Vault 7: Projects

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

© 2009 Microchip Technology Inc. DS39700C-page 6-15
Section 6. Oscillator
Oscillator
6
The course of a typical crystal or resonator start-up is shown in Figure 6-4. Notice that the time
to achieve stable oscillation is not instantaneous.
Figure 6-4: Example Oscillator/Resonator Start-up Characteristics
6.5.2.1.1 Primary Oscillator Start-up from Sleep Mode
The most difficult time for the oscillator to start-up is when waking up from Sleep mode. This is
because the load capacitors have both partially charged to some quiescent value and phase
differential at wake-up is minimal. Thus, more time is required to achieve stable oscillation. Also
remember that low-voltage, high temperatures and the lower frequency clock modes also impose
limitations on loop gain, which in turn, affects start-up.
Each of the following factors increases the start-up time:
Low-frequency design (with a Low Gain Clock mode)
Quiet environment (such as a battery operated device)
Operating in a shielded box (away from the noisy RF area)
Low voltage
High temperature
Wake-up from Sleep mode
Circuit noise, on the other hand, may actually help to “kick start” the oscillator and help to lower
the oscillator start-up time.
6.5.2.2 OSCILLATOR START-UP TIMER
In order to ensure that a crystal oscillator (or ceramic resonator) has started and stabilized, an
Oscillator Start-up Timer (OST) is provided. The OST is a simple, 10-bit counter that counts
1024 T
OSC cycles before releasing the oscillator clock to the rest of the system. This time-out
period is designated as T
OST. The amplitude of the oscillator signal must reach the VIL and VIH
thresholds for the oscillator pins before the OST can begin to count cycles.
The T
OST interval is required every time the oscillator has to restart (i.e., on POR, BOR and
wake-up from Sleep mode). The Oscillator Start-up Timer is applied to the XT and HS modes for
the Primary Oscillator, as well as the Secondary Oscillator, SOSC (see Section 6.7 “Secondary
Oscillator (SOSC)”).
Voltage
Crystal Start-up Time
Time
Device V
DD
Maximum VDD of System
0V
VIL
VIH

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