Crystal oscillator

A device comprising:

• a control circuit for controlling a value of a charge capacitor of a crystal oscillator, the control circuit comprising:
• a register configured to receive a clock signal and an internal value, and configured to, at each beginning of a period of the clock signal, update and store a control value from the internal value, the control value being supplied to the charge capacitor;
• a subtractor configured to subtract the control value from an operating target value to produce a subtraction result;
• a saturation circuit configured to receive the subtraction result from the subtractor and to produce a modification value, wherein:
• the modification value is equal to the subtraction result when the subtraction result is between a first positive threshold and a second negative threshold,
• the modification value is equal to the first positive threshold when the subtraction result is greater than or equal to the first positive threshold, and
• the modification value is equal to the second negative threshold when the subtraction result is smaller than or equal to the second negative threshold;
• and
• an adder configured to receive the modification value from the saturation circuit and to add the modification value to the control value to produce the internal value,
• wherein the first positive threshold and the second negative threshold have a same absolute value determined so that oscillations of the crystal oscillator are maintained at each update of the control value, the oscillations being maintained when a ratio gm/gmcrit is greater than or equal to 2, with gm being a transconductance of the crystal oscillator and gmcrit being a critical transconductance of the crystal oscillator determined by a current value of the charge capacitor, a frequency of the oscillations, an equivalent series resistance value of a crystal of the crystal oscillator, and a value of a shunt capacitor of the crystal of the crystal oscillator.

A method of controlling a value of a charge capacitor of a crystal oscillator, the method comprising:

• receiving, by a control circuit, an operating target value;
• at each beginning of a period of a clock signal, updating and storing, by the control circuit, a control value of the value of the charge capacitor from an internal value of the control circuit;
• and
• at each period of the clock signal:
• subtracting, by the control circuit, the control value from the operating target value to produce a subtraction result;
• determining, by the control circuit, a modification value equal to:
• the subtraction result when the subtraction result is between a first positive threshold and a second negative threshold,
• the first positive threshold when the subtraction result is greater than or equal to the first positive threshold, and
• the second negative threshold when the subtraction result is smaller than or equal to the second negative threshold;
• and
• calculating, by the control circuit, the internal value by adding the modification value to the control value,
• wherein the first positive threshold and the second negative threshold have a same absolute value determined so that oscillations of the crystal oscillator are maintained at each update of the control value, the oscillations being maintained when a ratio gm/gmcrit is greater than or equal to 2, with gm being a transconductance of the crystal oscillator and gmcrit being a critical transconductance of the crystal oscillator determined by a current value of the charge capacitor, a frequency of the oscillations, an equivalent series resistance value of a crystal of the crystal oscillator, and a value of a shunt capacitor of the crystal of the crystal oscillator.