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# arm_fir_decimate_fast_q15.c

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    00001 /* ----------------------------------------------------------------------
    00002 * Copyright (C) 2010 ARM Limited. All rights reserved.
    00003 *
    00004 * $Date:        29. November 2010
    00005 * $Revision:    V1.0.3
    00006 *
    00007 * Project:      CMSIS DSP Library
    00008 * Title:        arm_fir_decimate_fast_q15.c
    00009 *
    00010 * Description:  Fast Q15 FIR Decimator.
    00011 *
    00012 * Target Processor: Cortex-M4/Cortex-M3
    00013 *
    00014 * Version 1.0.3 2010/11/29
    00015 *    Re-organized the CMSIS folders and updated documentation.
    00016 *
    00017 * Version 1.0.2 2010/11/11
    00018 *    Documentation updated.
    00019 *
    00020 * Version 1.0.1 2010/10/05
    00021 *    Production release and review comments incorporated.
    00022 *
    00023 * Version 1.0.0 2010/09/20
    00024 *    Production release and review comments incorporated.
    00025 * -------------------------------------------------------------------- */
    00026
    00027 #include "arm_math.h"
    00028
    00060 void arm_fir_decimate_fast_q15(
    00061   const arm_fir_decimate_instance_q15 * S,
    00062   q15_t * pSrc,
    00063   q15_t * pDst,
    00064   uint32_t blockSize)
    00065 {
    00066   q15_t *pState = S->pState;                     /* State pointer */
    00067   q15_t *pCoeffs = S->pCoeffs;                   /* Coefficient pointer */
    00068   q15_t *pStateCurnt;                            /* Points to the current sample of the state */
    00069   q15_t *px;                                     /* Temporary pointer for state buffer */
    00070   q15_t *pb;                                     /* Temporary pointer coefficient buffer */
    00071   q31_t x0, c0;                                  /* Temporary variables to hold state and coefficient values */
    00072   q31_t sum0;                                    /* Accumulators */
    00073   uint32_t numTaps = S->numTaps;                 /* Number of taps */
    00074   uint32_t i, blkCnt, tapCnt, outBlockSize = blockSize / S->M;  /* Loop counters */
    00075
    00076
    00077   /* S->pState buffer contains previous frame (numTaps - 1) samples */
    00078   /* pStateCurnt points to the location where the new input data should be written */
    00079   pStateCurnt = S->pState + (numTaps - 1u);
    00080
    00081   /* Total number of output samples to be computed */
    00082   blkCnt = outBlockSize;
    00083
    00084   while(blkCnt > 0u)
    00085   {
    00086     /* Copy decimation factor number of new input samples into the state buffer */
    00087     i = S->M;
    00088
    00089     do
    00090     {
    00091       *pStateCurnt++ = *pSrc++;
    00092
    00093     } while(--i);
    00094
    00095     /*Set sum to zero */
    00096     sum0 = 0;
    00097
    00098     /* Initialize state pointer */
    00099     px = pState;
    00100
    00101     /* Initialize coeff pointer */
    00102     pb = pCoeffs;
    00103
    00104     /* Loop unrolling.  Process 4 taps at a time. */
    00105     tapCnt = numTaps >> 2;
    00106
    00107     /* Loop over the number of taps.  Unroll by a factor of 4.
    00108      ** Repeat until we've computed numTaps-4 coefficients. */
    00109     while(tapCnt > 0u)
    00110     {
    00111       /* Read the Read b[numTaps-1] and b[numTaps-2]  coefficients */
    00112       c0 = *__SIMD32(pb)++;
    00113
    00114       /* Read x[n-numTaps-1] and x[n-numTaps-2]sample */
    00115       x0 = *__SIMD32(px)++;
    00116
    00117       /* Perform the multiply-accumulate */
    00118       sum0 = __SMLAD(x0, c0, sum0);
    00119
    00120       /* Read the b[numTaps-3] and b[numTaps-4] coefficient */
    00121       c0 = *__SIMD32(pb)++;
    00122
    00123       /* Read x[n-numTaps-2] and x[n-numTaps-3] sample */
    00124       x0 = *__SIMD32(px)++;
    00125
    00126       /* Perform the multiply-accumulate */
    00127       sum0 = __SMLAD(x0, c0, sum0);
    00128
    00129       /* Decrement the loop counter */
    00130       tapCnt--;
    00131     }
    00132
    00133     /* If the filter length is not a multiple of 4, compute the remaining filter taps */
    00134     tapCnt = numTaps % 0x4u;
    00135
    00136     while(tapCnt > 0u)
    00137     {
    00138       /* Read coefficients */
    00139       c0 = *pb++;
    00140
    00141       /* Fetch 1 state variable */
    00142       x0 = *px++;
    00143
    00144       /* Perform the multiply-accumulate */
    00145       sum0 = __SMLAD(x0, c0, sum0);
    00146
    00147       /* Decrement the loop counter */
    00148       tapCnt--;
    00149     }
    00150
    00151     /* Advance the state pointer by the decimation factor
    00152      * to process the next group of decimation factor number samples */
    00153     pState = pState + S->M;
    00154
    00155     /* Store filter output , smlad returns the values in 2.14 format */
    00156     /* so downsacle by 15 to get output in 1.15 */
    00157     *pDst++ = (q15_t) ((sum0 >> 15));
    00158
    00159     /* Decrement the loop counter */
    00160     blkCnt--;
    00161   }
    00162
    00163   /* Processing is complete.
    00164    ** Now copy the last numTaps - 1 samples to the satrt of the state buffer.
    00165    ** This prepares the state buffer for the next function call. */
    00166
    00167   /* Points to the start of the state buffer */
    00168   pStateCurnt = S->pState;
    00169
    00170   i = (numTaps - 1u) >> 2u;
    00171
    00172   /* copy data */
    00173   while(i > 0u)
    00174   {
    00175     *__SIMD32(pStateCurnt)++ = *__SIMD32(pState)++;
    00176     *__SIMD32(pStateCurnt)++ = *__SIMD32(pState)++;
    00177
    00178     /* Decrement the loop counter */
    00179     i--;
    00180   }
    00181
    00182   i = (numTaps - 1u) % 0x04u;
    00183
    00184   /* copy data */
    00185   while(i > 0u)
    00186   {
    00187     *pStateCurnt++ = *pState++;
    00188
    00189     /* Decrement the loop counter */
    00190     i--;
    00191   }
    00192 }
    00193

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