Atmel SpW-10X Packet Blocking, Blocked destination, SpaceWire Router, User Manual, Preliminary

SpW-10X	Ref.:	UoD_SpW-10X_
SpaceWire Router		UserManual
	Issue:	3.4
User Manual
	Date:	11th July 2008

8.3.5 Packet Blocking

The Time-Out Enable bit (bit 0) of the router control register enables the watchdog timers on the ports. When this bit is set and the watchdog timers are enabled the router is in “Watchdog Timer” mode. When it is clear and the watchdog timers are disable then the router is in the “Blocking Allowed”mode.

In Blocking Allowed mode packets wait indefinitely on other packets to complete. An exception to this is when an output port that a packet is to be routed to is a SpaceWire port and that port is not started. In this case the packet waits as long as the timeout period and is then discarded if the output port has not started. If group adaptive routing is being used and at least one of the destination ports is running then the packet will wait indefinitely for that output port to become free or another one in the group to start.

In Watchdog Timer mode watchdog timers on the ports are used to clear packets from the network if they become blocked, either while being routed or while waiting on a port which is not granted to any other port. The watchdog timers are restarted every time a data character is transferred. They are stopped after an EOP and started again on the first data character of a packet. In this way the time to transfer a complete packet is not checked but instead the watchdog timers check if a packet has blocked (i.e. no data transfers).

A blocked packet is spilt by terminating the packet at the router output port with an EEP and spilling the remainder of the packet to be transmitted up to and including the EOP at the router input port. If the router output port is blocked (full) and cannot accept data then the EEP is added after the port is unblocked.

WARNING

Blocking Allowed mode is not recommended and should be used with caution.

When Blocking Allowed mode is used (Watchdog timers disabled) then it is important that provision is made for a network manager to detect blocking situations and to reset the nodes or routers causing the problem.

The various ways in which an input port can become blocked and the resulting actions taken by the router are considered in the following sub-sections:

8.3.5.1 Blocked destination

In a blocked destination scenario data cannot be transmitted to the destination port because there is no more transmit credit (no more FCTs received) in a SpaceWire port or an external port output FIFO has become full. Since the destination node is blocked the packet data is left strung out across the SpaceWire network from the packet source to the blockage. In this situation the tail of the packet is distributed across multiple routers and other network paths can become blocked waiting on the original blocked packet to complete.

Preliminary 95