GRSPW datasheet(6/48 Pages) ACTEL

6 / 48 page

GRSPW / GRSPW-FT

December 2008, Version 1.0.2

GAISLER

2.3.1

Link interface FSM

The FSM controls the link interface (a more detailed description is found in the SpaceWire standard).

The low-level protocol handling (the signal and character level of the SpaceWire standard) is handled

by the transmitter and receiver while the FSM in the host domain handles the exchange level.

The link interface FSM is controlled through the control register. The link can be disabled through the

link disable 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 link start bit is set or when a NULL character

has been received and the autostart bit is set.

The current state of the link interface determines which type of characters are allowed to be transmit-

ted which together with the requests made from the host interfaces determine what character will be

sent.

Time-codes are sent when the FSM is in the run-state and a request is made through the time-interface

(described in section 2.3.5).

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 for

the outstanding credit counter and the currently free space in the receiver N-Char FIFO. FCTs are sent

as long as the outstanding counter is less than or equal to 48 and there are at least 8 more empty FIFO

entries than the counter value.

N-Chars are sent in the run-state when they are available from the transmitter FIFO and there are

credits available. NULLs are sent when no other character transmission is requested or the FSM is in

a state where no other transmissions are allowed.

The credit counter (incoming credits) is automatically increased when FCTs are received and

decreased when N-Chars are transmitted. Received N-Chars are stored to the receiver N-Char FIFO

for further handling by the DMA interface. Received Time-codes are handled by the time-interface.

2.3.2

Transmitter

The state of the FSM, credit counters, requests from the time-interface and requests from the DMA-

interface are used to decide the next character to be transmitted. The type of character and the charac-

ter itself (for N-Chars and Time-codes) to be transmitted are presented to the low-level transmitter

which is located in a separate clock-domain.

This is done because one usually wants to run the SpaceWire link on a different frequency than the

host system clock. The core has a separate clock input which is used to generate the transmitter clock.

More information on transmitter clock generation is found in section 2.8.1. Since the transmitter often

runs on high frequency clocks (> 100 MHz) as much logic as possible has been placed in the system

clock domain to minimize power consumption and timing issues.

The transmitter logic in the host clock domain decides what character to send next and sets the proper

control signal and presents any needed character to the low-level transmitter as shown in figure 6. The

transmitter sends the requested characters and generates parity and control bits as needed. If no

requests are made from the host domain, NULLs are sent as long as the transmitter is enabled. Most of

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