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4.2
Time Stamping and Aging
Most time stamp and aging functionality is controlled through register DB(N)CFG0. The critical control bits
are listed in Table 41. Time stamping is not supported on the host port.
Table 41. Time Stamp and Aging Control Bits
BIT NAME
DESCRIPTION
DB(N)CFG0.TSRELEASE
For receive operation, this control bit holds a packet in the associated buffer until its
time stamp is equal to current cycle timer value. It then releases the packet to the
application.
For transmit operation, this control bit is used to play back prerecorded MPEG2 data. It
holds the data packet until the time stamp (without offset) is equal to the current cycle
time before transmitting the packet over 1394.
DB(N)CFG0.TSAGE
For receive operation, this control bit flushes all packets with expired time stamps in an
associated buffer.
For transmit operation, this control bit flushes packets waiting to be transmitted whose
time stamps equal the current cycle timer value. This is used to prevent ceLynx from
transmitting old MPEG2 packets over 1394.
DB(N)CFG0.TSINSERT
For transmit operations, this control bit adds a time stamp to the transmitted data
stream. This time stamp value is equal to the current cycle timer value plus a
programmable offset.
For receive operations, this control bit strips time stamps from the data in the receive
buffer. This control bit only operates if all other data headers are also stripped.
DB(N)CFG0.TSSTRIP
The ceLynx does not support time stamping of the formatted ISO packets transmitted or received through
the host interface. Time stamp operations are supported only through the HSDI.
4.2.1
Time Stamp and Aging for MPEG2 Data
The ceLynx uses time stamping to preserve the temporal relationship of MPEG2 (DirecTV
) packets in a
transport stream while being transmitted over 1394.
The transmitting ceLynx (transmitting onto 1394) places a time stamp on each MPEG2 cell it transmits. The
time stamp value is the sum of the current value of the 1394 cycle timer and a user programmable transmit
offset value. This value is programmed in register DB(N)CFG2.
The transmitting ceLynx can age a packet (or flush it from the FIFO) if it is not transmitted in time. This is
to avoid transmitting invalid time stamps over 1394. If the packet is not transmitted before the time stamp
plus transmit offset equals the cycle timer, the packet is aged. If transmit aging is used, a transmit offset must
be used.
See Figure 42 for an explanation of packet aging.
Transmit Offset
Packet + Time Stamp
Written To Transmit
Buffer
Packet Aged From
Transmit Buffer if it Has
Not Been Transmitted
Figure 42. MPEG2 Transmit and Aging
The receiving ceLynx decodes the time stamp upon receive. The MPEG2 packet is released to the
application when the incoming time stamp is equal to the current cycle timer. When the data is released to
the application, the HSDI will indicate data is available then the application is able to read data from the
buffer.