41594B datasheet(50/372 Pages) MICROCHIP

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PIC12LF1840T48A

DS41594B-page 50

Preliminary

 2011-2012 Microchip Technology Inc.

FIGURE 5-3:

QUARTZ CRYSTAL

OPERATION (LP, XT OR

HS MODE)

FIGURE 5-4:

CERAMIC RESONATOR

OPERATION

(XT OR HS MODE)

5.2.1.3

Oscillator Start-up Timer (OST)

If the oscillator module is configured for LP, XT or HS

modes, the Oscillator Start-up Timer (OST) counts

1024 oscillations from OSC1. This occurs following a

Power-on Reset (POR) and when the Power-up Timer

(PWRT) has expired (if configured), or a wake-up from

Sleep. During this time, the program counter does not

increment and program execution is suspended. The

OST ensures that the oscillator circuit, using a quartz

crystal resonator or ceramic resonator, has started and

is providing a stable system clock to the oscillator

module.

In order to minimize latency between external oscillator

start-up and code execution, the Two-Speed Clock

Start-up mode can be selected (see Section 5.4

“Two-Speed Clock Start-up Mode”).

5.2.1.4

4xPLL

The oscillator module contains a 4xPLL that can be

used with both external and internal clock sources to

provide a system clock source. The input frequency for

the 4xPLL must fall within specifications. See the PLL

Clock

Timing

Specifications

Section 31.0

“Electrical Specifications”.

The 4xPLL may be enabled for use by one of two

methods:

Program the PLLEN bit in Configuration Word 2

to a ‘1’.

Write the SPLLEN bit in the OSCCON register to

a ‘1’. If the PLLEN bit in Configuration Word 2 is

programmed to a ‘1’, then the value of SPLLEN

is ignored.

Note 1: Quartz

crystal

characteristics

vary

according

type,

package

and

manufacturer. The user should consult the

manufacturer data sheets for specifications

and recommended application.

2: Always verify oscillator performance over

the VDD and temperature range that is

expected for the application.

3: For oscillator design assistance, reference

the following Microchip Applications Notes:

• AN826, “Crystal Oscillator Basics and

Crystal Selection for rfPIC® and PIC®

Devices” (DS00826)

• AN849, “Basic PIC® Oscillator Design”

(DS00849)

• AN943, “Practical PIC® Oscillator

Analysis and Design” (DS00943)

• AN949, “Making Your Oscillator Work”

(DS00949)

Note 1: A series resistor (RS) may be required for

quartz crystals with low drive level.

2: The value of RF varies with the Oscillator mode

selected (typically between 2 M

 to 10 M.

Quartz

RS(1)

OSC1/CLKIN

RF(2)

Sleep

To Internal

Logic

PIC® MCU

Crystal

OSC2/CLKOUT

Note 1: A series resistor (RS) may be required for

ceramic resonators with low drive level.

2: The value of RF varies with the Oscillator mode

selected (typically between 2 M

 to 10 M.

3: An additional parallel feedback resistor (RP)

may be required for proper ceramic resonator

operation.

C2 Ceramic

RS(1)

OSC1/CLKIN

RF(2)

Sleep

To Internal

Logic

PIC® MCU

RP(3)

Resonator

OSC2/CLKOUT

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