41594B datasheet(49/372 Pages) MICROCHIP

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 2011-2012 Microchip Technology Inc.

Preliminary

DS41594B-page 49

PIC12LF1840T48A

5.2

Clock Source Types

Clock sources can be classified as external or internal.

External clock sources rely on external circuitry for the

clock source to function. Examples are: oscillator mod-

ules (EC mode), quartz crystal resonators or ceramic

resonators (LP, XT and HS modes) and Resis-

tor-Capacitor (RC) mode circuits.

Internal clock sources are contained internally within

the oscillator module. The internal oscillator block has

two internal oscillators and a dedicated Phase-Locked

Loop (HFPLL) that are used to generate three internal

system clock sources: the 16 MHz High-Frequency

Internal Oscillator (HFINTOSC), 500 kHz (MFINTOSC)

and the 31 kHz Low-Frequency Internal Oscillator

(LFINTOSC).

The system clock can be selected between external or

internal clock sources via the System Clock Select

(SCS) bits in the OSCCON register. See Section 5.3

“Clock Switching” for additional information.

5.2.1

EXTERNAL CLOCK SOURCES

An external clock source can be used as the device

system clock by performing one of the following

actions:

• Program the FOSC<2:0> bits in the Configuration

Word 1 to select an external clock source that will

be used as the default system clock upon a

device Reset.

• Write the SCS<1:0> bits in the OSCCON register

to switch the system clock source to:

- Timer1 Oscillator during run-time, or

- An external clock source determined by the

value of the FOSC bits.

See Section 5.3 “Clock Switching”for more informa-

tion.

5.2.1.1

EC Mode

The External Clock (EC) mode allows an externally

generated logic level signal to be the system clock

source. When operating in this mode, an external clock

source

connected

the

OSC1

input.

OSC2/CLKOUT is available for general purpose I/O or

CLKOUT. Figure 5-2 shows the pin connections for EC

mode.

EC mode has 3 power modes to select from through

Configuration Word 1:

• High power, 4-32 MHz (FOSC = 111)

• Medium power, 0.5-4 MHz (FOSC = 110)

• Low power, 0-0.5 MHz (FOSC = 101)

The Oscillator Start-up Timer (OST) is disabled when

EC mode is selected. Therefore, there is no delay in

operation after a Power-on Reset (POR) or wake-up

from Sleep. Because the PIC® MCU design is fully

static, stopping the external clock input will have the

effect of halting the device while leaving all data intact.

Upon restarting the external clock, the device will

resume operation as if no time had elapsed.

FIGURE 5-2:

EXTERNAL CLOCK (EC)

MODE OPERATION

5.2.1.2

LP, XT, HS Modes

The LP, XT and HS modes support the use of quartz

crystal resonators or ceramic resonators connected to

OSC1 and OSC2 (Figure 5-3). The three modes select

a low, medium or high gain setting of the internal

inverter-amplifier to support various resonator types

and speed.

LP Oscillator mode selects the lowest gain setting of the

internal inverter-amplifier. LP mode current consumption

is the least of the three modes. This mode is designed to

drive only 32.768 kHz tuning-fork type crystals (watch

crystals).

XT Oscillator mode selects the intermediate gain

setting of the internal inverter-amplifier. XT mode

current consumption is the medium of the three modes.

This mode is best suited to drive resonators with a

medium drive level specification.

HS Oscillator mode selects the highest gain setting of the

internal inverter-amplifier. HS mode current consumption

is the highest of the three modes. This mode is best

suited for resonators that require a high drive setting.

Figure 5-3 and Figure 5-4 show typical circuits for

quartz crystal and ceramic resonators, respectively.

OSC1/CLKIN

OSC2/CLKOUT

Clock from

Ext. System

PIC® MCU

FOSC/4 or I/O(1)

Note 1:

Output depends upon CLKOUTEN bit of the

Configuration Word 1.

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