Audio Interface Module (AIM)MCF5253 Reference Manual, Rev. 117-22 Freescale SemiconductorThe timing, as it applies to packet boundary, is extracted by hardware.The last UChannelRcvFullcorresponding to a given packet should be coincident with the last QChannelRcvFull. In this last U, Qchannel interrupt, symbols 95-98 are received, as are Q-channel bits 67-98. The interrupts are coincidentwith 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 thepacket), the ChannelLengthError interrupt is set. The application software should read out UChannelRcvand QchannelRcv registers, discard the value, and assume the start of a new packet.As previously mentioned, packet sync extraction is tolerant for single-symbol errors. Packet sync detectionis based on the recognition of the sequence data-sync-sync-data in the symbol stream, because this is theonly syncing sequence that is not affected by single errors. If the sync symbol is not found 98 symbols afterthe previous occurrence, it is assumed to be destroyed by channel error, and a new sync symbol isinterpolated.Normally, only data bytes are passed to the application software. Every data byte will have its mostsignificant bit set. If sync symbols are passed to the application software (i.e., the processor), they are seenas all-zero symbols. Sync symbols can only end up in the data stream due to channel error.17.6.1.11 Behavior of User Channel Receive Interface (non-CD data)This section details the behavior of the user channel receive interface on incoming non-CD data.This mode is selected if UsyncMode (bit 1) in register CD Text control is set ‘0’.In non-CD mode, the IEC958 User channel stream is recognized as a sequence of data symbols. No packetrecognition is done.Any sequence found in the IEC958 U-channel stream starting with a leading one, followed by 7information bits, is recognized as a data symbol. Subsequent data symbols are separated by pauses. Duringthe pause, zero bits are seen on the IEC958 U-channel.Four consecutive data symbols seen in the IEC958 U-Channel stream are grouped together into theUChannelRcv register. First symbol is left, last symbol is right aligned. Whenever UChannelRcv contains4 new data symbols, UChannelRcvFull is asserted.In this mode, the operation of QchannelRcv and associated interrupt QChannelRcvFull is reserved,undefined. Also reserved, undefined is the operation of ChannelLengthError and ChannelSyncFound.The U-channel is extracted and output by the IEC958 Receive block on EBURcvUChannelStream.Processing is done by the CD-Subcode as described in Section 17.7, “Processor Interface Overview.”17.6.2 IEC958 (SPDIF) Transmit InterfaceThe IEC958 interface provides the necessary features to allow transmitting of digital data according to theIEC958 specification with the exception that only 20-bit data is supported. The 4 LSB’s of the 24-bit dataword are always ‘0’. In addition to data, the interface allows for transmission of the C- and U-channels andcontrol over the Valid flag. Note: For the U-channel, only the CD User Data format is supported.
