CN8478/CN8474A/CN8472A/CN8471A
6.0 Basic Operation
Multichannel Synchronous Communications Controller (MUSYCC™)
6.3 Channel Operation
6.3.12 Receive Time Slot Map
Same as Transmit Time Slot Map.
6.3.13 Receive Subchannel Map
Same as Transmit Subchannel Map.
6.3.14 Receive Channel Configuration Descriptor
Same as Transmit Channel Configuration Descriptor.
6.3.15 Message Lists
Message lists contain data transmitted or received by MUSYCC. Message lists
always reside in shared memory. Upon channel activation, MUSYCC traverses
the list and either takes data from data buffers (Tx) or puts data into data buffers
(Rx). Each direction of a channel must be assigned a message list before the
direction of that channel is activated.
A message list is a singly-linked list of Message Descriptors. A Message
Descriptor consists of a Buffer Descriptor (1 dword), a Data Pointer (1 dword),
and a Next Descriptor Pointer (1 dword). (For further information, refer to
Table 5-19, Message Descriptor).
The Buffer Descriptor contains a set of bit fields which instruct MUSYCC
how to behave after the data is put in or taken out of a data buffer.
The Data Pointer contains an address in shared memory where MUSYCC can
take data to be transmitted or store data received from the corresponding channel.
The Next Descriptor Pointer contains an address of a Message Descriptor in
shared memory where MUSYCC can access the “next” Message Descriptor in the
linked list.
To terminate a message list, the contents of the Next Descriptor Pointer in the
last descriptor in a list can point either to the address of the last descriptor or to a
general purpose “terminate” Message Descriptor that can be used by any message
list to represent the end of the list. Thus, the OWNER bit field in the last
descriptor’s Buffer Descriptor must eventually indicate that the host owns the
buffer. This bit value is opposite for receive and transmit buffer ownership.
The OWNER bit field mechanism controls the termination of the message list
as MUSYCC reads in each Message Descriptor in the linked list: it first checks
the OWNER bit field to see if it, and not the host, owns the buffer. If it does own
the descriptor, after servicing the contents of this descriptor, MUSYCC reverses
the OWNER bit field to hand the descriptor back to the host; if it does not own the
buffer, the end of the message list is automatically concluded. The channel stays
active and, depending on other bit field values in the Buffer Descriptor,
MUSYCC either polls this last descriptor regularly to see if the OWNER bit value
has changed, or it idles the channel and awaits another channel activation or
channel jump request. The former is useful in continuing a message list while
retaining the original list. The latter is useful in starting a message list from the
top element in the list.
The host processor must never change a descriptor in a buffer to which it has
already granted MUSYCC ownership. The Owner bit is the only handshake
mechanism to prevent race conditions.
100660E
Conexant
6-19
CONEXANT [ CONEXANT SYSTEMS, INC ]
