Microchip Technology AC164112 Data Sheet

© 2009 Microchip Technology Inc.

DS41341E-page 93

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.

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 or high gain setting of 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.

OSC1/CLKIN

OSC2/CLKOUT

I/O

Clock from
Ext. System

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 V

DD

 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 

Devices” (DS00826)

• AN849, “Basic PIC

 Oscillator Design” 

(DS00849)

• AN943, “Practical PIC

Analysis and Design” (DS00943)

• AN949, “Making Your Oscillator Work” 

(DS00949)

A series resistor (R

S

) may be required for

quartz crystals with low drive level.

The value of R

F

 varies with the Oscillator mode

selected.

C1

C2

Quartz

R

S

 

OSC1/CLKIN

R

F

 

Sleep

To Internal 
Logic

Crystal

OSC2/CLKOUT