R31DS0087EU0101 Rev.1.01

Page 6

Sep.28.21

pcm4l Datasheet

2. Mean Path Delay, or <meanPathDelay>[2]

◦ = [(<syncEventIngressTimestamp> - <delayReqEventEgressTimestamp>) +

(<delayReqEventIngressTimestamp> – <originTimestamp>) – <correctedSyncCorrectionField> –

<correctedDelayRespCorrectionField>]/2

◦ = [(t2 - t3) + (t4 – t1) – <correctedSyncCorrectionField> – <correctedDelayRespCorrectionField>]/2

The timing diagram in Figure 2 shows a simplified example using the PTP timing messages exchange based on

an End-to-End (E2E) delay mechanism. Once the slave has all timestamps, it is in position to calculate the offset

from master, offsetFromMaster.

Figure 2. Example Synchronization Using the IEEE 1588 Timing Message Exchange

There are some simplifications assumed with these calculations (e.g., no corrections are made to account for any

path and/or data asymmetry or noise generation from the master clock). This is where more complex filtering

schemes will be required, which is part of the Renesas PTP Clock Manager (PCM). The PCM software also

includes other features required to meet the ITU-T Telecom Equipment Clock recommendations, including a

packet clock state machine. In complex networks with partially unknown and varying packet delays, such as

telecom applications, the patented filter algorithms help to improve synchronization accuracy with phase accuracy

of under 1 microsecond and frequency accuracy of under 16 parts-per-billion (ppb).

The tight requirements of network synchronization dictate the use of special hardware to generate the physical

clock. The Renesas family of System Synchronizer PLLs such as ClockMatrix provide precise, low-noise solutions

for this function. In order to accommodate ever-changing requirements of the network, the servo is implemented in

software running on an external processor that controls the PLL. The protocol and master selection algorithm are

also run on the external processor.

There are both open-source and commercial source options for the PTP stack. Most have very similar capabilities

because they are based on the IEEE 1588 standard to guarantee interoperability. For Linux-based systems, a

popular PTP stack is Linux PTP, or ptp4l. The ptp4l software provides all the required features per IEEE 1588-

2008, along with Profile support. It uses the Linux kernel SO_TIMESTAMPING socket option for timestamp

management of the PTP even packets and the PTP Hardware Clock infrastructure for clock control. v3.0 has

support for all three ITU-T Telecom profiles and added support for external filter/servos, such a pcm4l.

2. If using peer-to-peer delay mechanism, then this would be Mean Link Delay, or <meanLinkDelay>.