PIC16 datasheet(49/418 Pages) MICROCHIP

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 2011-2013 Microchip Technology Inc.

DS40001609C-page 49

PIC16(L)F1508/9

5.2

Clock Source Types

Clock sources can be classified as external, internal or

peripheral.

External clock sources rely on external circuitry for the

clock source to function. Examples are: oscillator mod-

ules (ECH, ECM, ECL modes), quartz crystal resona-

tors or ceramic resonators (LP, XT and HS modes) and

Resistor-Capacitor (EXTRC) mode circuits.

Internal clock sources are contained within the oscillator

module. The internal oscillator block has two internal

oscillators that are used to generate the internal system

clock sources: the 16 MHz High-Frequency Internal

Oscillator (HFINTOSC) and the 31 kHz Low-Frequency

Internal Oscillator (LFINTOSC).

The peripheral clock source is a nominal 600 kHz

internal RC oscillator, FRC. The FRC is traditionally

used with the ADC module, but is sometimes available

to other peripherals. See Section 5.2.2.4 “Peripheral

Clock Sources”.

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

Words 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:

- Secondary 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 three power modes to select from through

the FOSC bits in the Configuration Words:

• ECH – High-power, 4-20 MHz

• ECM – Medium-power, 0.5-4 MHz

• ECL – Low-power, 0-0.5 MHz

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.

Clock from

Ext. system

FOSC/4 or I/O

(1)

OSC1/CLKIN

PIC

® MCU

OSC2/CLKOUT

Note 1:

Output depends upon the CLKOUTEN bit

of the Configuration Words.

Rev. 10-000045A

7/30/2013

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