Ring oscillator based RC calibration circuit
A calibration operation adjusts a frequency of a ring oscillator to a desired frequency by adjusting programmable RC circuits in the stages of the ring oscillator. The programmable RC circuits have programmable capacitors, resistors, or both. The RC circuits account for most of the delay through the ring oscillator. Another circuit with its own RC time constant is calibrated based on the adjustments made to the RC circuits in the ring oscillator to achieve the desired frequency.
1. A method of performing a calibration operation comprising:
operating a ring oscillator with RC time constants in respective stages of the ring oscillator and generating an oscillator output signal, the RC time constants being adjustable;
dividing the oscillator output signal to generate a divided oscillator signal;
asserting a load signal responsive to assertion of an enable signal and a transition of the divided oscillator signal, the enable signal allowing the ring oscillator to oscillate;
loading a starting count value corresponding to a desired frequency into a counter responsive to the load signal;
counting down the starting count value during a period of the divided oscillator signal to determine if the ring oscillator is oscillating above or below the desired frequency;
if a count value of the counter goes to zero before an end of the period of the divided oscillator signal as indicated by a first value of a carry-out bit of the counter, thereby indicating an oscillation frequency of the ring oscillator is above the desired frequency, increasing the RC time constants to decrease the oscillation frequency of the ring oscillator, wherein after reaching zero the count value staying at zero during the period of the divided oscillator signal;
if the count value is non-zero at the end of the period of the divided oscillator signal as indicated by a second value of a carry-out bit of the counter, thereby indicating the oscillation frequency is below the desired frequency, decreasing the RC time constants to increase the oscillation frequency of the ring oscillator;
adjusting the RC time constants until the oscillation frequency of the ring oscillator is at the desired frequency, the RC time constants having a final value when the oscillation frequency is at the desired frequency; and
adjusting another RC time constant in another circuit based on the final value of the RC time constants.
2. The method as recited in claim 1 further comprising:
adjusting one or more variable capacitors in the respective stages of the ring oscillator to adjust the RC time constants.
3. The method as recited in claim 1 further comprising:
adjusting one or more variable resistors in the respective stages of the ring oscillator to adjust the RC time constants.
4. The method as recited in claim 1 further comprising:
wherein inverters contribute two percent or less delay through the ring oscillator at nominal values of the RC time constants.
5. The method as recited in claim 1 further comprising adjusting a capacitor forming a part of the other circuit to adjust the other RC time constant.
6. The method as recited in claim 1 , wherein the other circuit is a filter or a delta-sigma modulator.
7. The method as recited in claim 1 further comprising performing the calibration operation at power up of an integrated circuit on which the ring oscillator and the other circuit are disposed.
8. An integrated circuit comprising:
a ring oscillator including an odd number of inverter stages, each inverter stage including,
an inverter; and
an RC circuit having an RC time constant that is adjustable;
a divider to divide an output signal of the ring oscillator and provide a divided ring oscillator signal;
a counter to load an initial count value corresponding to a desired oscillation frequency and to count down the initial count value for a period of the divided ring oscillator signal;
wherein if a count value of the counter goes to zero before an end of the period of the divided ring oscillator signal, thereby indicating an oscillation frequency of the ring oscillator is above the desired oscillation frequency, the RC time constant of each inverter stage is increased to decrease the oscillation frequency of the ring oscillator, and wherein after reaching zero, the count value stays at zero during the period of the divided ring oscillator signal;
wherein if the count value is non-zero at the end of the period of the divided ring oscillator signal, thereby indicating the oscillation frequency is below the desired oscillation frequency, the RC time constant of each inverter stage is decreased to increase the oscillation frequency of the ring oscillator;
wherein the RC time constant of each inverter stage is adjusted until an oscillation frequency of the ring oscillator is at a desired oscillation frequency; and
a second circuit independent of the ring oscillator and having a variable circuit parameter calibrated based on adjustments made to the RC time constant of each inverter stage to achieve the desired oscillation frequency.
9. The integrated circuit as recited in claim 8 , wherein the variable circuit parameter is another RC time constant.
10. The integrated circuit as recited in claim 8 further comprising:
one or more variable capacitors in the RC circuit that are adjustable to adjust the RC time constant.
11. The integrated circuit as recited in claim 8 further comprising:
one or more variable resistors in the RC circuit that are adjustable to adjust the RC time constant.
12. The integrated circuit as recited in claim 8 wherein,
delay due to the RC time constant in each of the inverter stages dominates delay due to active elements in each of the inverter stages.
13. The integrated circuit as recited in claim 8 , wherein the ring oscillator and the second circuit are calibrated responsive to a power on condition.
14. The integrated circuit as recited in claim 8 ,
wherein the ring oscillator is coupled to an enable signal that allows the ring oscillator to oscillate responsive to the enable signal being asserted; and
wherein a load signal is asserted to load the initial count value into the counter responsive to the enable signal being asserted and a transition of the divided ring oscillator signal.
15. The integrated circuit as recited in claim 8 ,
wherein the count value going to zero before the end of the period of the divided ring oscillator signal is indicated by a first value of a carry-out bit of the counter and the count value being non-zero at the end of the period of the divided ring oscillator signal is indicated by a second value of the carry-out bit of the counter.
16. An integrated circuit comprising:
a ring oscillator including a plurality of inverting stages having programmable capacitor circuits disposed in each of the inverting stages, the ring oscillator supplying an oscillator output signal;
wherein the ring oscillator is coupled to an enable signal that allows the ring oscillator to oscillate responsive to the enable signal being asserted;
a calibration controller to measure an oscillation frequency of the oscillator output signal and to adjust the programmable capacitor circuits until the oscillation frequency is at a desired oscillation frequency, the calibration controller supplying the enable signal, the calibration controller including,
a divider to divide the oscillator output signal of the ring oscillator and provide a divided ring oscillator signal;
a counter to load an initial count value corresponding to the desired oscillation frequency and to count down the initial count value for a period of the divided ring oscillator signal;
wherein a load signal is asserted to load the initial count value into the counter responsive to the enable signal being asserted and a transition of the divided ring oscillator signal;
a functional circuit independent of the ring oscillator and having a variable capacitor circuit; and
wherein the variable capacitor circuit is calibrated based on adjustments made to the programmable capacitor circuits to achieve the desired oscillation frequency.
17. The integrated circuit as recited in claim 16 , wherein the desired oscillation frequency is programmable.
18. The integrated circuit as recited in claim 16 , wherein the functional circuit is a filter or a delta-sigma modulator.
19. The integrated circuit as recited in claim 16 ,
wherein if a count value of the counter goes to zero before an end of the period of the divided ring oscillator signal, thereby indicating the oscillation frequency is above the desired oscillation frequency, the calibration controller increases capacitance of the programmable capacitor circuits to decrease the oscillation frequency of the ring oscillator, and wherein after reaching zero the count value stays at zero during the period of the divided ring oscillator signal; and
wherein if the count value is non-zero at the end of the period of the divided ring oscillator signal thereby indicating the oscillation frequency is below the desired oscillation frequency, the calibration controller decreases the capacitance of the programmable capacitor circuits to increase the oscillation frequency of the ring oscillator.
20. The integrated circuit as recited in claim 19 , wherein the count value going to zero before the end of the period of the divided ring oscillator signal is indicated by a first value of a carry-out bit of the counter and the count value being non-zero at the end of the period of the divided ring oscillator signal is indicated by a second value of the carry-out bit of the counter.