MVTX2603
Data Sheet
24
Zarlink Semiconductor Inc.
6.0 Frame Engine
6.1 Data Forwarding Summary
When a frame enters the device at the RxMAC, the RxDMA will move the data from the MAC RxFIFO to the
FDB. Data is moved in 8-byte granules in conjunction with the scheme for the SRAM interface.
A switch request is sent to the Search Engine. The Search Engine processes the switch request.
A switch response is sent back to the Frame Engine and indicates whether the frame is unicast or multicast
and its destination port or ports.
A Transmission Scheduling Request is sent in the form of a signal notifying the TxQ manager. Upon
receiving a Transmission Scheduling Request, the device will format an entry in the appropriate
Transmission Scheduling Queue (TxSch Q) or Queues. There are 4 TxSch Q for each 10/100 port (and 8
per Gigabit port), one for each priority. Creation of a queue entry either involves linking a new job to the
appropriate linked list if unicast or adding an entry to a physical queue if multicast.
When the port is ready to accept the next frame, the TxQ manager will get the head-of-line (HOL) entry of
one of the TxSch Qs, according to the transmission scheduling algorithm (to ensure per-class quality of
service). The unicast linked list and the multicast queue for the same port-class pair are treated as one
logical queue. The older HOL between the two queues goes first. For 10/100 ports multicast queue 0 is
associated with unicast queue 0 and multicast queue 1 is associated with unicast queue 2. For Gigabit ports
multicast queue 0 is associated with unicast queue 0, multicast queue 1 with unicast queue 2, multicast
queue 2 with unicast queue 4 and multicast queue 3 with unicast queue 6.
The TxDMA will pull frame data from the memory and forward it granule-by-granule to the MAC TxFIFO of
the destination port.
6.2 Frame Engine Details
This section briefly describes the functions of each of the modules of the MVTX2603 frame engine.
6.2.1 FCB Manager
The FCB manager allocates FCB handles to incoming frames, and releases FCB handles upon frame departure.
The FCB manager is also responsible for enforcing buffer reservations and limits. The default values can be
determined by referring to Chapter 8. In addition, the FCB manager is responsible for buffer aging and for linking
unicast forwarding jobs to their correct TxSch Q. The buffer aging can be enabled or disabled by the bootstrap pin
and the aging time is defined in register FCBAT.
6.2.2 Rx Interface
The Rx interface is mainly responsible for communicating with the RxMAC. It keeps track of the start and end of
frame and frame status (good or bad). Upon receiving an end of frame that is good, the Rx interface makes a switch
request.
6.2.3 RxDMA
The RxDMA arbitrates among switch requests from each Rx interface. It also buffers the first 64 bytes of each
frame for use by the search engine when the switch request has been made.
6.2.4 TxQ Manager
First, the TxQ manager checks the per-class queue status and global reserved resource situation, and using this
information, makes the frame dropping decision after receiving a switch response. If the decision is not to drop, the
TxQ manager requests that the FCB manager link the unicast frame’s FCB to the correct per-port-per-class TxQ. If
multicast, the TxQ manager writes to the multicast queue for that port and class. The TxQ manager can also trigger