PIC16F722-ESLASHML datasheet(93/302 Pages) MICROCHIP

93 / 302 page

DS41341E-page 93

PIC16F72X/PIC16LF72X

7.6

External Clock Modes

7.6.1

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 on the OSC1 pin before the device is

released from Reset. 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.

7.6.2

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 7-2 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 7-2:

EXTERNAL CLOCK (EC)

MODE OPERATION

7.6.3

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 7-3). The mode selects a low,

medium

high

gain

setting

the

internal

inverter-amplifier to support various resonator types

and speed.

LP 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 best suited

to drive resonators with a low drive level specification, for

example, tuning fork type crystals.

XT 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.

HS 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 7-3 and Figure 7-4 show typical circuits for

quartz crystal and ceramic resonators, respectively.

FIGURE 7-3:

QUARTZ CRYSTAL

OPERATION (LP, XT OR

HS MODE)

OSC1/CLKIN

OSC2/CLKOUT(1)

I/O

Clock from

Ext. System

PIC® MCU

Note

Alternate pin functions are described in

Section 6.1 “Alternate Pin Function”.

Note 1: Quartz crystal characteristics vary according

to type, package and manufacturer. The

user should consult the manufacturer data

sheets for specifications and recommended

application.

2: Always verify oscillator performance over

the VDD and temperature range that is

expected for the application.

3: For oscillator design assistance, reference

the following Microchip Applications Notes:

• AN826, “Crystal Oscillator Basics and

Crystal Selection for rfPIC® and PIC®

Devices” (DS00826)

• AN849, “Basic PIC® Oscillator Design”

(DS00849)

• AN943, “Practical PIC® Oscillator

Analysis and Design” (DS00943)

• AN949, “Making Your Oscillator Work”

(DS00949)

Note 1:

A series resistor (RS) may be required for

quartz crystals with low drive level.

The value of RF varies with the Oscillator mode

selected.

Quartz

RS(1)

OSC1/CLKIN

RF(2)

Sleep

To Internal

Logic

PIC® MCU

Crystal

OSC2/CLKOUT

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