MT90883 Zarlink Semiconductor, MT90883 Datasheet - Page 49

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

Context Look-Up Table field definitions
Context ID
V
NV
F
NF
Context Setup
To setup a new context, the context lookup table is programmed to enter the new context ID against all the
members of the context. The New Valid Channel bit is set for all members, and the New First Channel bit is set
to indicate the first channel in the sequence. The Valid Channel and First Channel bits are cleared.
When context setup is complete, the Update bit in the Context Memory is set. This bit initiates the operation of
the context. The timeslot pointer cycles round until it finds the first channel of the given context. At this point the
state machine copies the New Valid Channel and New First Channel bits across into the Valid Channel and First
Channel bits. The timeslot data is read and placed into the data cache.
Each time the timeslot pointer finds more channels that are valid members of the context, the bits are copied
across in the same way, and the data read into the cache. When the pointer has circulated back round to the
first channel, the context setup is complete, and the Update bit in context memory is cleared.
The first channel bit is required because the CPU cannot exactly time the programming of the Update bit to the
start of a frame. Therefore, the first channel bit is used as a marker to indicate that this is the first channel in the
context, following a frame boundary. This provides the necessary synchronisation to the start of a frame.
Note that context setup is not an isolated operation, and will normally need to be co-ordinated between the
WAN Receiver and the remote WAN Transmit Controller. The sequence for co-ordinating context setup is
shown in Table 15 on page 37. In general, the application should ensure that the receiving context in the WAN
Transmit Controller is updated and ready to receive the packets prior to setting the update bit in the WAN
Receiver.
Context Modification
The WAN Receive Controller supports dynamic addition and deletion of channels into contexts without
interruption to the existing channels within that context. This is achieved by use of the New Valid Channel and
New First Channel bits to program the alteration in advance of the actual update where the changes take place.
This allows the CPU time to setup the changes and inform the receiving device before actually initiating the
modification.
Note that such changes will normally need to be applied at both the local WAN Receiver and the remote WAN
Transmit Controller, and that it is necessary to observe the correct sequence order to avoid mis-operation (see
Table 16 on page 38). The application should ensure that following the lookup table modifications, the receiving
The identity of the context that the channel is a member of. Value is in the range 0-127.
Valid Channel bit. A '1' indicates that the channel is an active member of the context, and to be
New Valid Channel bit. A '1' indicates that the channel will become active in the next context
First Channel bit. A '1' indicates that this is the first channel of the indicated context (as indicated
New First Channel bit. A '1' indicates that this will become the first channel of the indicated context
included in the packet payload. A '0' indicates that the channel is inactive.
update. A '0' indicates that the channel will be removed in the next context update.
by the sequence shown in Figure 17). Each context has one and only one "first channel".
at the next context update.
31
24 23
Figure 25 - Context Look-Up Table
Zarlink Semiconductor Inc.
MT90880/1/2/3
19
N
F
F N
V
49
16 15
V
7 6
Context ID
0
Data Sheet