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  1. /* ----------------------------------------------------------------------
  2.  * Project:      CMSIS DSP Library
  3.  * Title:        arm_sqrt_q15.c
  4.  * Description:  Q15 square root function
  5.  *
  6.  * $Date:        27. January 2017
  7.  * $Revision:    V.1.5.1
  8.  *
  9.  * Target Processor: Cortex-M cores
  10.  * -------------------------------------------------------------------- */
  11. /*
  12.  * Copyright (C) 2010-2017 ARM Limited or its affiliates. All rights reserved.
  13.  *
  14.  * SPDX-License-Identifier: Apache-2.0
  15.  *
  16.  * Licensed under the Apache License, Version 2.0 (the License); you may
  17.  * not use this file except in compliance with the License.
  18.  * You may obtain a copy of the License at
  19.  *
  20.  * www.apache.org/licenses/LICENSE-2.0
  21.  *
  22.  * Unless required by applicable law or agreed to in writing, software
  23.  * distributed under the License is distributed on an AS IS BASIS, WITHOUT
  24.  * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  25.  * See the License for the specific language governing permissions and
  26.  * limitations under the License.
  27.  */
  28.  
  29. #include "arm_math.h"
  30. #include "arm_common_tables.h"
  31.  
  32.  
  33. /**
  34.  * @ingroup groupFastMath
  35.  */
  36.  
  37. /**
  38.  * @addtogroup SQRT
  39.  * @{
  40.  */
  41.  
  42.   /**
  43.    * @brief  Q15 square root function.
  44.    * @param[in]   in     input value.  The range of the input value is [0 +1) or 0x0000 to 0x7FFF.
  45.    * @param[out]  *pOut  square root of input value.
  46.    * @return The function returns ARM_MATH_SUCCESS if the input value is positive
  47.    * and ARM_MATH_ARGUMENT_ERROR if the input is negative.  For
  48.    * negative inputs, the function returns *pOut = 0.
  49.    */
  50.  
  51. arm_status arm_sqrt_q15(
  52.   q15_t in,
  53.   q15_t * pOut)
  54. {
  55.   q15_t number, temp1, var1, signBits1, half;
  56.   q31_t bits_val1;
  57.   float32_t temp_float1;
  58.   union
  59.   {
  60.     q31_t fracval;
  61.     float32_t floatval;
  62.   } tempconv;
  63.  
  64.   number = in;
  65.  
  66.   /* If the input is a positive number then compute the signBits. */
  67.   if (number > 0)
  68.   {
  69.     signBits1 = __CLZ(number) - 17;
  70.  
  71.     /* Shift by the number of signBits1 */
  72.     if ((signBits1 % 2) == 0)
  73.     {
  74.       number = number << signBits1;
  75.     }
  76.     else
  77.     {
  78.       number = number << (signBits1 - 1);
  79.     }
  80.  
  81.     /* Calculate half value of the number */
  82.     half = number >> 1;
  83.     /* Store the number for later use */
  84.     temp1 = number;
  85.  
  86.     /* Convert to float */
  87.     temp_float1 = number * 3.051757812500000e-005f;
  88.     /*Store as integer */
  89.     tempconv.floatval = temp_float1;
  90.     bits_val1 = tempconv.fracval;
  91.     /* Subtract the shifted value from the magic number to give intial guess */
  92.     bits_val1 = 0x5f3759df - (bits_val1 >> 1);  /* gives initial guess */
  93.     /* Store as float */
  94.     tempconv.fracval = bits_val1;
  95.     temp_float1 = tempconv.floatval;
  96.     /* Convert to integer format */
  97.     var1 = (q31_t) (temp_float1 * 16384);
  98.  
  99.     /* 1st iteration */
  100.     var1 = ((q15_t) ((q31_t) var1 * (0x3000 -
  101.                                      ((q15_t)
  102.                                       ((((q15_t)
  103.                                          (((q31_t) var1 * var1) >> 15)) *
  104.                                         (q31_t) half) >> 15))) >> 15)) << 2;
  105.     /* 2nd iteration */
  106.     var1 = ((q15_t) ((q31_t) var1 * (0x3000 -
  107.                                      ((q15_t)
  108.                                       ((((q15_t)
  109.                                          (((q31_t) var1 * var1) >> 15)) *
  110.                                         (q31_t) half) >> 15))) >> 15)) << 2;
  111.     /* 3rd iteration */
  112.     var1 = ((q15_t) ((q31_t) var1 * (0x3000 -
  113.                                      ((q15_t)
  114.                                       ((((q15_t)
  115.                                          (((q31_t) var1 * var1) >> 15)) *
  116.                                         (q31_t) half) >> 15))) >> 15)) << 2;
  117.  
  118.     /* Multiply the inverse square root with the original value */
  119.     var1 = ((q15_t) (((q31_t) temp1 * var1) >> 15)) << 1;
  120.  
  121.     /* Shift the output down accordingly */
  122.     if ((signBits1 % 2) == 0)
  123.     {
  124.       var1 = var1 >> (signBits1 / 2);
  125.     }
  126.     else
  127.     {
  128.       var1 = var1 >> ((signBits1 - 1) / 2);
  129.     }
  130.     *pOut = var1;
  131.  
  132.     return (ARM_MATH_SUCCESS);
  133.   }
  134.   /* If the number is a negative number then store zero as its square root value */
  135.   else
  136.   {
  137.     *pOut = 0;
  138.     return (ARM_MATH_ARGUMENT_ERROR);
  139.   }
  140. }
  141.  
  142. /**
  143.  * @} end of SQRT group
  144.  */
  145.