Texas Instruments TMS320C6457 DSP manual Shared-Processing SP Mode, Parameter Termination

Shared-Processing (SP) Mode www.ti.com

The CRC will process one sub-block at time using the data stored from the previous sub-block. The decision bit will be used by a CRC block. After all sub-blocks have been processed, the CRC bits in the CRC block are checked and compared with the last crc_length bits of the frame. If they all match, then the CRC passes.

4.4.3Parameter Termination

The parameters min_iter and max_iter need to be set prior to decode. The decoder must execute min_iter number of iterations. This parameter can be set from 0 to 31. The decoder will stop executing when the iteration count equals max_iter. Max_iter can be set from 0 to 31 and must be equal or greater than min_iter. A zero for max is equal to 32 iterations. A zero for min is equal to 1 iteration.

4.4.3.1Maximum Iterations

Turbo decoders execute the MAP decoder twice per iteration. One execution is for non-interleaved data and the other execution is for interleaved data. This parameter sets the maximum number of decoder iterations for each block of data. Valid sizes are 0 to 31. If either the CRC passed or the SNR stopping criteria threshold has been exceeded, then the decoder will stop early. The last iteration will only process the MAP decoder for the non-interleaved data.

4.4.3.2Minimum Iterations

Turbo decoders execute the MAP decoder twice per iteration. One execution is for non-interleaved data and the other execution is for interleaved data. This parameter sets the minimum number of decoder iterations for each block of data. Valid sizes are 0 to 31 and the min_iter must be less than or equal to the max_iter. The CRC unit will not be enabled until the decoder iteration count is equal or greater than the min_iter parameter. The turbo decoder will not process the CRC, re-encode, or write to the output RAM until the minimum number of iterations has been reached.

5Shared-Processing (SP) Mode

In shared-processing (SP) mode, the DSP sends systematic and parity data, and a priori data. The TCP performs one single MAP decode and outputs extrinsic data. A priori data for MAP1 is obtained by de-interleaving the extrinsic data from the previous MAP2, and a priori data for MAP2 is obtained by interleaving the extrinsics data from the previous MAP1. An overview of the SP mode is shown in Figure 16. Note that the systematic and parity data to be sent to the TCP has to be demultiplexed from the original flow described in Section 5.1. The DSP must perform the input data demultiplexing, interleaving, deinterleaving operations, hard decision calculation, and any stopping criteria algorithm.

18 TMS320C6457 Turbo-Decoder Coprocessor 2 SPRUGK1–March 2009

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