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Sheet目录3873

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PIC18F2XK20/4XK20

DS41303G-page 30

2010 Microchip Technology Inc.

2.3

Clock Source Modes

Clock Source modes can be classified as external or

internal.

External Clock modes rely on external circuitry for

the clock source. Examples are: Clock modules

(EC mode), quartz crystal resonators or ceramic

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

Capacitor (RC mode) circuits.

Internal clock sources are contained internally

within the Oscillator block. The Oscillator block

has two internal oscillators: the 16 MHz High-

Frequency Internal Oscillator (HFINTOSC) 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<1:0>) bits of the OSCCON register. See

Section 2.9 “Clock Switching” for additional informa-

tion.

2.4

External Clock Modes

2.4.1

OSCILLATOR START-UP TIMER

(OST)

When 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. When switching between clock sources, a

delay is required to allow the new clock to stabilize.

These oscillator delays are shown in Table 2-1.

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 2.10

“Two-Speed Clock Start-up Mode”).

TABLE 2-1:

OSCILLATOR DELAY EXAMPLES

2.4.2

EC MODE

The External Clock (EC) mode allows an externally

generated logic level as the system clock source. When

operating in this mode, an external clock source is

connected to the OSC1 input and the OSC2 is available

for general purpose I/O. Figure 2-2 shows the pin

connections for EC mode.

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 2-2:

EXTERNAL CLOCK (EC)

MODE OPERATION

Switch From

Switch To

Frequency

Oscillator Delay

Sleep/POR

LFINTOSC

HFINTOSC

31 kHz

250 kHz to 16 MHz

Oscillator Warm-Up Delay (TWARM)

Sleep/POR

EC, RC

DC – 64 MHz

2 instruction cycles

LFINTOSC (31 kHz)

EC, RC

DC – 64 MHz

1 cycle of each

Sleep/POR

LP, XT, HS

32 kHz to 40 MHz

1024 Clock Cycles (OST)

Sleep/POR

HSPLL

32 MHz to 64 MHz

1024 Clock Cycles (OST) + 2 ms

LFINTOSC (31 kHz)

HFINTOSC

250 kHz to 16 MHz

s (approx.)

OSC1/CLKIN

OSC2/CLKOUT(1)

I/O

Clock from

Ext. System

PIC MCU

Note

Alternate pin functions are listed in

Section 1.0 “Device Overview”.

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