Wireline transceiver with internal and external clock generation

1. An integrated circuit device (IC) having functional circuitry driven by a clock signal, the IC comprising onboard clock generation circuitry, the onboard clock generation circuitry comprising:

an input configured to accept a frequency reference signal;

at least one variable loading capacitor coupled to the input for converting the frequency reference signal into a calibrated clock signal;

calibration circuitry configured to calibrate the at least one variable loading capacitor based on a reference voltage, the calibration circuitry including:

a source of constant reference voltage,

a source of constant current, and

a pair of complementary switches coupled to the constant current source and to a variable reference capacitor, the complementary switches configured to be clocked according to the frequency reference signal to act as a resistor to convert the constant current to a derived voltage that charges a selected variable capacitor; and

trimming circuitry configured to convert the derived voltage into the calibrated clock signal.

2. The IC of claim 1 wherein the input configured to accept a frequency reference signal is configured to accept a crystal resonator signal.

3. The IC of claim 1 wherein:

the frequency reference signal is a differential signal;

the at least one variable loading capacitor comprises a respective variable loading capacitor coupled to each respective differential leg of the differential frequency reference signal; and

the calibration circuitry is configured to calibrate both of the respective variable loading capacitors.

4. The IC of claim 1 wherein the circuitry configured to convert the derived voltage into the calibrated clock signal comprises:

calibration logic circuitry configured to vary a capacitance setting of the selected variable capacitor; and

a comparator configured to change states when the derived voltage passes through the reference voltage, the calibration logic circuitry setting the at least one variable loading capacitor to the capacitance of the selected variable capacitor when the comparator changes state.

5. The IC of claim 4 wherein the selected variable capacitor is one of the at least one variable loading capacitor.

6. The IC of claim 5 wherein:

the frequency reference signal is a differential signal;

the at least one variable loading capacitor comprises a respective variable loading capacitor coupled to each respective differential leg of the differential frequency reference signal; and

the calibration logic circuitry sets each respective variable loading capacitor to the capacitance of the selected variable capacitor when the comparator changes state.

7. The IC of claim 4 wherein the selected variable capacitor is a replica of one of the at least one variable loading capacitor.

8. The IC of claim 7 wherein:

the frequency reference signal is a differential signal;

the at least one variable loading capacitor comprises a respective variable loading capacitor coupled to each respective differential leg of the differential frequency reference signal; and

the calibration logic circuitry sets each respective variable loading capacitor to the capacitance of the replica capacitor when the comparator changes state.

9. The IC of claim 4 wherein the calibration logic circuitry is on the integrated circuit device.

10. The IC of claim 4 wherein the calibration logic circuitry is external to the integrated circuit device.

11. The IC of claim 4 wherein the calibration logic circuitry implements a finite state machine.

12. The IC of claim 4 wherein the complementary switches are configured to be clocked at a frequency of the frequency reference signal.

13. The IC of claim 4 wherein the complementary switches are configured to be clocked at a frequency derived from the frequency reference signal.

14. A method of generating a clock signal onboard an integrated circuit device, the method comprising:

accepting input of a frequency reference signal;

calibrating at least one variable loading capacitor coupled to the frequency reference signal by:

coupling a pair of complementary switches to a constant current source and to a variable reference capacitor, and

clocking the complementary switches according to the frequency reference signal to act as a resistor to convert the constant current to a derived voltage that charges a selected variable capacitor; and

converting the frequency reference signal into a calibrated clock signal using the at least one variable loading capacitor.

15. The method of claim 14 wherein accepting input of a frequency reference signal comprises accepting input of a crystal resonator signal.

16. The method of claim 14 wherein:

accepting input of a frequency reference signal comprises accepting input of a differential frequency reference signal;

converting the frequency reference signal into a calibrated clock signal using a loading capacitor coupled to the frequency reference signal comprises using a respective variable loading capacitor coupled to each respective differential leg of the differential frequency reference signal; and

calibrating the at least one variable loading capacitor comprises calibrating both of the respective variable loading capacitors.

17. The method of claim 14 wherein calibrating the at least one variable loading capacitor further comprises:

varying a capacitance setting of the selected variable capacitor;

comparing magnitude of the derived voltage to a reference voltage; and

setting the at least one variable loading capacitor to the capacitance of the selected variable capacitor when the magnitude of the derived voltage passes through a reference voltage.

18. The method of claim 17 wherein clocking the complementary switches according to the frequency reference signal to act as the resistor to convert the constant current to the derived voltage that charges the selected variable capacitor comprises clocking the complementary switches according to the frequency reference signal to act as the resistor to convert the constant current to the derived voltage that charges the at least one variable loading capacitor.

19. The method of claim 18 wherein, when the frequency reference signal is a differential signal, setting the at least one variable loading capacitor to the capacitance of the selected variable capacitor when the magnitude of the derived voltage passes through a reference voltage comprises setting capacitance of each one of a respective variable loading capacitor coupled to each respective differential leg of the differential frequency reference signal to the capacitance of the selected variable capacitor when the magnitude of the derived voltage passes through the reference voltage.

20. The method of claim 17 wherein coupling the pair of complementary switches to the constant current source and to the variable reference capacitor comprises coupling the pair of complementary switches to the constant current source and to a replica of one of the at least one variable loading capacitor.

21. The method of claim 20 wherein, when the frequency reference signal is a differential signal, setting the at least one variable loading capacitor to the capacitance of the selected variable capacitor when the magnitude of the derived voltage passes through the reference voltage comprises setting a respective variable loading capacitor coupled to each respective differential leg of the differential frequency reference signal to the capacitance of the replica capacitor when the magnitude of the derived voltage passes through the reference voltage.

22. The method of claim 17 wherein clocking the complementary switches according to the frequency reference signal comprises clocking the complementary switches at a frequency of the frequency reference signal.

23. The method of claim 17 wherein clocking the complementary switches according to the frequency reference signal comprises clocking the complementary switches at a frequency derived from the frequency reference signal.