Using SpaceWire Time-codes for Global Synchronization of PLL-Based Local Clocks

Kurzfassung

Abstract In a control application all sensors and actuators have to be able to perform their functions in a specific time to guarantee the desired control cycle. To achieve an exact actual system state a synchronization of the different sensor measurements is necessary. This guarantees that all acquired values are in a very small period of time and not spread over the whole control cycle. Otherwise, a noiser system state would result. The simplest way to synchronize those measurements is with cyclic triggering. This can be either an external request via communication or generated locally by a timer. Both approaches work well in a monolithic system approach with a central unit due to the existence of a common clock domain. However, a distributed system consists of multiple monolithic systems connected via communication, each with a separate clock domain. Due to the communication delay from unit to unit, and the differences between the local clocks, a synchronization according to the presented approaches is only possible by introducing a global clock domain. With time-codes SpaceWire already includes a mechanism to distribute time over a network. However, the communication delay regarding the time-code distribution is not considered. In systems consisting of large networks which have to be synchronized in very small cycles (e.g. high dynamic robots see [1]) this aspect is not negligible. This approach introduces a global time distribution by synchronization and runtime compensation. The synchronization is based on digital phase locked loops (PLLs) which align the units’ local time to the occurence of SpaceWire time-codes. The runtime compensation of the time- code distribution in the network is based on statistical evaluation.