In-band Time Synchronization in Real-Time systems

Abstract

Real-time (RT) systems are widespread over different industries, e.g., healthcare, robotics, manufacturing, machine vision, etc. These systems consist of a hardware and software part that execute an RT application. These systems require a bounded and predictable time response on incoming events and execute all input and output (IO) tasks simultaneously. To ensure this concurrent behavior, the different IO devices are synchronized to achieve a common time notion. The implementation of time notion in systems can differ and therefore
various types of synchronization exist, e.g. in-band and out-of-band. In-band synchronization utilizes the general communication channel to distribute time information, contrary to out-of-band synchronization which uses an external wire to transfer the time signals. As the in-band type implies the synchronization transactions flow together with the general traffic, the synchronization mechanisms are often implemented in the interconnect protocol. This integration limits the choice for a feasible synchronization protocol as this choice is dependent on that of the interconnect due to restricting communication requirements of RT applications. Current synchronization protocols are often limited to sub-microsecond (μs) latency variations and/or partially rely on an out-of-band principle. The goal of this master thesis is to find a solution that is able to achieve in-band synchronization with nanosecond (ns) range jitter. The proposed design is optimized towards nanosecond-scale jitter, by taking implementation challenges of Precision Time Protocol (PTP) into account, and is separate from the choice of interconnect. PTP is an existing synchronization mechanism which retrieves the differences in time notion (offsets) across multiple devices while accounting for transmission latency to each individual device. The implementation of PTP can result in limited performance in terms of jitter. The design focuses on minimizing this jitter with increasing the accuracy and robustness of the PTP synchronization algorithm by improving the precision of timestamps and filtering the calculated offsets for outliers. The synchronization mechanism was evaluated through simulation and validation in hardware. This master thesis presents a Proof of Concept (PoC) that can be implemented into real-world RT systems. It consists of two devices synchronizing to one reference device using the proposed synchronization mechanism. The PoC achieves down to 7 ns jitter, which was not reached by feasible existing in-band synchronization yet.