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# arm_sqrt_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_sqrt_q15.c
    00009 *
    00010 * Description:  Q15 square root function.
    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 #include "arm_common_tables.h"
    00029
    00030
    00048 arm_status arm_sqrt_q15(
    00049   q15_t in,
    00050   q15_t * pOut)
    00051 {
    00052   q31_t out;
    00053   q31_t prevOut;
    00054   q15_t oneByOut;
    00055   uint32_t sign_bits;
    00056
    00057   if(in > 0)
    00058   {
    00059     /* run for ten iterations */
    00060
    00061     /* Take initial guess as half of the input and first iteration */
    00062     out = ((q31_t) in >> 1u) + 0x3FFF;
    00063
    00064     /* Calculation of reciprocal of out */
    00065     /* oneByOut contains reciprocal of out which is in 2.14 format
    00066        and oneByOut should be upscaled by signBits */
    00067     sign_bits = arm_recip_q15((q15_t) out, &oneByOut, armRecipTableQ15);
    00068
    00069     /* 0.5 * (out) */
    00070     out = out >> 1u;
    00071     /* prevOut = 0.5 * out + (in * (oneByOut << signBits))) */
    00072     prevOut = out + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00073
    00074     /* Third iteration */
    00075     sign_bits = arm_recip_q15((q15_t) prevOut, &oneByOut, armRecipTableQ15);
    00076     prevOut = prevOut >> 1u;
    00077     out = prevOut + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00078
    00079     sign_bits = arm_recip_q15((q15_t) out, &oneByOut, armRecipTableQ15);
    00080     out = out >> 1u;
    00081     prevOut = out + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00082
    00083     /* Fifth iteration */
    00084     sign_bits = arm_recip_q15((q15_t) prevOut, &oneByOut, armRecipTableQ15);
    00085     prevOut = prevOut >> 1u;
    00086     out = prevOut + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00087
    00088     sign_bits = arm_recip_q15((q15_t) out, &oneByOut, armRecipTableQ15);
    00089     out = out >> 1u;
    00090     prevOut = out + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00091
    00092     /* Seventh iteration */
    00093     sign_bits = arm_recip_q15((q15_t) prevOut, &oneByOut, armRecipTableQ15);
    00094     prevOut = prevOut >> 1u;
    00095     out = prevOut + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00096
    00097     sign_bits = arm_recip_q15((q15_t) out, &oneByOut, armRecipTableQ15);
    00098     out = out >> 1u;
    00099     prevOut = out + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00100
    00101     sign_bits = arm_recip_q15((q15_t) prevOut, &oneByOut, armRecipTableQ15);
    00102     prevOut = prevOut >> 1u;
    00103     out = prevOut + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00104
    00105     /* tenth iteration */
    00106     sign_bits = arm_recip_q15((q15_t) out, &oneByOut, armRecipTableQ15);
    00107     out = out >> 1u;
    00108     *pOut = out + (((q15_t) (((q31_t) in * oneByOut) >> 16)) << sign_bits);
    00109
    00110     return (ARM_MATH_SUCCESS);
    00111   }
    00112   else
    00113   {
    00114
    00115     *pOut = 0;
    00116     return (ARM_MATH_ARGUMENT_ERROR);
    00117   }
    00118
    00119 }
    00120

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