PIC16F684 datasheet(23/192 Pages) MICROCHIP

23 / 192 page

DS41202F-page 21

PIC16F684

3.3

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: Oscillator mod-

ules (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 module. The Oscillator

module has two internal oscillators: the 8 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) bit of the OSCCON register. See Section 3.6

“Clock Switching” for additional information.

3.4

External Clock Modes

3.4.1

OSCILLATOR START-UP TIMER (OST)

If 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 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 3.7

“Two-Speed Clock Start-up Mode”).

TABLE 3-1:

OSCILLATOR DELAY EXAMPLES

3.4.2

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 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 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 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

Oscillator Warm-Up Delay (TWARM)

Sleep/POR

EC, RC

DC – 20 MHz

2 cycles

LFINTOSC (31 kHz)

EC, RC

DC – 20 MHz

1 cycle of each

Sleep/POR

LP, XT, HS

32 kHz to 20 MHz

1024 Clock Cycles (OST)

LFINTOSC (31 kHz)

HFINTOSC

125 kHz to 8 MHz

μs (approx.)

OSC1/CLKIN

OSC2/CLKOUT(1)

I/O

Clock from

Ext. System

PIC® MCU

Note

Alternate pin functions are listed in the

Section 1.0 “Device Overview”.

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