PIC12F635ESLASHSSQTP datasheet(32/196 Pages) MICROCHIP

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PIC12F635/PIC16F636/639

DS41232B-page 30

Preliminary

3.2

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 oscillator 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

PIC12F635/PIC16F636/639. The device has two inter-

nal oscillators: the 8 MHz High-Frequency Internal

Oscillator (HFINTOSC) and 31 kHz Low-Frequency

Internal Oscillator (LFINTOSC).

The system clock can be selected between external or

internal clock sources via the System Clock Selection

(SCS) bit (see Section 3.5 “Clock Switching”).

3.3

External Clock Modes

3.3.1

OSCILLATOR START-UP TIMER (OST)

If the PIC12F635/PIC16F636/639 is configured for LP,

XT or HS modes, the Oscillator Start-up Timer (OST)

counts 1024 oscillations from the OSC1 pin following a

Power-on Reset (POR) and 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 PIC12F635/

PIC16F636/639.

When switching between clock sources, a delay is

required to allow the new clock to stabilize. Table 3-1

shows oscillator delay examples.

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 3.6 “Two-Speed

Clock Start-up Mode”).

TABLE 3-1:

OSCILLATOR DELAY EXAMPLES

Switch From

Switch To

Frequency

Oscillator Delay

Sleep/POR

LFINTOSC

HFINTOSC

31 kHz

125 kHz-8 MHz

μs-10 μs (approx.)

CPU Start-up

Sleep/POR

EC, RC

DC – 20 MHz

LFINTOSC (31 kHz)

EC, RC

DC – 20 MHz

Sleep/POR

LP, XT, HS

31 kHz-20 MHz

1024 Clock Cycles (OST)

LFINTOSC (31 kHz)

HFINTOSC

125 kHz-8 MHz

μs (approx.)

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