Vault 7: Projects

This publication series is about specific projects related to the Vault 7 main publication.

PIC24F Family Reference Manual
DS39700C-page 6-14 © 2009 Microchip Technology Inc.
6.5.1 Selecting a Primary Oscillator Mode
The main difference between the XT and HS modes is the gain of the internal inverter of the oscil-
lator circuit, which allows the different frequency ranges. The XT 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 XT Oscillator modes.
The EC and HS modes that use the PLL circuit provide the highest device operating frequencies.
The oscillator circuit will consume the most current in these modes because the PLL is enabled
to multiply the frequency of the oscillator by 4.
In general, users should select the oscillator option with the lowest possible gain that still meets
their specifications. 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 output pin. When the OSC2 pin is used as a clock output pin, the output
frequency is F
OSC/2.
6.5.2 Crystal Oscillators and Ceramic Resonators
In XT and HS modes, a crystal or ceramic resonator is connected to the OSC1 and OSC2 pins
to establish oscillation (Figure 6-3). The PIC24F 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.
6.5.2.1 OSCILLATOR/RESONATOR START-UP
As the device voltage increases from VSS, the oscillator will start its oscillations. The time
required for the oscillator 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 gain of the internal oscillator inverter)
Crystal quality
Oscillator circuit layout
System noise

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