Patent Application
20080031312
On the transmit side, skew can be injected on a per-lane basis to compensate for any skew added by the system, such as Field Programmable Gate Array (FPGA) startup conditions. This injected skew achieves compliancy as specified by applicable standards, such as SxI-5. To determine how much skew should be injected to meet these standards, the following algorithm is implemented.
The present invention consists of N+1 transceivers for the purposes of data transmission and reception. The system is designed such that a deskewing algorithm can be used to determine the necessary per-lane skew to be added for total lane alignment.
The term “deskewing algorithm”, as used herein, refers to a calibration method to achieve total lane alignment with respect to a desired lane-to-lane skew goal, usually defined by standards such as SxI-5. These standards often specify skew related characteristics in terms of the unit interval, or UI. UI is calculated via 1/(data bit rate). The method uses an iterative process to determine the correct combination of skew injection parameters for each transmitting lane.
In order to determine this suitable combination, skew is systematically injected into each lane. On the receive side, a feedback signal is necessary to give status updates as to alignment of the lane. This feedback signal may be specific to the particular lane or can be an aggregation of some or all of the lanes. When alignment is successful, the feedback signal will indicate this success, thus indicating that the correct amount of skew has been injected and proper deskewing has occurred.
If the feedback signal is particular to a single lane, then the process of injecting skew will have to be performed for each lane. For an aggregated feedback signal, the process of injecting skew will occur on all lanes, as only one combination will satisfy the aggregated status. This process will take much longer as the search space of combinations will be much larger.
The system of the present invention enables the deskewing of lanes through the use of a deskewing algorithm. The following examples will illustrate the workings of possible systems in more detail. These examples will deal with smaller 3 transceiver systems. The 3 transceiver example is easily expandable into an SFI-5 system with 17 transceivers, or any other transceiver-based system. Transceivers are herein referred to also as MGTs (Multi-Gigabit Transceivers).
The following examples are set forth to gain a better understanding of the algorithm portion of the invention described herein. These examples are provided for illustrative purposes only and they should not limit the scope of this invention in any way.
The system in this case has a feedback signal for each individual lane that reports on the successful, error-free reception of the data stream on that particular lane. Error free reception is expected when an appropriate amount of skew has been injected on the individual lane relative to a reference lane. When one lane's feedback signal reports the correct alignment, the process begins with the next lane, until all lanes have been aligned and thus deskewed. This system is shown in
In Example 1, there is a plurality of feedback signals—one for each lane as illustrated in
For the three MGT system, there is an out-of-alignment alarm for each lane, labeled OOA—0, OOA—1, and OOA—2. Each alarm will report an error should one occur on its respective lane. These alarms are expected to remain set if the correct alignment has not occurred, and are expected to clear within a specified time (t) if correct alignment has been achieved.
The search space, S, refers to the maximum UI that a lane may be expected to be out of alignment. With reference to the applicable standard, there is typically a specification as to the maximum allowable skew tolerance in order to achieve compliancy with said standard. For the SxI-5 standard, that maximum allowable skew tolerance between lanes is set at 5 UI. The search space interval, i, must be set at a value less than this skew tolerance. To achieve the fastest possible search times, it is best to set the search space interval at 1 UI below the skew tolerance level specified in the applicable standard. Thus, with reference to the SxI-5 standard, the search space interval, i, should be set to 4.
For the purposes of this example, the search space, S, will be set at 20 UI, which is an arbitrary choice, with the search space interval, i, set to 4 UI. This gives five possible sets of interval choices for each lane, i.e. from 0-4, 8-12, 12-16, and 16-20.
The process begins with Lane 0 set in the first interval position. If the out-of-alignment alarm, OOA—0, indicates an alarm condition after a specified wait-time (t), then the lane realigns to the next interval position. Again, the OOA—0 alarm is checked and if the alarm condition is still active the next interval position is tested. This process continues until the OOA—0 alarm indicates alignment for a specific interval position or the entire search space has been exhausted. For this reason it is important to choose the search space such that it is large enough to take in the maximum expected lane skew deviation.
Assuming the OOA—0 alarm indicates that Lane 0 has achieved alignment, the process begins for the next lane, Lane 1. When the alarm for Lane 1, OOA—1, indicates successful alignment, the process continues for Lane 3. When the alarms for each lane have achieved alignment, the process ends.
The maximum amount of search intervals for this example that have to be traversed through is 15, i.e. five intervals for each of the three lanes. In order to determine the maximum amount of search intervals for a different system, the following variables need to be known:
n+1, the total number of lanes
S, the search space size
i, the search space interval
Therefore, the maximum amount of search intervals, C, can be determined through the following formula:
C=(n+1)*S/i
In order to determine an approximate maximum time for the searching process, the maximum amount of search intervals must be multiplied by the time, t, necessary to wait for an OOA alarm check. Thus the maximum wait time, T, is given by:
T=t*(n+1)*S/i
By following this process, all lanes can be deskewed sequentially to provide total lane alignment in compliancy with relevant standards, such as SxI-5.
The system in this case has a feedback signal that reports the aggregated status across all lanes, i.e. there is only one feedback signal for the entire system. Thus only when every lane has been deskewed will the feedback signal report success. This system is shown in
In Example 2, there is one aggregated feedback signal, i.e. the combination of all feedback signals as illustrated in
For the three MGT system, there is an aggregated out-of-alignment alarm combining the status for each lane, labeled OOA. The alarm will report an error should one occur on any lane.
The search space, S, refers to the maximum UI that a lane may be expected to be out of alignment. With reference to the applicable standard, there is typically a specification as to the maximum allowable skew tolerance in order to achieve compliancy with said standard. For the SxI-5 standard, that maximum allowable skew tolerance between lanes is set at 5 UI. The search space interval, i, must be set at a value less than this skew tolerance. To achieve the fastest possible search times, it is best to set the search space interval at 1 UI below the skew tolerance level specified in the applicable standard. Thus, with reference to the SxI-5 standard, the search space interval, i, should be set to 4.
For the purposes of this example, the search space, S, will be set at 20 UI, which is an arbitrary choice, with the search space interval, i, set to 4 UI. This gives five possible sets of interval choices for each lane, i.e. from 0-4, 8-12, 12-16, and 16-20.
The process begins with all lanes set in the first interval position. If the out-of-alignment alarm, OOA, indicates an alarm condition after a specified wait-time then the first lane realigns to the next interval position. Again, the OOA alarm is checked and if the alarm condition is still active the next interval position for the first lane is tested. This process continues for all five interval positions. If the OOA alarm still indicates an alarm condition then the next lane can be iterated through, while still iterating through the first lane. This process can be lengthy as since there is only one global alarm making the range of combinations much larger. The process ends when the OOA alarm indicates alignment for a specific interval position on each lane or the entire search space has been exhausted. For this reason it is important to choose the search space such that it is large enough to take in the maximum expected lane skew deviation.
The maximum amount of search intervals for this example that have to be traversed through is 125, i.e. five intervals for each of the three lanes. In order to determine the maximum amount of search intervals for a different system, the following variables need to be known:
n+1, the total number of lanes
S, the search space size
i, the search space interval
Therefore, the maximum amount of search intervals, C, can be determined through the following formula:
C═(S/i)(n+1)
In order to determine an approximate maximum time for the searching process, the maximum amount of search intervals must be multiplied by the time, t, necessary to wait for an OOA alarm check. Thus the maximum wait time, T, is given by:
T=t*(S/i)(n+1)
By following this process, all lanes can be deskewed to provide total lane alignment in compliancy with relevant standards, such as SxI-5.
The previously explained deskewing methods are only considered as examples. Different configurations of each are possible as long as deskewing is achieved. Combined configurations, such as multiple aggregated signals for groups of lanes, are also possible as long as deskewing is achieved.
In order to inject the appropriate amount of skew into each lane to traverse the different search space intervals, the unaligned transmit (TX) data lines enter a buffer for the purposes of lane deskewing. The skew values to be injected for each lane are supplied to the buffer. The buffer then bit shifts each lane the appropriate amount to move to a different alignment position as specified by the search space intervals. A representation of a possible embodiment of this injection system is shown in
