Research On Precise Clock Synchronization Method Based On FPGA

In current industrial control field, co-existing of several fieldbus standards promotes rapid development of industrial Ethernet impersonally, and many industrial Ethernet protocols, such as HSE, Profinet, Modbus TCP/IP, Ethernet/IP, Ethernet Powerlink, EtherCAT, have been developed in recent years. It is the non-real-time feature of Ethernet that is the biggest obstacle in applying traditional commercial Ethernet to field level devices in control systems. Clock synchronization technology is the basis of solving real-time problem in industrial Ethernet protocols.IEEE 1588 defines a protocol called Precision Time Protocol which enabling precise synchronization of clocks in measurement and control systems implemented with technologies such as network communication, local computing, and distributed objects. The protocol is applicable to systems communicating by local area networks supporting multicast messaging including, but not limited to, Ethernet. The protocol enables heterogeneous systems that include clocks of various inherent precision, resolution, and stability to synchronize. The protocol supports system-wide synchronization accuracy in the sub-microsecond range with minimal network and local clock computing resources.Clock synchronization method based on PC software, such as NTP, only reaches accuracy in the millisecond range in the best case. Method based on embedded software which places clock synchronization module in the driver level in operating system, reaches accuracy in the microsecond range. This accuracy level will satisfy real-time requirements to all industrial control systems but systems requiring more precise timing, e.g. motion control system. Problems such as uncertainty of interrupt response delay in operating system and frequency drift of oscillator are the main obstacle to reach accuracy in sub-microsecond range for method based on embedded software.A clock synchronization method based on FPGA is designed and implemented with IEEE 1588 as its protocol and Ethernet as its communication network. This method implements TCP/IP communication in embedded software and clock synchronization module in FPGA. This method utilizes FPGA to realize functions such as precisely capturing message timestamp and dynamically compensating frequency drift of oscillator, gains better effect than method based on embedded software and reaches synchronization accuracy in the sub-microsecond range over 10Mbps Ethernet connected by hub.