Low Latency 40-Gbps Ethernet IP Core User Guide

ID 683745
Date 3/08/2021
Document Table of Contents PTP Transmit Functionality

When you send a 1588 PTP packet to a LL 40GbE core with Enable 1588 PTP turned on in the parameter editor, you should assert the following respective input signals with the TX SOP signal to tell the IP core the PTP operations or processes that the IP core should perform to the packet:

  • tx_egress_timestamp_request_valid: assert this signal to tell the IP core to process the current packet in two-step processing mode.
  • tx_etstamp_ins_ctrl_timestamp_insert: assert this signal to tell the IP core to process the current packet in one-step processing mode and to insert the exit timestamp for the packet in the packet (insertion mode).
  • tx_etstamp_ins_ctrl_residence_time_update: assert this signal to tell the IP core to process the current packet in one-step processing mode and to update the timestamp in the packet by adding the latency through the IP core (the residence time in the IP core) to the cumulative delay field maintained in the packet (correction mode). This mode supports transparent clock systems.
Note: If tx_etstamp_ins_ctrl_residence_time_update is asserted, you should not assert tx_egress_timestamp_request_valid or tx_etstamp_ins_ctrl_timestamp_insert as the result will be undefined.

The IP core transmits the 1588 PTP packet in an Ethernet frame after PTP processing.

Figure 33. PTP Transmit Block Diagram

In one-step mode, the IP core either overwrites the timestamp information provided at the user-specified offset with the packet exit timestamp (insertion mode), or adds the residence time in this system to the value at the specified offset (correction mode). You tell the IP core how to process the timestamp by asserting the appropriate signal with the TX SOP signal. You must specify the offset of the timestamp in the packet (tx_etstamp_ins_ctrl_offset_timestamp) in insertion mode, or the offset of the correction field in the packet (tx_etstamp_ins_ctrl_offset_correction_field) in correction mode. In addition, the IP core zeroes out or updates the UDP checksum, or leaves the UDP checksum as is, depending on the mutually exclusive tx_etstamp_ins_ctrl_checksum_zero and tx_etstamp_ins_ctrl_checksum_correct signals.

Two-step PTP processing ignores the values on the one-step processing signals. In two-step processing mode, the IP core does not modify the current timestamp in the packet. Instead, the IP core transmits a two-step derived timestamp on the separate tx_egress_timestamp_96b_data[95:0] or tx_egress_timestamp_64b_data[63:0] bus, when it begins transmitting the Ethernet frame. The value on the tx_egress_timestamp_{96b,64b}_data bus is the packet exit timestamp. The tx_egress_timestamp_{96b,64b}_data bus holds a valid value when the corresponding tx_egress_timestamp_{96b,64b}_valid signal is asserted.

In addition, to help the client to identify the packet, you can specify a fingerprint to be passed by the IP core in the same clock cycle with the timestamp. To specify the number of distinct fingerprint values the IP core can handle, set the Timestamp fingerprint width parameter to the desired number of bits W. You provide the fingerprint value to the IP core in the tx_egress_timestamp_request_fingerprint[(W–1):0] signal. The IP core then drives the fingerprint on the appropriate tx_egress_timestamp_{96b,64b}_fingerprint[(W–1):0] port with the corresponding output timestamp, when it asserts the tx_egress_timestamp_{96b,64b}_valid signal.

The IP core calculates the packet exit timestamp.

exit TOD = entry TOD + IP core maintained expected latency + user-specified extra latency

  • entry TOD is the value in tx_time_of_day_96b_data or tx_time_of_day_64b_data when the packet enters the IP core.
  • The expected latency through the IP core is a static value. The IP core maintains this value internally.
  • The IP core reads the user-specified extra latency from the TX_PTP_EXTRA_LATENCY register. This option is provided for user flexibility.

The IP core provides the exit TOD differently in different processing modes.

  • In two-step mode, the IP core drives the exit TOD on tx_egress_timestamp_96b_data and on tx_egress_timestamp_64b_data, as available.
  • In one-step processing insertion mode, the IP core inserts the exit TOD in the timestamp field of the packet at the offset you specify in tx_etstamp_ins_ctrl_offset_timestamp.
  • In one-step processing correction mode, the IP core calculates the exit TOD and uses it only to calculate the residence time.

In one-step processing correction mode, the IP core calculates the updated correction field value:

exit correction field value = entry correction field value + residence time + asymmetry extra latency

  • residence time = exit TOD – entry (ingress) timestamp.
  • entry (ingress) timestamp is the value on tx_etstamp_ins_ctrl_ingress_timestamp_{95,64}b in the SOP cycle when the IP core received the packet on the TX client interface. The application is responsible to drive this signal with the correct value for the cumulative calculation. The correct value depends on system configuration.
  • The IP core reads the asymmetry extra latency from the TX_PTP_ASYM_DELAY register if the tx_egress_asymmetry_update signal is asserted. This option is provided for additional user-defined precision. You can set the value of this register and set the tx_egress_asymmetry_update signal to indicate the register value should be included in the latency calculation.