Prepackaged crystal oscillators are a well known and widely used commodity within the electronics industry. Conventional prepackaged crystal oscillators have as their minimum electrical interface four pins: power (VCC), ground (GND), output enable (OE), and oscillator output (XO). The components comprising an industry standard prepackaged crystal oscillator system include a crystal (to provide the frequency reference), oscillator amplifier (to excite the crystal to resonance), buffer amplifier (to isolate the output load from the oscillator), output limiting resistor (to limit power dissipation in the crystal), trim capacitors (to properly trim the resulting crystal oscillator output frequency), and a package enclosure having a minimum of four interface pins for the VCC, GND, OE, and XO connections. Common package enclosures for prepackaged crystal oscillators include hermetically sealed steel enclosures and a variety of molded epoxy/plastic configurations. By the very nature of the construction process used in industry standard crystal oscillators the calibration and configuration of the oscillator must be performed prior to enclosure or encapsulation.
VCC and GND pins provide power for the oscillator circuitry. The XO pin is driven by an internal crystal oscillator and represents the usable output of the crystal oscillator system. This XO output is usually the output of a digital buffer which isolates the internal crystal oscillator system from the load being driven by the prepackaged crystal oscillator system. The OE pin controls the activity state of the crystal oscillator system. When the OE pin is at VCC potential (logic high level), the oscillator is enabled and the XO pin is driven by the internal crystal oscillator. When the OE pin is at GND potential (logic low level), the oscillator is disabled and the crystal oscillator system is placed in a low power standby mode.
It is widely known and accepted by those skilled in the art that the OE pin can be configured to have a negative logic sense to the one just described, such that OE at VCC potential (logic high level) disables the crystal oscillator operation and OE at GND potential (logic low level) enables the crystal oscillator operation. In this configuration the OE signal is usually renamed to STANDBY (SB) to indicate the logic sense reversal. This variation in logic sense is one of interface convenience and has no fundamental impact on the overall operation of the prepackaged crystal oscillator system other than the fact that an additional digital inverter is necessary to convert the external SB signal with active high sense to an internal OE signal of active high sense. Throughout the remainder of this document the nomenclature logically active and logically inactive will be used to denote the activity state of various signals with the implicit assertion that the signals in question may be implemented with either a positive logic sense in which VCC voltage levels represent a logic 1 (logically active/asserted) and GND voltage levels represent a logic 0 (logically inactive/deasserted), or correspondingly a negative logic sense in which VCC voltage levels represent a logic 0 (logically inactive/deasserted) and GND voltage levels represent a logic 1 (logically active/asserted)