R31DS0087EU0101 Rev.1.01

Page 11

Sep.28.21

pcm4l Datasheet

2.1.2

Adaptive Time Reference Tracker

An Adaptive Time Reference Tracker is designed to be used in a network where the PDV is large. It periodically

analyzes the PDV statistics, and based on the PDV analysis, the Adaptive Time Reference Tracker determines

the required time to collect the time stamps to achieve the target frequency or time estimation accuracy. After

collecting enough time stamps, the Adaptive Time Reference Tracker will select a frequency estimation algorithm

and/or a time estimation algorithm based on the PDV analysis. The frequency and/or a time offset are then

estimated and local oscillator correction requests will be sent to the reference tracker supervisor.

Similar to the Write Phase Reference Tracker, an outlier filter is implemented in software. When the reference

tracker collects the time stamps, the outlier filter analyzes the time stamps and drops the time stamps that are far

away from the average ones.

The Adaptive Time Reference Tracker has three stages when it starts tracking a master clock or after it has been

reset by the RTS:

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Stage 1: Rough Time And Frequency Correction Stage. This stage takes 10 seconds or less depending on

the configuration. In this stage, the Adaptive Time reference Tracker makes a rough time estimation and sends

a phase correction request to the reference tracker supervisor. If a frequency correction is configured, a rough

frequency estimation is performed and a frequency correction request will be sent to the supervisor as well.

Then, the Adaptive Time Reference Tracker enters the second stage.

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Stage 2: High Precision Frequency Correction Stage. This stage takes roughly one minute to several

minutes depending on the configuration and PDV condition. The maximum time spent in this stage is

determined by highPrecisionFrequencyCorrectionTimeMinutes. If the PDV is very small, this stage will take

about one minute. If the PDV is not very small, the tracker will analyze the PDV and either determine how long

it will need to collect time stamps or will take the configured highPrecisionFrequencyCorrectionTimeMinutes

duration time to collect time stamps, depending on the lesser of the two time periods. After collecting the

required time stamps, the reference tracker will perform a high-precision frequency and time estimation.

Frequency and time correction requests will be sent to the reference tracker supervisor. After these corrections,

the local clock should be very closely synchronized with the master clock. The tracker then transfers to the next

stage.

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Stage 3: ToD Correction Stage. In this stage, the Adaptive Time Reference Tracker runs like an adaptive PLL.

It analyzes PDV statistics, collects enough time stamps, makes a ToD offset estimation, and requests a

frequency correction. The reference tracker will stay in this stage until it is reset by the reference tracker

supervisor, or a large frequency or ToD offset is detected. If one of these events happens, the Adaptive Time

Reference Tracker will signal the reference tracker supervisor that the lock state is lost and will then transfer to

the Stage 2 - High Precision Frequency Correction Stage.

2.1.3

Assisted Time Reference Tracker

The Assisted Time Reference Tracker is designed to be used in a network where PTP will be a backup to a local

time reference, such as GNSS. It performs all the functions of the Adaptive Time Reference Tracker in addition to

calibrating for asymmetries using information from the local time reference (via the APTS Supervisor).The

frequency and/or a time offset are then estimated and local oscillator correction requests will be sent to the

reference tracker supervisor.

The Assisted Time Reference Tracker has four stages when it starts tracking a master clock or after it has been

reset by the APTS Supervisor:

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Stage 1: Idle Stage. In this stage, the Assisted Time reference Tracker is waiting for a PTP clock source. If the

PTP source is available prior to a local time reference being available, phase and frequency corrections may

occur in this stage for initial frequency and phase snapping (equivalent to stages 1 and 2 for Adaptive Time

Reference Tracker).

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Stage 2: PTP Out of Specification Stage. This stage is entered when there is only a PTP clock source (no

local time reference). Any previous asymmetry compensation will be disabled and the Assisted Time tracker will

provide phase and frequency corrections (equivalent to stage 3 for Adaptive Time Reference Tracker). The