File content as of revision 4:04bb0cfea187:

//--------------------------------------------------------------
// Biquad filter for IIR filter of cascade structure
// 2014/10/21, Copyright (c) 2014 MIKAMI, Naoki
//--------------------------------------------------------------

#ifndef IIR_BIQUAD_HPP
#define IIR_BIQUAD_HPP

#include "mbed.h"

// 2nd order IIR filter
class Biquad
{
public:
    struct Coefs { float a1, a2, b1, b2; };

    Biquad() {}     // Default constructore

    Biquad(const Coefs ck)
        : a1_(ck.a1), a2_(ck.a2), b1_(ck.b1), b2_(ck.b2)
    { Clear(); }       

    float Execute(float xn)
    {
        float un = xn + a1_*un1_ + a2_*un2_;
        float yn = un + b1_*un1_ + b2_*un2_;
        
        un2_ = un1_;
        un1_ = un;
        
        return yn;
    }

    void Clear() { un1_ = un2_ = 0; }

private:
    float a1_, a2_, b1_, b2_;
    float un1_, un2_;

    Biquad(const Biquad&);
};

#endif  // IIR_BIQUAD_HPP