Microchip Technology DM183037 Data Sheet
PIC18F97J94 FAMILY
DS30575A-page 50
2012 Microchip Technology Inc.
3.5.1
SELECTING A PRIMARY
OSCILLATOR MODE
OSCILLATOR MODE
The main difference between the MS and HS modes is
the gain of the internal inverter of the oscillator circuit,
which allows the different frequency ranges. The MS
mode is a medium power, medium frequency mode.
HS mode provides the highest oscillator frequencies
with a crystal. OSC2 provides crystal feedback in both
HS and MS Oscillator modes.
The EC and HS modes that use the PLL circuit provide
the highest device operating frequencies. The oscilla-
tor circuit will consume the most current in these modes
because the PLL is enabled to multiply the frequency of
the oscillator.
In general, users should select the oscillator option with
the lowest possible gain that still meets their specifica-
tions. This will result in lower dynamic currents (I
the gain of the internal inverter of the oscillator circuit,
which allows the different frequency ranges. The MS
mode is a medium power, medium frequency mode.
HS mode provides the highest oscillator frequencies
with a crystal. OSC2 provides crystal feedback in both
HS and MS Oscillator modes.
The EC and HS modes that use the PLL circuit provide
the highest device operating frequencies. The oscilla-
tor circuit will consume the most current in these modes
because the PLL is enabled to multiply the frequency of
the oscillator.
In general, users should select the oscillator option with
the lowest possible gain that still meets their specifica-
tions. This will result in lower dynamic currents (I
DD
).
The frequency range of each oscillator mode is the
recommended frequency cutoff, but the selection of a
different gain mode is acceptable as long as a thorough
validation is performed (voltage, temperature and
component variations, such as resistor, capacitor and
internal oscillator circuitry).
The oscillator feedback circuit is disabled in all EC
modes. The OSC1 pin is a high-impedance input and
can be driven by a CMOS driver.
If the Primary Oscillator is configured for an external
clock input, the OSC2 pin is not required to support the
oscillator function. For these modes, the OSC2 pin can
be used as an additional device I/O pin or a clock out-
put pin. When the OSC2 pin is used as a clock output
pin, the output frequency is F
recommended frequency cutoff, but the selection of a
different gain mode is acceptable as long as a thorough
validation is performed (voltage, temperature and
component variations, such as resistor, capacitor and
internal oscillator circuitry).
The oscillator feedback circuit is disabled in all EC
modes. The OSC1 pin is a high-impedance input and
can be driven by a CMOS driver.
If the Primary Oscillator is configured for an external
clock input, the OSC2 pin is not required to support the
oscillator function. For these modes, the OSC2 pin can
be used as an additional device I/O pin or a clock out-
put pin. When the OSC2 pin is used as a clock output
pin, the output frequency is F
OSC
/4.
3.6
Crystal Oscillators and Ceramic
Resonators
Resonators
In MS and HS modes, a crystal or ceramic resonator is
connected to the OSC1 and OSC2 pins to establish
oscillation (
connected to the OSC1 and OSC2 pins to establish
oscillation (
). The PIC18F oscillator design
requires the use of a parallel cut crystal. Using a series
cut crystal may give a frequency out of the crystal
manufacturer’s specifications.
cut crystal may give a frequency out of the crystal
manufacturer’s specifications.
3.6.1
OSCILLATOR/RESONATOR
START-UP
START-UP
As the device voltage increases from V
SS
, the oscillator
will start its oscillations. The time required for the oscil-
lator to start oscillating depends on many factors,
including:
• Crystal/resonator frequency
• Capacitor values used
• Series resistor, if used, and its value and type
• Device V
lator to start oscillating depends on many factors,
including:
• Crystal/resonator frequency
• Capacitor values used
• Series resistor, if used, and its value and type
• Device V
DD
rise time
• System temperature
• Oscillator mode selection of device (selects the
• Oscillator mode selection of device (selects the
gain of the internal oscillator inverter)
• Crystal quality
• Oscillator circuit layout
• System noise
The course of a typical crystal or resonator start-up is
shown in
• Oscillator circuit layout
• System noise
The course of a typical crystal or resonator start-up is
shown in
. Notice that the time to achieve
stable oscillation is not instantaneous.
FIGURE 3-4:
EXAMPLE OSCILLATOR/RESONATOR START-UP CHARACTERISTICS
Voltage
Crystal Start-up Time
Time
Device V
DD
Maximum V
DD
of System
0V
V
IL
V
IH