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GR716-DS-UM, May 2019, Version 1.29
342
www.cobham.com/gaisler
GR716
34
SpaceWire - Time Distribution Protocol
34.1
Overview
This interface implements the SpaceWire - Time Distribution Protocol (TDP). The protocol provides
capability to transfer time values and synchronise them between onboard users of SpaceWire network.
The time values are transferred as CCSDS Time Codes and synchronisation is performed through
SpaceWire Time-Codes. The core also provides datation services. The core operates in an AMBA
APB bus system. The AMBA APB bus is used for configuration, control and status handling. The
interface is coupled with a SpaceWire node with AMBA AHB master and RMAP target implementa-
tion.
34.2
Protocol
The initiator and target maintain their own time locally. The Time Distribution Protocol provides the
means for transferring time of initiator to targets and for providing a synchronization point in time.
The time is transferred by means of an RMAP write command carrying a CCSDS Time Code (time
message). The synchronization event is signaled by means of transferring a SpaceWire Time-Code.
The transfer of the SpaceWire Time-Code is synchronized with time maintained by the initiator. To
distinguish which SpaceWire Time-Code is to be used for synchronization, the value of SpaceWire
Time-Code is transferred from initiator to target by means of an RMAP write command prior to actual
transmission of SpaceWire Time-Code itself.
34.3
Functionality
The block diagram below explains the complete system.
Figure 53.
Block diagram
The system can act as initiator (time master) and target being able to send and receive SpaceWire
Time-Codes. The initiator requires SpaceWire link interface implements an RMAP initiator. The Tar-
get requires SpaceWire link interface implements an RMAP target. The SPWTDP component is a part
of this system providing SpaceWire Time-Codes, CCSDS Time Codes, datation, time-stamping of
distributed interrupts, support for transmission of CCSDS Time Codes through RMAP and support
for latency measurement and correction. In this implementation the CCSDS Time Codes carried
between the SpaceWire network is based on CCSDS Unsegmented Code format. The table below
Processor
AMBA
AHB
Time-Code/Interrupt
SpaceWire
Interface
AHB
APB
AMBA
APB
SPWTDP
Slave