PIC18F1XK50 datasheet(18/422 Pages) MICROCHIP

18 / 422 page

PIC18F/LF1XK50

DS41350D-page 18

Preliminary

 2010 Microchip Technology Inc.

FIGURE 2-3:

CERAMIC RESONATOR

OPERATION

(XT OR HS MODE)

2.3.3

EXTERNAL RC

The External Resistor-Capacitor (RC) mode supports

the use of an external RC circuit. This allows the

designer maximum flexibility in frequency choice while

keeping costs to a minimum when clock accuracy is not

required. In RC mode, the RC circuit connects to OSC1,

allowing OSC2 to be configured as an IO or as

CLKOUT. The CLKOUT function is selected by the

FOSC bits of the CONFIG1H Configuration register.

When OSC2 is configured as CLKOUT, the frequency

at the pin is the frequency of the RC oscillator divided by

4. Figure 2-4 shows the external RC mode connections.

FIGURE 2-4:

EXTERNAL RC MODES

The RC oscillator frequency is a function of the supply

voltage, the resistor REXT, the capacitor CEXT and the

operating temperature. Other factors affecting the

oscillator frequency are:

• Input threshold voltage variation

• Component tolerances

• Variation in capacitance due to packaging

2.3.4

EXTERNAL CLOCK

The External Clock (EC) mode allows an externally

generated logic level clock to be used as the system’s

clock source. When operating in this mode, the

external clock source is connected to the OSC1

allowing OSC2 to be configured as an I/O or as

CLKOUT. The CLKOUT function is selected by the

FOSC bits of the CONFIG1H Configuration register.

When OSC2 is configured as CLKOUT, the frequency

at the pin is the frequency of the EC oscillator divided

by 4.

Three different power settings are available for EC

mode. The power settings allow for a reduced IDD of the

device, if the EC clock is known to be in a specific

range. If there is an expected range of frequencies for

the EC clock, select the power mode for the highest

frequency.

Low power

0 – 250 kHz

Medium power 250 kHz – 4 MHz

High power

4 – 48 MHz

2.4

Secondary External Oscillator

The Secondary External Oscillator is designed to drive

an external 32.768 kHz crystal. This oscillator is

enabled or disabled by the T1OSCEN bit of the T1CON

information.

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

OSC2/CLKOUT(1)

CEXT

REXT

PIC® MCU

OSC1/CLKIN

FOSC/4 or

Internal

Clock

VDD

VSS

Recommended values: 10 k

  REXT  100 k

CEXT > 20 pF

Note 1:

Alternate pin functions are listed in

Section 1.0 “Device Overview”.

Output depends upon RC or RCIO clock mode.

I/O(2)

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