2009 Microchip Technology Inc.
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
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
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.
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
1:
Alternate pin functions are described in
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.
2:
The value of RF varies with the Oscillator mode
selected.
C1
C2
Quartz
RS(1)
OSC1/CLKIN
RF(2)
Sleep
To Internal
Logic
PIC MCU
Crystal
OSC2/CLKOUT