PIC16F18345 datasheet(66/442 Pages) MICROCHIP

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PIC16(L)F18313/18323

DS40001799A-page 66

Preliminary

 2015 Microchip Technology Inc.

6.2

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

modules (ECH, ECM, ECL mode), quartz crystal

resonators or ceramic resonators (LP, XT and HS

modes).

There is also a secondary oscillator block which is

optimized for a 32.768 kHz external clock source,

which can be used as an alternate clock source.

There are two internal oscillator blocks:

- HFINTOSC

- LFINTOSC

The HFINTOSC can produce clock frequencies from

1-16 MHz. The LFINTOSC generates a 31 kHz clock

frequency.

There is a PLL that can be used by the external

oscillator. See Section 6.2.1.4 “4x PLL” for more

details. Additionally, there is a PLL that can be used by

the HFINTOSC at certain frequencies. Section 6.2.2.2

“2x PLL”

for more details.

6.2.1

EXTERNAL CLOCK SOURCES

An external clock source can be used as the device

system clock by performing one of the following

actions:

• Program the RSTOSC<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 NOSC<2:0> and NDIV<3:0> bits in the

OSCCON1 register to switch the system clock

source

See Section 6.3 “Clock Switching” for more

information.

6.2.1.1

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

CLKIN

input.

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

CLKOUT. Figure 6-2 shows the pin connections for EC

mode.

EC mode has three power modes to select from through

Configuration Words:

• ECH – High power, 8-32 MHz

• ECM – Medium power, 0.1-8 MHz

• ECL – Low power, 0-0.1 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 6-2:

EXTERNAL CLOCK (EC)

MODE OPERATION

6.2.1.2

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 6-3). The three modes select

a low, medium or high gain setting of the internal

inverter-amplifier to support various resonator types

and speed.

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).

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.

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 6-3 and Figure 6-4 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

Output depends upon CLKOUTEN bit of the

Configuration Words.

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