Frequency locking oscillator
A delay line of individually selectable delay elements can operate as an oscillator in an open loop mode to track process variation or drive a clock signal that varies with temperatures and voltages in the system. The delay line oscillator can also operate in a closed loop mode to match a frequency given by a tuner ratio and a reference clock. The delay line can also be used for measuring clock jitter or duty cycle.
1. A system comprising an oscillator, wherein the oscillator comprises:
a delay line comprising:
a plurality of delay elements connected in a cascade from a first delay element to a last delay element, wherein each delay element after the first delay element is configured to time delay a signal by a time period twice as long as the time period of the prior delay element in the cascade; and
a plurality of selecting elements configured to, for each delay element following the first delay element in the cascade, add or remove the delay element to a delay path through the delay line;
a delay select register configured to store a value indicative of a total delay for the delay line, wherein the delay select register is coupled to each of the selecting elements; and
an inverter configured to receive an output of the last delay element, wherein the inverter comprises an output coupled to the first delay element and the delay select register.
2. The system of claim 1 , further comprising a tuner coupled to the delay select register and the output of the inverter, wherein the tuner comprises:
a reference clock input;
a tuner ratio input; and
a comparator configured to determine a difference between a frequency of an oscillator signal from the output of the inverter divided by a tuner ratio value from the tuner ratio input and a frequency of a reference clock signal from the reference clock input;
wherein the tuner is configured to adjust the value on the delay select register based on the difference determined by the comparator.
3. The system of claim 2 , wherein the tuner is configured to decrease the value on the delay select register if the difference determined by the comparator indicates that the frequency of the oscillator signal divided by the tuner ratio value is less than the frequency of the reference clock signal, and wherein the tuner is configured to increase the value on the delay select register if the difference determined by the comparator indicates that the frequency of the oscillator signal divided by the tuner ratio value is greater than the frequency of the reference clock signal.
4. The system of claim 1 , wherein each delay element is non-inverting.
5. The system of claim 1 , wherein the first delay element is configured to time delay a signal by a minimum time delay, wherein the minimum time delay is longer than a critical time delay between the delay select register and outputs of the selecting elements.
6. The system of claim 1 , further comprising a target device coupled to the output of the inverter, wherein each delay element comprises a standard cell selected from a library of standard cells, and wherein the target device comprises a plurality of cells selected from the library of standard cells.
7. The system of claim 1 , further comprising a selecting element having an output coupled to the input of the inverter, wherein the selecting element is configured to select between the output of the last delay element and a measurement clock signal.
8. The system of claim 7 , further comprising a counting circuit coupled to output of the last delay element and the measurement clock signal.
9. The system of claim 8 , wherein the counting circuit comprises:
a first flip-flop coupled to the measurement clock signal and the output of the last delay element;
logic coupled to an output of the first flip-flop; and
a second flip-flop coupled to the measurement clock signal and the output of the logic, wherein an output of the second flip-flip is coupled to the logic.
10. A system comprising an oscillator, wherein the oscillator comprises:
a delay line comprising:
a plurality of delay elements connected in a cascade from a first delay element to a last delay element; and
a plurality of selecting elements configured to, for each delay element following the first delay element in the cascade, add or remove the delay element to a delay path through the delay line;
a delay select register configured to store a value indicative of a total delay for the delay line, wherein the delay select register is coupled to each of the selecting elements;
an inverter configured to receive an output of the last delay element, wherein the inverter comprises an output coupled to the first delay element and the delay select register;
a selecting element having an output coupled to the input of the inverter, wherein the selecting element is configured to select between the output of the last delay element and a measurement clock signal;
a counting circuit coupled to output of the last delay element and the measurement clock signal, wherein the counting circuit comprises:
a first flip-flop coupled to the measurement clock signal and the output of the last delay element;
logic coupled to an output of the first flip-flop; and
a second flip-flop coupled to the measurement clock signal and the output of the logic, wherein an output of the second flip-flip is coupled to the logic; and
a frequency meter circuit configured to measure the number of times that the second flip-flop toggles over a given number of clock cycles of the measurement clock.
11. A system comprising an oscillator, wherein the oscillator comprises:
a delay line comprising:
a plurality of delay elements connected in a cascade from a first delay element to a last delay element; and
a plurality of selecting elements configured to, for each delay element following the first delay element in the cascade, add or remove the delay element to a delay path through the delay line;
a delay select register configured to store a value indicative of a total delay for the delay line, wherein the delay select register is coupled to each of the selecting elements;
an inverter configured to receive an output of the last delay element, wherein the inverter comprises an output coupled to the first delay element and the delay select register; and
a selecting element having an output coupled to the input of the inverter, wherein the selecting element is configured to select between the output of the last delay element and an output of a scanning flip-flop.
12. A method performed by a system comprising a oscillator, the oscillator comprising a plurality of delay elements connected in a cascade, each delay element comprising a standard cell selected from a library of standard cells, the method comprising:
receiving a delay value;
configuring a plurality of selecting elements to select delay elements to be included in a delay path through the delay elements, the delay path having a total delay based on the delay value, wherein the delay elements are connected in a cascade from a first delay element to a last delay element, and each delay element after the first delay element is configured to time delay a signal by a time period twice as long as the time period of the prior delay element in the cascade;
receiving an input signal at an input to the oscillator;
inverting an output signal from an output of the oscillator and providing the inverted output signal to the input of the oscillator; and
providing the output signal of the oscillator to a target device comprising another standard cell from the library of standard cells.
13. The method of claim 12 , further comprising:
receiving a tuner ratio value and a reference clock signal;
determining a difference between the frequency of the output signal divided by the tuner ratio value and the frequency of the reference clock signal; and
configuring the selecting elements to define a new delay path based on the difference.
14. The method of claim 13 , wherein configuring the selecting elements to define the new delay path based on the difference comprises:
adding delay elements to the delay path if the frequency of the output signal divided by the tuner ratio value exceeds the frequency of the reference clock signal; and
removing delay elements from the delay path if the frequency of the output signal divided by the tuner ratio value is less than the frequency of the reference clock signal.
15. The method of claim 13 , further comprising executing a binary search to configure the selecting elements to minimize the difference.