Clock distribution and alignment using a common trace

1. A method for clock alignment in a network element, the method comprising:

distributing, by a timing device, a clock signal to a line card via a trace of a backplane;

sending, by the timing device, a pulse to the line card at a first time, wherein the pulse is being sent to the line card via the trace, and the clock signal and the pulse are received at a same port of the line card;

receiving, by the timing device, a return pulse from the line card at a second time, wherein the return pulse is received via the trace;

determining, by the timing device, a time difference between the first time in which the pulse is being sent from the timing device and the second time in which the return pulse is received by the timing device, wherein the time difference is indicative of a propagation delay associated with the line card and the trace; and

sending, by the timing device, the time difference to the line card, wherein a phase delay offset of the line card is adjustable by the line card using the time difference.

2. The method of claim 1 , wherein sending the time difference to the line card comprises sending, by the timing device, the time difference to the line card using the trace.

3. The method of claim 1 , further comprising:

in response to sending the pulse to the line card:

deactivating, by the timing device, an output mode of the timing device; and

activating, by the timing device, an input mode of the timing device;

in response to receiving the return pulse from the line card:

deactivating, by the timing device, an input mode of the timing device; and

activating, by the timing device, an output mode of the timing device.

4. The method of claim 1 , wherein determining the time difference comprises:

operating a time-to-digital converter (TDC) to detect a rising edge of the pulse being sent from the timing device;

operating the TDC to detect a rising edge of the return pulse being received by the timing device; and

operating the TDC to convert the detected rising edge of the pulse and the detected rising edge of the return pulse into a digital representation of the time difference.

5. A system comprising:

a timing device;

a backplane coupled to the timing device;

a line card coupled to the backplane;

the timing device being configured to:

distribute a clock signal to a line card via a trace of a backplane;

send a pulse to a line card at a first time, wherein the pulse is being sent to the line card via the trace;

receive a return pulse from the line card at a second time, wherein the return pulse is received via the trace;

determine a time difference between the first time in which the pulse is being sent from the timing device and the second time in which the return pulse is received by the timing device, wherein the time difference is indicative of a propagation delay associated with the line card and the trace; and

send the time difference to the line card;

the line card being configured to:

receive the clock signal and the pulse at a same port;

receive the time difference from the timing device; and

adjust a phase delay offset of the line card using the time difference.

6. The system of claim 5 , wherein the timing device is configured to send the time difference to the line card using the trace.

7. The system of claim 5 , wherein the line card is configured to invert the pulse to generate the return pulse.

8. The system of claim 5 , wherein the timing device is further configured to:

in response to the pulse being sent to the line card:

deactivate an output mode of the timing device; and

activate an input mode of the timing device;

in response to the receipt of the return pulse from the line card:

deactivate an input mode of the timing device; and

activate an output mode of the timing device.

9. The system of claim 5 , wherein the line card is further configured to:

in response to the pulse being received at the line card:

deactivate an input mode of the line card; and

activate an output mode of the line card;

in response to the output of the return pulse from the line card:

deactivate an output mode of the line card; and

activate an input mode of the line card.

10. The system of claim 5 , wherein the line card is further configured to determine a residence time indicating a duration in which the pulse resides in the line card, wherein the determination of the phase delay offset is based on time difference and the residence time.

11. The system of claim 5 , wherein the timing device includes a time-to-digital converter (TDC) configured to:

detect a rising edge of the pulse being sent from the timing device;

detect a rising edge of the return pulse being received by the timing device; and

convert the detected rising edge of the pulse and the detected rising edge of the return pulse into a digital representation of the time difference.

12. A system comprising:

a timing device;

a backplane coupled to the timing device;

a buffer coupled to the timing device and the backplane;

a plurality of line cards coupled to the buffer and the backplane;

the timing device being configured to:

send a clock signal to the buffer via a first trace of a backplane;

send a pulse to the buffer at a first time, wherein the pulse is being sent to the buffer via a first trace of the backplane;

the buffer being configured to:

distribute the clock signal to the plurality of line cards via a plurality of traces of the backplane;

send the pulse to the plurality of line cards via the plurality of traces of the backplane;

receive a return pulse from a line card via a second trace, wherein the line card is among the plurality of line cards, and the second trace is among the plurality of traces;

send the return pulse to the timing device via the first trace;

the timing device being further configured to:

receive the return pulse, at a second time, from the buffer via the first trace;

determine a time difference between the first time in which the pulse is being sent from the timing device and the second time in which the return pulse is received by the timing device, wherein the time difference is indicative of a propagation delay associated with the line card, the buffer, the first trace, and the second trace; and

send the time difference to the line card;

the line card being configured to:

receive the clock signal and the pulse at a same port;

receive the time difference from the timing device; and

adjust a phase delay offset of the line card using the time difference.

13. The system of claim 12 , wherein the timing device is configured to send the time difference to the line card using the trace.

14. The system of claim 12 , wherein the line card is configured to invert the pulse to return the pulse to the timing device as the return pulse.

15. The system of claim 12 , wherein the timing device is further configured to:

in response to the pulse being sent to the line card:

deactivate an output mode of the timing device; and

activate an input mode of the timing device;

in response to the receipt of the return pulse from the line card:

deactivate an input mode of the timing device; and

activate an output mode of the timing device.

16. The system of claim 12 , wherein the line card is further configured to:

in response to the pulse being received at the line card:

deactivate an input mode of the line card; and

activate an output mode of the line card;

in response to the output of the return pulse from the line card:

deactivate an output mode of the line card; and

activate an input mode of the line card.

17. The system of claim 12 , wherein the line card is further configured to determine a residence time indicating a duration in which the pulse resides in the line card, wherein the determination of the phase delay offset is based on the time difference and the residence time.

18. The system of claim 12 , wherein the buffer is further configured to select the line card from the plurality of line cards using a round-robin approach.

19. The system of claim 12 , wherein:

the return pulse is a first return pulse;

the line card is a first line card,

the time difference is a first time difference; and

the buffer is further configured to:

receive a second return pulse from a second line card via a third trace, wherein the second line card is among the plurality of line cards, and the third trace is among the plurality of traces;

send the second return pulse to the timing device via the first trace;

the timing device being further configured to:

receive the second return pulse, at a third time, from the buffer via the first trace;

determine a second time difference between the first time and the third time, wherein the second time difference is indicative of a propagation delay associated with the second line card, the buffer, the first trace, and the third trace; and

send the second time difference to the second line card;

the second line card being configured to adjust a phase delay offset of the second line card using the second time difference.

20. The system of claim 12 , wherein the timing device includes a time-to-digital converter (TDC) configured to:

detect a rising edge of the pulse being sent from the timing device;

detect a rising edge of the return pulse being received by the timing device; and

convert the detected rising edge of the pulse and the detected rising edge of the return pulse into a digital representation of the time difference.