GR740-UM-DS, Nov 2017, Version 1.7
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GR740
13.2.22.1 Autoscrub
With autoscrubbing enabled, the routing table and port setup registers are periodically read and rewrit-
ten. This is done to prevent a buildup of SEUs, which can cause an uncorrectable error in the memo-
ries. It runs in the background and has no timing impact on SpaceWire traffic but can delay
configuration accesses by two cycles.
The scrubber starts at address 0 and simultaneously writes one location in the port setup memory and
the routing table memory. It then waits for a timeout period before writing the next word. Eventually,
the last location is reached and the process starts over from address 0.
The period between each word refresh is approximately 2
26
system clock cycles (system clock used
for AMBA system). The scrubber uses a free time slot, in which data traffic does not perform a table
lookup to read and write the memories that causes a small nondeterminism in the period.
13.3
SpaceWire ports
Each SpaceWire port comprises a SpaceWire codec that implements an encoder-decoder compliant to
ECSS-E-ST-50-12C
[SPW]. The interface to the router’s switch matrix consists of a transmit FIFO,
receive FIFO, and control and status signals, see Figure 12. The transmit and receive FIFOs can both
store up to 64 N-chars. All the configuration parameters and status information for the ports are acces-
sible through the router’s configuration port, either through RMAP or AMBA AHB (see section
13.5.3).
13.3.1 Link-interface FSM
The link-interface FSM controls the link interface (a more detailed description is found in [SPW]).
The low-level protocol handling (the signal and character level) is handled by the transmitter and
receiver while the FSM handles the exchange level.
The link-interface FSM is controlled through the control signals provided in the RTR.PCTRL regis-
ters. The link can be disabled through the RTR.PCTRL.LD bit, which depending on the current state,
either prevents the link-interface from reaching the started-state or forces it to the error-reset state.
When the link is not disabled, the link interface FSM is allowed to enter the started-state when either
the RTR.PCTRL.LS bit is set, or the link-start-on-request feature is trying to start the port, or when a
NULL character has been received and the RTR.PCTRL.AS bit is set.
The current state of the link-interface determines which type of characters are allowed to be transmit-
ted. When the link-interface is in the connecting- or run-state, it is allowed to send FCTs. FCTs are
sent automatically by the link-interface when possible. This is done based on the maximum value of
56 outstanding credits and the currently free space in the receive FIFO. FCTs are sent out as long as
Figure 12.
Block diagram
TRANSMITTER
TXCLK
LINK INTERFACE
TRANSMITTER
D(1:0)
S(1:0)
FSM
RECEIVER0
FIFO
RECEIVER
FIFO
D0
S0
PHY
D
DV
CONTROL SIGNALS
STATUS SIGNALS