© 2004 Microchip Technology Inc. DS70054C-page 7-11
Section 7. Oscillator
Oscillator
7
7.6.2 Tuning the Oscillator Circuit
Since Microchip devices have wide operating ranges (frequency, voltage and temperature;
depending on the part and version ordered) and external components (crystals, capacitors,...) of
varying quality and manufacture, validation of operation needs to be performed to ensure that
the component selection will comply with the requirements of the application.
There are many factors that go into the selection and arrangement of these external components.
These factors include:
• amplifier gain
• desired frequency
• resonant frequency(s) of the crystal
• temperature of operation
• supply voltage range
• start-up time
• stability
• crystal life
• power consumption
• simplification of the circuit
• use of standard components
• component count
7.6.3 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.
Remember also 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:
• a low frequency design (with a Low Gain Clock mode)
• a 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
Noise actually helps lower the oscillator start-up time since it provides a “kick start” to the
oscillator.
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