GR740-UM-DS, Nov 2017, Version 1.7
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GR740
31
SpaceWire - Time Distribution Protocol Controller
31.1
Overview
This core provides basic time keeping functions such as Elapsed Time counter according to the
CCSDS Unsegmented Code specification. It provides support for setting and sampling the Elapsed
Time counter. It also includes a frequency synthesizer with which a binary frequency is generated to
drive the Elapsed Time counter. This interface also 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 AMBA APB bus is used for configuration, control and status handling.
31.2
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. When there is more than one target the CCSDS Time
Code need to be transferred to each individual target separately [SPWCUC].
31.3
Functionality
The block diagram below shows how the controller is connected to the system.
Figure 43.
Block diagram
The foreseen usage of this core is to distribute and synchronise time between an initiator SPWTDP
core and one or more target SPWTDP (slave) cores using the SpaceWire interface for communication
between them.
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
Processor
AMBA
AHB
Time-Code/Interrupt
SpaceWire
Router
AHB
APB
AMBA
APB
SPWTDP
Slave