Digital Audio Interface (EBU)
MOTOROLA
Audio Functions
17-17
When a long pause is seen between two subsequent data symbols, the IEC958 receiver assumes the
reception of one or more sync symbols.
Table 17-17 shows this functionality.
The recognition of the number of sync symbols derives from the fact that the U-channel transmitter in the
CD channel decoder will transmit one symbol on average every 12 IEC958 channel bits. On this average
rate, there is a tolerance of maximum 5%.
The IEC958 receiver is tolerant on symbol error. Due to the physical nature of the transmission of the data
over the CD disc, not more than one out of any 5 consecutive user channel symbols may be in error. The
error may cause a change in data value, which is not treated by this interface, or it may cause a data
symbol to be seen as a sync symbol, or a sync symbol to be seen as a data symbol. However, not more
than one out of any 5 consecutive user channel symbols can be affected in this way.
The IEC958 User channel circuitry will recognize the 98-symbol packet structure, and send the 96 symbol
payload to the ColdFire application. The 96 symbol payload is transmitted to the ColdFire using 2 registers:
The UChannelRcv register — In this register, data is presented 4 symbols at a time to the ColdFire
processor. Every time 4 new valid symbols, received on the IEC958 U-Channel, are present, the
UChannelRcvFull interrupt is asserted. For one 98-symbol packet, 96 symbols are carried across
UChannelRcv. To transfer all this data, 24 UChannelRcvFull interrupts are generated.
The QChannelRcv register — In this register, only the Q bit of the packet is accumulated. Operation
is similar to UChannelRcv. Because only Q-bit is transferred, only 96 Q-bits are transferred for any
98-symbol packet. To transfer this data, 3 QChannelRcvFull interrupts are generated. When
QChannelRcvFull occurs, it is coincident with UChannelRcvFull. There is only one QChannelRcvFull
for every 8 UChannelRcvFull. The convention is that the most significant data is transmitted first, and
is left-aligned in the registers.
The timing, as it applies to packet boundary, is extracted by hardware.The last UChannelRcvFull
corresponding to a given packet should be coincident with the last QChannelRcvFull. In this last U, Q
channel interrupt, symbols 95-98 are received, as are Q-channel bits 67-98. The interrupts are coincident
with ChannelSyncFound, flagging the last symbols of the current frame.
When the start of a new packet is found before the current packet is complete (less than 98 symbols in the
packet), the ChannelLengthError interrupt is set. The application software should read out UChannelRcv
and QchannelRcv registers, discard the value, and assume the start of a new packet.
Table 17-17 Correlation Between Zero Bits and Sync Symbols
NO OF U
CHANNEL ZERO
BITS
CORRESPONDING NUMBER
OF SYNC SYMBOLS
0-1
unpredictable, not allowed
2-10
0
11-22
1
23-34
2
35-45
3
> 45
unpredictable, not allowed
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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