Using crystals in embedded applications - Part 3

In the previous articles, we have discussed about external components used around crystal for stable output frequency signal generation. We also, discussed about the load capacitance and how to match this load capacitance with the external reactance components. The input-output capacitance, output resistance determine the reactive components outside. Here are some few more points to be considered in this aspect:

• The internal CMOS inverter gate inside the processors/controllers and other main integrated circuits are generally operated in active region. The gate must be designed such that it must overcome loss in the external feedback resistor and frequency decisive components (Damping resistor, external load capacitance).
• As the CMOS inverter gate is operated as a linear amplifier, the characteristics like phase and propagation delay also to be considered.
• Just to throw a small a small light on CMOS inverter, it is a combination of P-MOSFET and N-Mosfet.

• The above circuit is nothing but a unbuffered inverter circuit.

Parameters to consider for crystal oscillator selection:

• Operating frequency
• Supply voltage (decides logic level)
• Operating temperature
• Storage temperature
• Tolerance/Stability
• Rise/Fall time
• Load drive capability (in terms of capacitance, pF)
• Desired output logic type
• RMS Jitter
• Start up time