Jitter producing circuitry and methods
To facilitate measurement of the jitter tolerance of circuitry such as serializer/deserializer (SERDES) circuitry, test circuitry is provided that can add jitter to a data signal. The jitter added is preferably controllable and variable with respect to such parameters as jitter frequency (i.e., how rapid is the jitter) and/or amplitude (i.e., how large or great is the amount of the jitter).
1. A method facilitating testing of jitter tolerance of a serializer/deserializer (SERDES) subcircuit of an integrated circuit device comprising:
deriving a periodic signal from a reference clock signal;
counting cycles of the periodic signal so that a count of the cycles alternately increments and decrements between upper and lower limits;
delaying the reference clock signal by an amount that is responsive to the count to produce a clock signal having cyclically varying jitter;
selectively applying the clock signal having cyclically varying jitter to a first SERDES subcircuit of the device to cause the first SERDES subcircuit to output a serial data signal having cyclically varying jitter; and
selectively applying the serial data signal output by the first SERDES subcircuit to a second SERDES subcircuit of the device.
2. The method defined in claim 1 wherein the selectively applying the clock signal comprises:
selectively applying either the reference clock signal or the clock signal having cyclically varying jitter to the first SERDES subcircuit.
3. The method defined in claim 1 wherein the deriving comprises:
dividing a frequency of the reference clock signal by a divisor parameter.
4. The method defined in claim 3 wherein the divisor parameter is variable.
5. The method defined in claim 1 wherein at least one of the upper and lower limits is variable.
6. The method defined in claim 1 wherein the selectively applying the serial data signal output by the first SERDES subcircuit is partly performed external to the device.
7. The method defined in claim 1 further comprising:
selectively applying the clock signal having cyclically varying jitter to the second SERDES subcircuit to cause the second SERDES subcircuit to output a serial data signal having cyclically varying jitter; and
selectively applying the serial data signal output by the second SERDES subcircuit to the first SERDES subcircuit.
8. An integrated circuit device for facilitating testing of jitter tolerance of serializer/deserializer (SERDES) means of the device comprising:
means for deriving a periodic signal from a reference clock signal;
means for counting cycles of the periodic signal so that a count of the cycles alternately increments and decrements between upper and lower limits;
means for delaying the reference clock signal by an amount that is responsive to the count to produce a clock signal having cyclically varying jitter;
first and second SERDES means for performing SERDES functions;
means for selectively applying the clock signal having cyclically varying jitter to the first SERDES means to cause the first SERDES means to output a serial data signal having cyclically varying jitter; and
means for applying a serial data input signal, which can be derived from the serial data signal output by the first SERDES means, to the second SERDES means.
9. The device defined in claim 8 wherein the means for selectively applying comprises:
means for selectively applying either the reference clock signal or the clock signal having cyclically varying jitter to the first SERDES means.
10. The device defined in claim 8 wherein the means for deriving comprises:
means for dividing a frequency of the reference clock signal by a divisor parameter.
11. The device defined in claim 10 wherein the divisor parameter is variable.
12. The device defined in claim 8 wherein at least one of the upper and lower limits is variable.
13. The device defined in claim 8 wherein the means for applying the serial data input signal to the second SERDES means is operable to externally route the serial data signal output by the first SERDES means so that it becomes the serial data input signal of the second SERDES means.
14. The device defined in claim 8 further comprising:
means for selectively applying the clock signal having cyclically varying jitter to the second SERDES means to cause the second SERDES means to output a serial data signal having cyclically varying jitter; and
means for applying a serial data input signal, which can be derived from the serial data signal output by the second SERDES means, to the first SERDES means.
15. An integrated circuit device for facilitating testing of jitter tolerance of a serializer/deserializer (SERDES) subcircuit of the device comprising:
processing circuitry that derives a periodic signal from a reference clock signal;
counter circuitry that counts cycles of the periodic signal so that a count of the cycles alternately increments and decrements between upper and lower limits;
delay circuitry that delays the reference clock signal by an amount that is responsive to the count to produce a clock signal having cyclically varying jitter;
first and second SERDES subcircuits;
clock signal circuitry that selectively applies the clock signal having cyclically varying jitter to the first SERDES subcircuit to cause the first SERDES subcircuit to output a serial data signal having cyclically varying jitter; and
input circuitry that applies a serial data input signal, which can be derived from the serial data signal output by the first SERDES subcircuit, to the second SERDES subcircuit.
16. The device defined in claim 15 wherein the clock signal circuitry comprises:
selection circuitry that applies either the reference clock signal or the clock signal having cyclically varying jitter to the first SERDES subcircuit.
17. The device defined in claim 15 wherein the processing circuitry comprises:
frequency dividing circuitry that divides a frequency of the reference clock signal by a divisor parameter.
18. The device defined in claim 17 wherein the divisor parameter is variable.
19. The device defined in claim 15 wherein at least one of the upper and lower limits is variable.
20. The device defined in claim 15 wherein the input circuitry that applies the serial data input signal to the second SERDES subcircuit is configured to externally route the serial data signal output by the first SERDES subcircuit so that it becomes the serial data input signal of the second SERDES subcircuit.
21. The device defined in claim 15 further comprising:
further clock signal circuitry that selectively applies the clock signal having cyclically varying jitter to the second SERDES subcircuit to cause the second SERDES subcircuit to output a serial data signal having cyclically varying jitter; and
further input circuitry that applies a serial data input signal, which can be derived from the serial data signal output by the second SERDES subcircuit, to the first SERDES subcircuit.