Calculate the Root Mean Sqaure

Official Hardware Mega Due
1

hello,

I'm using the library (... \ Arduino \ hardware \ arduino \ sam \ system \ CMSIS \ CMSIS \ DSP_Lib \ Source \ StatisticsFunctions) to calculate the RMS value of the elements in the input vector. but the value I get is 0.0.

code used:

#define ARM_MATH_CM3
#include <arm_math.h>

float32_t data[5] = {1, 2, 3, 4, 5};
float32_t *d = data;
float32_t *pResult;

void setup() {

Serial.begin(9600); 

}

void loop() {

arm_rms_f32(d, 5, pResult);

Serial.println(*pResult);

while(1){

}

}

output

0.0

2

after a detailed analysis found that the problem is in the following line of code library arm_rms_f32.c:

/ * Compute Rms and store the result in the destination * /
   arm_sqrt_f32 (sum / (float32_t) blockSize, pResult);

if we have:

arm_sqrt_f32 (7.5 pResult);

pResult = 2.74

if we have:

float32_t sum = 30

uint32_t blockSize = 4

arm_sqrt_f32 (sum / (float32_t) blockSize, pResult);

pResult = 0.0

any solution?

3

Hello pedrolopes746,

Try this:

#define ARM_MATH_CM3
#include <arm_math.h>

float32_t data[5] = {1, 2, 3, 4, 5};
float32_t *d = data;
float32_t *pResult;

void setup() {

Serial.begin(9600); 

}

void loop() {

arm_rms_f32(d, 5, pResult);

while(1){

}

}

void arm_rms_f32(
  float32_t * pSrc,
  uint32_t blockSize,
  float32_t * pResult)
{
  float32_t sum = 0.0f;                          /* Accumulator */
  float32_t in;                                  /* Tempoprary variable to store input value */
  uint32_t blkCnt;                               /* loop counter */

#ifndef ARM_MATH_CM0

  /* Run the below code for Cortex-M4 and Cortex-M3 */

  /* loop Unrolling */
  blkCnt = blockSize >> 2u;

  /* First part of the processing with loop unrolling.  Compute 4 outputs at a time.   
   ** a second loop below computes the remaining 1 to 3 samples. */
  while(blkCnt > 0u)
  {
    /* C = A[0] * A[0] + A[1] * A[1] + A[2] * A[2] + ... + A[blockSize-1] * A[blockSize-1] */
    /* Compute sum of the squares and then store the result in a temporary variable, sum  */
    in = *pSrc++;
    sum += in * in;
    in = *pSrc++;
    sum += in * in;
    in = *pSrc++;
    sum += in * in;
    in = *pSrc++;
    sum += in * in;

    /* Decrement the loop counter */
    blkCnt--;
  }

  /* If the blockSize is not a multiple of 4, compute any remaining output samples here.   
   ** No loop unrolling is used. */
  blkCnt = blockSize % 0x4u;

#else

  /* Run the below code for Cortex-M0 */

  /* Loop over blockSize number of values */
  blkCnt = blockSize;

#endif /* #ifndef ARM_MATH_CM0 */

  while(blkCnt > 0u)
  {
    /* C = A[0] * A[0] + A[1] * A[1] + A[2] * A[2] + ... + A[blockSize-1] * A[blockSize-1] */
    /* Compute sum of the squares and then store the results in a temporary variable, sum  */
    in = *pSrc++;
    sum += in * in;

    /* Decrement the loop counter */
    blkCnt--;
  }

  /* Compute Rms, store the result in the destination and print it */
  Serial.println(sqrt(sum / (float32_t) blockSize));
}

Output:

3.32

Regards!

4

thanks Palliser