PM0014 datasheet(37/46 Pages) STMICROELECTRONICS

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PM0014

Signal processing library functions

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6.3.2

fir_q15_q31_q15

fir_q15_q31_q15 (struct Filter *FirPtr).

Description:

Real FIR filter of 16-bit fractional inputs and outputs. Fractional

coefficients are on 32 bits.

Argument:

FirPtr

Filter pointer.

Algorithm:

y(n) =

bi * x(n - i)

Note:

The delay line must be stored in DPRAM. Its size is 2*2*nb_coeff.

Before the first call to the function, the delay line must be initialized

to 0.

There is a saturation to 1 (respectively -1) if the computed output

sample is greater than 1 (respectively lower than -1).

Example of the use for fir_q15_q31_q15:

#define FirNumberCoeffs NumberOfCoefficients

#define NbOutput NumberOfOutputSamples

iram short Delayline[FirNumberCoeffs];

iram short Coeff[2*FirNumberCoeffs];//coef are 32-bit wide

void main ()

{

// Data definition

short Input[NbOutput];

short Out[NbOutput];

// Here should be the coef[] and input[] initialization.

See the Test part...

// Declare a filter structure

struct Filter *Filt;

// Initialisation of filt

Filt.NumberCoeffs = FirNumberCoeffs;

Filt.NumberOutputSamples = NbOutput;

Filt.CoeffPtr = Coeff;

Filt.InputPtr = Input;

Filt.OutputPtr = Out;

Filt.DelayLinePtr = Delayline;

//Filter processing

fir_q15_q31_q15(&Filt);

}

Coefficients must be stored in memory as follow:

addr

Filt.CoeffPtr

----> a0 lsb

addr

Filt.CoeffPtr + 2 ----> a0

msb

...

addr

Filt.CoeffPtr + 4 *(FirNumberCoeffs -2)---->

aFirNumberCoeffs-1 msb

Assembly source

code:

fir_q15_q31_q15.asm

Code size and cycles: Size: 130 bytes

Instruction Cycles: 34 + NbOutput * (16 + 2 * FirNumberCoeffs)

nbcoeff

–

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