Bias control circuit for crystal oscillator

A crystal oscillator system comprising:

• a crystal oscillator circuit including:
• a buffer and filter circuit configured to provide a filtered reference signal,
• a peak detector circuit configured to generate a peak envelope signal,
• a bias amplifier coupled to generate a bias current based on a bias current code, the filtered reference signal, and the peak envelope signal,
• a crystal oscillator amplifier configured to generate oscillations for a crystal based on the bias current generated by the bias amplifier, and
• a Schmitt trigger circuit coupled to the crystal oscillator amplifier and configured to generate clock signals;
• and
• a bias control circuit coupled to the crystal oscillator circuit, the bias control circuit including:
• a clock cycle counter configured to count a number of cycles of the clock signals and generate a trigger signal,
• a clock comparator configured to compare the peak envelope signal to a programmable reference swing voltage in response to the trigger signal, and
• a bias current modifier circuit configured to generate an optimized bias current code based on an output of the clock comparator.

A method for controlling a crystal oscillator, the method comprising:

• initiating a digital calibration process at startup of the crystal oscillator by setting a bias current code to a maximum value;
• counting a number of oscillation cycles of the crystal oscillator;
• comparing, after the number of oscillation cycles, a measured oscillation swing to a reference swing;
• decreasing the bias current code in response to the measured oscillation swing being greater than the reference swing;
• increasing the bias current code in response to the measured oscillation swing being less than the reference swing, wherein, in response to the bias current code being at the maximum value and the measured oscillation swing being less than the reference swing, the bias current code is set to the maximum value;
• and
• exiting the digital calibration process in response to the measured oscillation swing being less than the reference swing or the bias current code equals a minimum value.