Oscillator amplifier with input clock detection and method therefor

A method for providing an oscillating output signal in a circuit, the method comprising:

• providing a crystal oscillator amplifier having only two clock terminals which are respectively an input clock terminal and an output clock terminal, the input clock terminal for receiving either a first clock signal generated from a crystal or receiving a second clock signal generated by a non-crystal source;
• selectively amplifying with the crystal oscillator amplifier the first clock signal or the second clock signal to provide an amplified clock signal;
• using the amplified clock signal to generate a control signal;
• comparing with a comparator circuit either the first clock signal or the second clock signal with a third clock signal to provide the oscillating output signal;
• and
• using the control signal to ensure that signal inputs to the comparator circuit repeatedly cross each other over time.

An oscillator circuit implemented in an integrated circuit, comprising:

• a crystal oscillator amplifier having only two clock terminals, one of the only two clock terminals being an input terminal and the other being an output terminal, the input terminal receiving either a first clock signal generated from a crystal or a second clock signal generated by a non-crystal source;
• control circuitry coupled to the output terminal of the crystal oscillator amplifier, the control circuitry comprising an inverter having an input coupled to the output terminal of the crystal oscillator amplifier and an output, a capacitor having a first terminal coupled to a first power supply voltage terminal and a second terminal coupled to an output node, a first transistor having a first current electrode coupled to the first power supply voltage terminal, a control electrode coupled to the output of the inverter, and a second current electrode coupled to the output node, and a resistive device coupled in series with the capacitor and first transistor between the output node and a second power supply voltage terminal, the control circuitry providing a control signal at the output node;
• and
• clock generation circuitry coupled to the crystal oscillator amplifier for providing an oscillating output signal in response to an enable signal.