MT90883 Zarlink Semiconductor, MT90883 Datasheet - Page 64

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

6.9.1
The majority of the memory is used by the jitter buffer, which compensates for delay variation (PDV) in the
packet network. The amount of memory required to compensate for a given PDV depends on the mean size of
the packet payload. This is because for small packets, a large proportion of the packet is actually header. The
graph in Figure 27 shows the amount of packet delay variation that can be handled given the mean packet
payload size and the size of the external packet memory.
6.9.2
Memory on the MT9088x family is dynamically allocated in 128 byte blocks, known as "granules". Each granule
contains a 16-byte header section, used primarily for linking granules together into chains, and 112 bytes of
data. The granular organization of memory is responsible for the discontinuities in the graph shown in Figure
27, as the packet size grows to consume another granule.
The granule header contains information passed between the various internal blocks indicating how to handle
the information contained within the granule. Most importantly, it contains a pointer field indicating the location
of the next granule. This is used to chain granules together, initially to create large packets, and subsequently to
link packets together into queues.
To simplify flow of packets between functional blocks within the device, pointers are passed between blocks
indicating the location of the packet in memory. These pointers actually contain the location of the first granule
in a packet. Subsequent granules are indicated by the pointer field in the granule header.
Free granules are held centrally in a pool managed by a granule manager block. When granules are required
(e.g. during payload assembly or packet reception), the relevant block makes a request to the granule manager,
which returns a pointer to a free granule. Similarly, when a granule (or chain of granules) is no longer required
(e.g. the packet has been sent out of one of the output interfaces), it is released back to the free pool by
sending a granule release request to the granule manager.
6.9.3
The MT9088x is capable of interfacing to two types of external memory device, pipelined burst SRAM (such as
Micron
which remove the turnaround cycles required when changing from a read to a write cycle. For most
applications, one external device is sufficient, as shown in the graph in Figure 27. All four memory capacities
shown in this graph can be implemented using one device (either 4, 8 or 16 Mbs). However, for more
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Figure 27 - PDV Supported for Different Memory and Packet Payload Sizes
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Packet Payload size, bytes
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Zarlink Semiconductor Inc.
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