of page.

Note 1: Higher capacitance increases the stability

of the oscillator, but also increases the start-

up time.

2: Since each resonator/crystal has its own

characteristics, the user should consult the

resonator/crystal manufacturer for appropri-

ate values of external components.

3: Rs may be required in HS mode to avoid

overdriving crystals with low drive level

specification.

4: When migrating from other PICmicro

devices, oscillator performance should be

verified.

5: Users should consult the USB Specification

1.1 to ensure their resonator/crystal oscilla-

tor meets the required jitter limits for USB

operation.

745cov.book Page 100 Wednesday, August 2, 2000 8:24 AM

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