Oscillator calibration

A system comprising:

• an integrated circuit including a clock tree;
• a frequency locked loop circuit that generates a system clock that drives the clock tree;
• wherein the frequency locked loop circuit has an oscillator circuit comprising:
• a first oscillator supplied with a first supply voltage;
• a second oscillator supplied with a second supply voltage;
• a calibration circuit operable to calibrate the first and second oscillators by:
• (a) setting a frequency control of the second oscillator at a target frequency;
• (b) after (a), adjusting a voltage control of the second supply voltage incrementally until a first control value is identified at which a second oscillator output frequency matches the target frequency;
• (c) after (b), setting a voltage control of the first supply voltage to the first control value;
• and
• (d) after (c), adjusting the voltage control for the first supply voltage incrementally until a second control value is identified at which a first oscillator output frequency is as close to the second oscillator output frequency as is achievable, but does not exceed it.

A method of calibrating an oscillator circuit comprising:

• (a) setting a frequency control input of a first oscillator to a target frequency;
• (b) setting a frequency control input of a second oscillator to the target frequency;
• (c) after (b), adjusting a voltage control of a second supply voltage, which supplies the second oscillator, incrementally until a first control value is identified at which a second oscillator output frequency matches the target frequency;
• (d) after (c), setting a voltage control of a first supply voltage, supplying the first oscillator, to the first control value;
• and
• (e) after (d), adjusting the voltage control for a first supply voltage incrementally until a second control value is identified at which a first oscillator output frequency is as close to the second oscillator output frequency as is achievable, but does not exceed it.