AT7911E
4. Interfaces
The AT7911E provides an interface between three SpaceWire links according to the SpaceWire
standard ECSS-E-50-12A specification and a data processing node consisting of a Central Pro-
cessing Unit and a communication data memory.
The AT7911E consists of the following blocks (See Figure 1):
• Three bidirectional SpaceWire channels
• Communication Memory Interface (COMI)
• Host Control Interface (HOCI)
• Protocol Command Interface (PRCI)
• JTAG Test Interface
4.1
Three bidirectional SpaceWire channels,
Three bidirectional SpaceWire channels, all comprising the DS-link SpaceWire cell, receive and
transmit sections (each including FIFOs) and a protocol processing unit (PPU). Each channel
allows full duplex communication up to 200 Mbit/s in each direction. With protocol command
execution a higher level of communication is supported. Link disconnect detection and parity
check at character level are performed. A checksum generation for a check at packet level can
be enabled.
The transmit rate is selectable between 1.25 and 200 Mbit/s. The startup transmit rate is 10
Mbit/s. For special applications the data transmit rate can be programmed to values even below
10 Mbit/s; the lowest possible SpaceWire transmit rate is 1.25 Mbit/s (the next values are 2.5
and 5 Mbit/s).
4.1.1
PPU Functional Description
Since the Protocol Processing Unit (PPU) determines a major part of the AT7911E functionality,
the principal blocks of the PPU and their function are described here. The PPU unit functionality
is provided for each SpaceWire channel of the AT7911E.
4.1.1.1
Protocol Execution Unit
This unit serves as the main controller of the PPU block. It receives the character from the
SpaceWire cell and interprets (in protocol mode) the four header data characters received after
an EOP control character. If the address field matches the link channel address and the com-
mand field contains a valid command then forwarding of data into the receive FIFO is enabled. If
the command field contains a "simple control command" then the execution request is forwarded
to the command execution unit.
The protocol execution unit also enables forwarding of header data characters to the acknowl-
edge generator and provides an error signal in case of address mismatch, wrong commands or
disabled safety critical "simple control commands".
The protocol execution unit is disabled in "transparent" or “wormhole routing” operation mode.
4.1.1.2
Receive, Transmit, Acknowledge
The transmit and receive FIFOs decouple the SpaceWire link related operations from the
AT7911E related operations in all modes and such allows to keep the speed of the different units
even when the source or sink of data is temporarily blocked.
7
7737B–AERO–05/08
ATMEL [ ATMEL ]
