LAN9250 datasheet(298/421 Pages) MICROCHIP

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LAN9250

DS00001913A-page 298

 2015 Microchip Technology Inc.

14.0

IEEE 1588

14.1

Functional Overview

The device provides hardware support for the IEEE 1588-2008 Precision Time Protocol (PTP), allowing clock synchro-

nization with remote Ethernet devices, packet time stamping, and time driven event generation.

Note:

Support for the IEEE 1588-2002 (v1) packet format is not provided.

The device may function as a master or a slave clock per the IEEE 1588-2008 specification. End-to-end and peer-to-

peer link delay mechanisms are supported as are one-step and two-step operations.

A 32-bit seconds and 30-bit nanoseconds tunable clock is provided that is used as the time source for all PTP timestamp

related functions. A 1588 Clock Events sub-module provides 1588 Clock comparison based interrupt generation and

timestamp related GPIO event generation. GPIO pins can be used to trigger a timestamp capture when configured as

an input, or output a signal based on a 1588 Clock Target compare event.

All features of the IEEE 1588 unit can be monitored and configured via their respective configuration and status regis-

ters. A detailed description of all 1588 CSRs is included in Section 14.8, "1588 Registers".

14.1.1

IEEE 1588-2008

IEEE 1588-2008 specifies a Precision Time Protocol (PTP) used by master and slave clock devices to pass time infor-

mation in order to achieve clock synchronization. Ten network message types are defined:

• Sync

• Follow_Up

• Delay_Req

• Delay_Resp

• PDelay_Req

• PDelay_Resp

• PDelay_Resp_Follow_Up

• Announce

• Signaling

• Management

The first seven message types are used for clock synchronization. Using these messages, the protocol software may

calculate the offset and network delay between timestamps, adjusting the slave clock frequency as needed. Refer to

the IEEE 1588-2008 protocol for message definitions and proper usage.

A PTP domain is segmented into PTP sub-domains, which are then segmented into PTP communication paths. Within

each PTP communication path there is a maximum of one master clock, which is the source of time for each slave clock.

The determination of which clock is the master and which clock(s) is(are) the slave(s) is not fixed, but determined by

the IEEE 1588-2008 protocol. Similarly, each PTP sub-domain may have only one master clock, referred to as the

Grand Master Clock.

PTP communication paths are conceptually equivalent to Ethernet collision domains and may contain devices which

extend the network. However, unlike Ethernet collision domains, the PTP communication path does not stop at a net-

work switch, bridge, or router. This leads to a loss of precision when the network switch/bridge/router introduces a vari-

able delay. Boundary clocks are defined which conceptually bypass the switch/bridge/router (either physically or via

device integration). Essentially, a boundary clock acts as a slave to an upstream master, and as a master to a down

stream slave. A boundary clock may contain multiple ports, but a maximum of one slave port is permitted.

Although boundary clocks solve the issue of the variable delay influencing the synchronization accuracy, they add clock

jitter as each boundary clock tracks the clock of its upstream master. Another approach that is supported is the concept

of transparent clocks. These devices measure the delay they have added when forwarding a message (the residence

time) and report this additional delay either in the forwarded message (one-step) or in a subsequent message (two-

step).

The PTP relies on the knowledge of the path delays between the master and the slave. With this information, and the

knowledge of when the master has sent the packet, a slave can calculate its clock offset from the master and make

appropriate adjustments. There are two methods of obtaining the network path delay. Using the end-to-end method,

packets are exchanged between the slave and the master. Any intermediate variable bridge or switch delays are com-

pensated by the transparent clock method described above. Using the round trip time and accounting for the residence

time reported, the slave can calculate the mean delay from the master. Each slave sends and receives its own mes-

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