VSC7440 datasheet(301/444 Pages) MICROSEMI

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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

284

3.29.1.3 RSRV30

Similar to the mode RSRV32, but the reference point is moved to the time of departure from the egress

unit. The time value to use in the reserved bytes field only is the TOD_nsec value instead of NSX32.

3.29.1.4 ADDS48

The interconnect link in this mode carries frames where the ingress time in NSX48 format is subtracted

from the CF field. Likewise, the egress unit will add the NSX48 at time of departure. This is accomplished

by the following steps.

•

Ingress rewriter: CF is added time from ingress port to rewriter (NSF delta)

•

Ingress rewriter: CF is subtracted NSX value when doing the rewrite

•

Egress rewriter: CF is added NSX value when doing the rewrite

•

Egress port: CF is added time from rewriter to departure (NSF delta)

3.29.1.5 ADDS44

This mode operates as the ADDS48, only NSX44 is used instead. In this mode the egress unit must be

able to see that the NSX44 has wrapped since the ingress unit did the subtraction, ie. if the NSX44 at

ingress was 0xFFF.FFFF.FFFF, and at egress is 0x000.0000.0132, a larger value was subtracted than

added. This is handled by the ingress unit setting CF(46) to the NSX48(44) value, and the egress unit

checking the same to see if NSX48(44) has changed. In that case 244 is added to CF.

3.29.1.6 MONITOR

This mode is used for an egress port where we want the frame to be updated to the time of reception

only. This is used for CPU ports, where software must be able to compare the frame time with the local

time stamp, in order to detect drift. It can also be used for mirroring ports where it is desired to see what

time was carried on frames received from an interconnect. When a frame is transmitted on a MONITOR

port, the frame will be calculated up to the ingress time stamping point. The frame will be added selected

delay (IDLY/EDLY), but will not take any chip internal residence time in as a contributor to the CF.

3.29.1.7 DISABLED

A disabled PTP port is not updating PTP frames at all. This mode only applies to the destination port. If

the ingress PTP mode is set to disabled, it is treated as being a FRONT port.

3.29.2

Calculation Overview

The modes of ingress and egress ports determine which PTP calculations to do. The calculations are

split in an ingress half updating the frame to the time at which it passes the central rewriter and an egress

half, where residence time from the rewriter is prepared and finalized in the egress port module or on the

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