PIC16F684-ESLASHST datasheet(22/164 Pages) MICROCHIP

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PIC16F684

DS41202C-page 20

Preliminary

 2004 Microchip Technology Inc.

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 the PIC16F684. The PIC16F684 has two

internal 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 PIC16F684 is configured for LP, XT or HS modes,

the Oscillator Start-up Timer (OST) counts 1024 oscil-

lations 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 PIC16F684. When switching

between clock sources a delay is required to allow the

new clock to stabilize. These oscillator delays are

shown in Table 3-1.

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 3.6

“Two-Speed Clock Start-up Mode”).

TABLE 3-1:

OSCILLATOR DELAY EXAMPLES

3.3.2

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 pin and the RA5 pin is

available for general purpose I/O. Figure 3-2 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 PIC16F684 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 3-2:

EXTERNAL CLOCK (EC)

MODE OPERATION

Switch From

Switch To

Frequency

Oscillator Delay

Sleep/POR

LFINTOSC

HFINTOSC

31 kHz

125 kHz to 8 MHz

µs-10 µs (approx.) CPU Start-up(1)

Sleep/POR

EC, RC

DC – 20 MHz

LFINTOSC (31 kHz)

EC, RC

DC – 20 MHz

Sleep/POR

LP, XT, HS

31 kHz to 20 MHz

1024 Clock Cycles (OST)

LFINTOSC (31 kHz)

HFINTOSC

125 kHz to 8 MHz

µs (approx.)

Note 1:

The 5

µs to 10 µs start-up delay is based on a 1 MHz system clock.

OSC1/CLKIN

I/O (OSC2)

RA4

Clock from

Ext. System

PIC16F684

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