R31DS0087EU0101 Rev.1.01

Page 13

Sep.28.21

pcm4l Datasheet

2.1.5

Programmable Loop Filter

Each ReferenceTacker has a programmable Proportional + Integrator (PI) loop filter that is used with the Adaptive

Time and Adaptive Frequency modes. It provides access to the Minimum Response Time, Maximum FFO

Correction, Integral Branch Gain, and Bandwidth Scalar. Generally, these should be left at their defaults because

they are optimized for G.8261 and G.8271.2 networks when using the Adaptive Reference Tracker(s).

▪

Minimum Response Time Seconds, dcoLoopFilter.minResponseTimeSeconds, is the target duration (in

seconds) for the DCO filter to bring the Time of Day offset back to zero, if the PDV conditions permit.

▪

Maximum FFO Correction, dcoLoopFilter.maxFfoCorrection, is the absolute value of the maximum allowed

FFO (fractional frequency offset) correction that the proportional branch of the loop filter can apply to the DCO.

It can also be equated to a phase slope limit (PSL) in ns/s.

▪

Integral Branch Gain, dcoLoopFilter.integralBranchGain, is the gain of the integral branch of the loop filter

relative to the proportional branch.

▪

Bandwidth Scalar, dcoLoopFilter.bandwidthScalar, is the DCO loop filter bandwidth scalar value.

For more information on usage, please contact Renesas.

2.2 Reference Tracker Supervisor (RTS)

The RTS manages the switching between RTs. This includes concepts like hitless switching, revertive and non-

revertive switching, and phase slope limiting. The use of multiple RTs is generally for Profiles that support multiple

GM instances to run in parallel, such as ITU-T G.8265.1 Profile, or that support alternate master monitoring, such

as ITU-T G.8275.2 Profile.

When using with a PTP stack that supports multiple OC instances, a Master Selection Algorithm (MSA) is

available to take advantage of additional G.8265 features, such as Wait-to-Restore and Master Lock-out.

2.2.1

Hitless Reference Switching

The hitless reference switching feature is configured in the JSON configuration file. Setting the value of

“ptpPacketReferenceSwitch.referenceSwitchType” to “Hitless” will enable this feature. When a reference switch

between RTs is triggered, the time offset of the selected/target master against the slave is measured. The servo

then automatically compensates for this offset, so the phase transient at the clock outputs can be minimized when

servo switches to another RT.

In order for hitless reference switching to work, the following conditions must be met:

▪ The slave time offset to the target master must be within the time lock threshold,

timeLockThresholdNanoseconds (i.e., in the time locked state).

▪ The slave frequency offset to target master must be within frequency lock threshold,

frequencyLockThresholdPpb (i.e., in the frequency locked state).

When all three conditions are satisfied, the servo will estimate the offset to the target master at the time of the

reference switch, and this offset will be compensated at the clock output. The compensated offset, if any, at the

original RT will be automatically cleared when the servo switches away from it.

An option to clear any absorbed phase offset can be triggered by calling the management API

mngApi_HitlessSwitchTieReset. When this API function is called, the servo will remove any phase offset

compensation and start to track the master at zero phase offset.