Cortex M7 DSP moving average UMAAL

Mr_M_from_G over 5 years ago

Hello,

I am looking to do a moving average function using DSP instructions of ARM Cortex M7. Unfortunately I couldn't find a direct example. My goal is to have variables for

- the sum

- the new value

- the oldest value

Then the algorithm is sum = sum + new value - oldest value and average = sum / ( number of values between oldest and new), ie two instructions

I think instruction UMAAL could be good for this, I found it in instruction set summary, but it is not in CMSIS library. Why is that? Where can I find details about it and how can I create my own C asm caller?

Thanks for any help and hints or other ideas on this topic, I hope it also helps other developers.

Kind regards

Martin

(I posted this also in Processor discussions, because I missed the embedded forum. Please delete it there, thanks)

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0 Simon Craske over 5 years ago

Optimised as you describe, yields the same division, where an optimising compiler should choose the optimal implementation based on it being able to deduce that WINDOW is a constant, and the particular characteristics of the operand types;

#include <stdint.h>

#define WINDOW 73

typedef uint16_t sample_t;
typedef uint32_t accum_t;

sample_t samples[WINDOW];
accum_t sum;
unsigned int position;

sample_t add_and_get_average(sample_t sample)
{
  if(position >= WINDOW)
    position = 0;

  sum -= samples[position];
  sum += sample;
  samples[position] = sample;
  position++;

  return sum / WINDOW;
}

Simon.

Reply

0 Simon Craske over 5 years ago

#include <stdint.h>

#define WINDOW 73

typedef uint16_t sample_t;
typedef uint32_t accum_t;

sample_t samples[WINDOW];
accum_t sum;
unsigned int position;

sample_t add_and_get_average(sample_t sample)
{
  if(position >= WINDOW)
    position = 0;

  sum -= samples[position];
  sum += sample;
  samples[position] = sample;
  position++;

  return sum / WINDOW;
}

Simon.

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