Microchip Technology AC244045 Data Sheet

 2010-2012 Microchip Technology Inc.

DS41440C-page 57

Clock sources can be classified as external or internal. 
External clock sources rely on external circuitry for the
clock source to function. 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 oscillator module. The internal oscillator block has
two internal oscillators and a dedicated Phase-Lock
Loop (HFPLL) that are used to generate three internal
system clock sources: the 16 MHz High-Frequency
Internal Oscillator (HFINTOSC), 500 kHZ (MFINTOSC)
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) bits in the OSCCON register. See 

 for additional information. 

5.2.1

EXTERNAL CLOCK SOURCES

An external clock source can be used as the device
system clock by performing one of the following
actions:
• Program the FOSC<2:0> bits in the Configuration 

Word 1 to select an external clock source that will 
be used as the default system clock upon a 
device Reset.

• Write the SCS<1:0> bits in the OSCCON register 

to switch the system clock source to:
- Timer1 Oscillator during run-time, or
- An external clock source determined by the 

value of the FOSC bits.

See 

for more

information.       

5.2.1.1

EC Mode

The External Clock (EC) mode allows an externally
generated logic level signal to be the system clock
source. When operating in this mode, an external clock
source is connected to the OSC1 input.
OSC2/CLKOUT is available for general purpose I/O or
CLKOUT. 

 shows the pin connections for EC

mode.
EC mode has three power modes to select from through
Configuration Word 1:
• High power, 4-32 MHz (FOSC = 111)
• Medium power, 0.5-4 MHz (FOSC = 110)
• Low power, 0-0.5 MHz (FOSC = 101)

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.

5.2.1.2

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 (

). The three modes select

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 designed to
drive only 32.768 kHz tuning-fork type crystals (watch
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.

 show typical circuits for

quartz crystal and ceramic resonators, respectively.

OSC1/CLKIN

OSC2/CLKOUT

Clock from
Ext. System

F

OSC

/4 or I/O

Output depends upon CLKOUTEN bit of the 
Configuration Word 1.