topic Efficient circular buffer for use with CMSIS-DSP lib K64 in Kinetis Microcontrollers

Efficient circular buffer for use with CMSIS-DSP lib K64

jparrish88 — Thu, 06 Aug 2015 22:55:52 GMT

Hello all,

I'm trying to figure out an efficient method to pass my (our any version of circular buffer) to the CMSIS-DSPs functions. Specifically, I'm targeting the arm_cfft_f32 function. The issue here is that it requires a source pointer to a contiguous memory space. Yet, the design of a ring or circular buffer makes that difficult.

I'm using a K64, using 32 bit floats, with 32 point input buffer.

My current buffer code is:

#define BUFF_SIZE 32
#define BUFF_SIZE_MASK (BUFF_SIZE-1)
typedef struct filbuffer{
    float buff[BUFF_SIZE];
    int writeIndex;
}filbuffer;
void initBuffer(filbuffer* buffer){
  int i;
  buffer->writeIndex = 0;
  for(i=0;i<BUFF_SIZE;i++){
  buffer->buff[i]=0;
  }
}
/* Write to the buffer */
void write(filbuffer* buffer, float value){
    buffer->buff[(buffer->writeIndex++) & BUFF_SIZE_MASK] = value;
}
/* Read from the buffer -- probably not needed */
float readn(filbuffer* buffer, int Xn){
    return buffer->buff[(buffer->writeIndex + (~Xn)) & BUFF_SIZE_MASK];
}

Re: Efficient circular buffer for use with CMSIS-DSP lib K64

xiangjun_rong — Fri, 07 Aug 2015 04:31:36 GMT

Hi, Friends,

I see that you would like to use the following api function to compute the complex FFT which is defined in arm_cfft_f32.c.

If it is the case, you use the function:

void arm_cfft_f32(

const arm_cfft_instance_f32 * S,

float32_t * p1,

uint8_t ifftFlag,

uint8_t bitReverseFlag)

* The FFT functions operate in-place. That is, the array holding the input data

* will also be used to hold the corresponding result. The input data is complex

* and contains <code>2*fftLen</code> interleaved values as shown below.

* <pre> {real[0], imag[0], real[1], imag[1],..} </pre>

* The FFT result will be contained in the same array and the frequency domain

* values will have the same interleaving.

as the above direction, you can define a structure:

typedef struct {

float32_t real;

float32_t image;

} complex;

complex array[64];

after you initialize the array[] and call the arm_cfft_f32(), it is okay.

Xiangjun Rong