USB Retimer Link Delay Management for PTM
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Solution Overview
Problem
Existing USB retimers introduce highly variable delays, rendering precision time management (PTM) unusable for devices in their path, as they fail to accurately synchronize and re-drive packets across longer trace lines and cables.
Innovation Solution
The implementation of retimers that participate in link delay management (LDM) exchanges by sending and responding to LDM requests on both upstream and downstream ports, allowing them to accurately determine and adjust link delays, thereby eliminating asymmetries and measurement errors, and enabling accurate timestamp adjustments for isochronous timestamp packets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If retimers are used to extend USB range through longer trace lines and cables, then the physical reach of USB is improved, but packet delay variability increases making PTM unusable
Solution Approach 1:
The retimer performs link delay management exchanges before normal data transmission to pre-determine the exact delay characteristics of the link. This preliminary measurement allows the retimer to calculate compensation values that will be applied to timestamps during actual data transmission, ensuring PTM remains accurate despite the extended physical length
Solution Approach 2:
The retimer implements a feedback mechanism where it measures the actual packet delay through the extended link and uses this information to adjust timestamp corrections dynamically. The LDM exchanges provide continuous feedback about link conditions, allowing the retimer to compensate for delay variability and maintain PTM accuracy throughout the extended USB segment
2Reliability
If retimers re-synchronize and re-drive packets on extended links, then signal integrity over longer cables is improved, but timestamp accuracy deteriorates due to variable delays
Solution Approach 1:
Before processing data packets, the retimer performs preliminary LDM exchanges to measure the exact timing characteristics of the extended link. This preliminary action allows the retimer to pre-calculate the delay compensation values needed to maintain timestamp accuracy while still providing signal re-synchronization for longer cables
Solution Approach 2:
The retimer changes the timing parameters of packets by applying calculated delay compensations to timestamps. By adjusting these temporal parameters based on the measured link characteristics, the retimer maintains both signal integrity through re-synchronization and timestamp accuracy through parameter correction
3Measurement precision
If retimers introduce delay compensation for LDM exchanges, then link delay measurement precision is improved, but device complexity increases
Solution Approach 1:
The retimer implements LDM exchange functionality that serves multiple purposes: measuring link delay characteristics, characterizing timing behavior, and enabling PTM compensation. By making this mechanism universal, the retimer achieves precise delay measurement without requiring separate complex subsystems, as the same LDM infrastructure handles all timing-related tasks
Data Source
AI summary
A system and method of conducting precision time management in a universal serial bus system with a retimer. The method includes initiating, from the retimer, a link delay management request on an upstream-facing port of the retimer. The method further includes receiving, at a downstream-facing port of the retimer, a link delay management request and responding to the request received on the downstream-facing port.