MT90883 Zarlink Semiconductor, MT90883 Datasheet - Page 39

• Manufacturer Part Number: MT90883
• Description: (MT90880 - MT90883) TDM to Packet Processors
• Manufacturer: Zarlink Semiconductor

5.5.4
Context removal is required when all active channels are removed from a context. Normally, channel deletion is
signalled in the first packet of the modified context. However, when the last channel is deleted from a context,
no packet will be transmitted since there is no payload. Therefore the receiver will not be signalled that the
channel deletion has taken place, and the context will underrun, waiting for further packets to arrive.
Therefore, it is not permitted to tear down a context simply by deleting all of its contents. Table 5 below
describes the context removal procedure, known as "context teardown", used to allow contexts to be deleted
cleanly.
5.5.5
A second means of context removal, known as "context cleardown", is provided for situations where a system
error has occurred, and the context has not been removed properly. The cleardown mechanism prevents
expired contexts from using up system resources and memory.
For example, during context teardown, if the last packet sent by the PTX is not received by the PRX, the context
will still exist at the PRX when it has already been deleted by the PTX. In this situation, a timeout informs the
PRX Host that the context teardown notified by the PTX Host has not taken place.
The PRX host then needs to take two actions:
1. Instruct the queue manager to flush the queue of packets for this context. Following this, the queue manager will
2. Program a context cleardown, which immediately clears any state associated with the context.
1
2
3
4
5
6
7
discard any packets that arrive using the same context identifier.
PTX Host signals PRX Host to expect a context
removal or "teardown"
PTX host activates the context teardown.
The next new packet is the last one to be transmitted
for this context.
The context switch bit in the header is toggled to
signal the context removal.
Context Removal
Context Cleardown
Packet Transmit End (PTX)
Table 17 - Context Teardown Process
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MT90880/1/2/3
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PRX Host programs MT9088x device to expect a
context teardown.
PRX Host sends an acknowledgement back to the
PTX Host.
Last packet received for the context to be removed,
identified by the toggling of the context switch bit.
This automatically activates the previously
programmed context teardown.
The PRX Queue manager discards any future
packets that arrive with the same context ID.
Packet Receive End (PRX)
Data Sheet