VSC7511 datasheet(155/324 Pages) MICROSEMI

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Functional Descriptions

VMDS-10488 VSC7511 Datasheet Revision 4.2

140

The following table shows the resulting rewriter actions for the origin PTP action, depending on the

ingress and egress backplane mode settings.

The following describes each of the PTP time stamping rewriter actions.

Residence Time. The frame’s residence time is calculated as Tx time stamp - Rx time stamp. For

information about how the Rx and Tx time stamps are derived, see Rx and Tx Time Stamps, page 21.

The residence time is adjusted for an optional egress delay (REW:PORT:PTP_DLY1_CFG) enabled

through IS2_ACTION.REW_OP[5] = 1. The resulting residence time is added to the correction field value

in the PTP header. The result is written back into the frame.

Add/subtract. Add the frame’s Tx time to the correction field or subtract the frame’s Rx time from the

correction field. This is used in backplane applications. This mode requires a rollover protection mode to

handle the situation where the nanosecond counter rolls over during the backplane transfer. The rollover

protection mode is configured in SYS::PTP_CFG.PTP_CF_ROLL_MODE and must be the same in both

the ingress and the egress backplane unit.

Time-of-Day. The time-of-day consists of a 48-bit seconds part and a 32-bit nanoseconds part. The

current time-of-day is derived by sampling the time-of-day seconds counter (SYS::PTP_TOD_LSB,

SYS::PTP_TOD_MSB) and the nanoseconds counter (Tx time stamp) at the time of the frame’s

departure from the switch. Note that the time-of-day value is adjusted for I/O path delay. For more

information, see Rx and Tx Time Stamps, page 21.

Set Reserved Bytes. This action write the frame’s Rx time stamp into the reserved bytes of the PTP

header.

Clear Reserved Bytes. This action clears the reserved byte of the PTP header by setting the four bytes

to 0.

In addition, the UDP checksum is handled for IPv4 frames and IPv6 frames after any PTP modifications

by the rewriter. For IPv4, the UDP checksum is cleared while for IPv6, the two bytes immediately

following the PTP header are updated so that the UDP checksum remains correct.

All internal calculations on nanoseconds time stamps are signed and use 48 bits of precision. When

enabling a port for backplane mode, it is possible specify the number of bits to use from the 48-bit

counter. This is specified in SYS::PTP_CFG.PTP_STAMP_WID. This means that the time stamp values

carried in the reserved bytes or in the correction field in the PTP header can be for instance 30 bits

instead of the default 32 bits.

3.13.5

Special Rewriter Operations

VCAP IS2 can trigger a special rewriter operation through IS2_ACTION.REW_OP[3:0] = 8 (rewriter

special), where the frame’s MAC addresses are swapped:

•

The original destination MAC address becomes the new source MAC address.

•

The original source MAC address becomes the new destination MAC address.

Bit 40 is cleared in the new source MAC address to prevent the possibility of transmitting an illegal frame

with a multicast source MAC address.

Table 112 •

PTP Time Stamping for Origin PTP

PTP Action

(from IS2)

Ingress

Backplane

Egress

Backplane

PTP time stamp Rewriter Actions

Origin

Disabled

Time-of-day: Write the time-of-day into the PTP origin

time stamp field.

Origin

Disabled

Enabled

None.

Origin

Enabled

Disabled

Time-of-day: Write the time-of-day into the PTP origin

time stamp field.

Origin

Enabled

Does not apply.

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