R31DS0087EU0101 Rev.1.01

Page 7

Sep.28.21

pcm4l Datasheet

1.2 Clock Recovery Techniques

For network clock synchronization, equipment clock systems predominantly rely on a linear-based clock recovery

technique (e.g., a low-pass loop filter). This filters the noise on the recovered clock (i.e., from CDR) and

suppresses high-frequency noise, or jitter. The ITU-T has many equipment clock recommendations for TDM,

legacy or non-packet network types (e.g., ITU-T G.813 for SDH/SONET networks and ITU-T G.8262 for Ethernet

networks). However, with the transition to packet-switched networks, more advanced techniques may be required

as Ethernet is built based on an asynchronous physical layer clock (some Telecom networks have been

transitioned to Synchronous Ethernet, per ITU-T G.8262; however, there are still many networks that are based

solely on IEEE 802.3). There are typically two ways to recover a clock when sent over a packet-based network:

adaptive and differential.

Per ITU-T G.8260, adaptive clock recovery is a technique that does not require the support of a network-wide

synchronization signal to regenerate the timing. In this case, the timing recovery process is based on the (inter-)

arrival time of the packets using timestamps. The information carried by the packets could be used to support this

operation, depending on the packet delay variation (PDV) each timing packet experiences. Either two-way or one-

way protocols can be used, which is why IEEE 1588 is a good protocol choice.

Differential clock recovery is another technique, typically used for Circuit Emulation Services (CES), to recover

clocks based on the difference between (inter-)arrival time of the packets using a traceable clock at both the

sending end and the receiving end. This ensures the recovered clock is not affected by the packet delay variation

(PDV) each timing packet experiences. However, a traceable clock may not be supported [or available] at the

receiving end, so adaptive clock recovery must be used. More details on differential methods can be found in

section 8.2 of ITU-T G.8261.

The Renesas pcm4l software supports both adaptive and linear clock recovery techniques, with the linear clock

recovery being offloaded to the Renesas network synchronization devices, such a ClockMatrix device.

1.2.1

Assisted Clock Recovery

The ITU-T has proposed an architecture that uses GNSS as the primary local time source, with PTP acting as a

backup – primarily to maintain a time holdover on the loss of GNSS. This is described in G.8273.4 as Assisted

Partial Time Support (APTS).

Two methods can be used to implement APTS:

1. Select the local time source as the best clock for local PTP clock (i.e., ITU-T virtual PTP port), and monitor the

PTP T-GM using ITU-T alternate master.

2. Maintain two independent clock channels (GNSS and PTP) and correct for asymmetry on the active PTP T-GM

The latter works best for T-BC-A implementations, to maintain PTP clock traceability to the T-GM. The former can

work well for either T-BC-A or T-TSC-A, but the PTP stack must support the G.8275.2 alternate master to allow the

PTP T-GM’s to be monitored and corrected for asymmetry prior to GNSS failure.

2. Software Architecture Overview

The Renesas software is provided under a no-fee license. The terms of that license can be found and agreed to

here: pcm4l Software (Note: user login is required).

The Renesas PCM software solution covers network clock tracking, filtering, and a clock state machine. It is

meant to interface an IEEE 1588 compliant PTP stack, such as Linux PTP (ptp4l). The PCM for Linux (pcm4l)

software package has already been integrated to Linux, and can run on any Linux Distribution v3.0 or later.

The software runs on an external processor running the Linux OS and interfaces the clock and timestamper unit(s)

(TSU) through the PTP Clock Manage (PHC) Infrastructure. The raw PTP timestamps are provided by the IEEE

1588-2019 Slave Event Monitoring channel and either filtered by Renesas’ patented PDV noise filtering algorithm

or by hardware using Renesas’ network synchronization PLLs, such as ClockMatrix. The PCM has a local

oscillator state machine (LOSM) that supports the ITU-T PTP clock operating modes.