void computeIMU()
{
    uint8_t axis;
    static int16_t gyroADCprevious[3] = { 0, 0, 0 };
    int16_t gyroADCp[3];
    int16_t gyroADCinter[3];
    static uint32_t timeInterleave = 0;
#if defined(TRI)
    static int16_t gyroYawSmooth = 0;
#endif

    if (MAG)
        Mag_getADC();
    if (BARO)
        Baro_update();

    //we separate the 2 situations because reading gyro values with a gyro only setup can be acchieved at a higher rate
    //gyro+nunchuk: we must wait for a quite high delay betwwen 2 reads to get both WM+ and Nunchuk data. It works with 3ms
    //gyro only: the delay to read 2 consecutive values can be reduced to only 0.65ms
    if (!ACC && nunchuk) {
        annexCode();
        while ((micros() - timeInterleave) < INTERLEAVING_DELAY);       //interleaving delay between 2 consecutive reads
        timeInterleave = micros();
        WMP_getRawADC();
        getEstimatedAttitude(); // computation time must last less than one interleaving delay
#if BARO
        getEstimatedAltitude();
#endif
        while ((micros() - timeInterleave) < INTERLEAVING_DELAY);       //interleaving delay between 2 consecutive reads
        timeInterleave = micros();
        while (WMP_getRawADC() != 1);   // For this interleaving reading, we must have a gyro update at this point (less delay)

        for (axis = 0; axis < 3; axis++) {
            // empirical, we take a weighted value of the current and the previous values
            // /4 is to average 4 values, note: overflow is not possible for WMP gyro here
            gyroData[axis] = (gyroADC[axis] * 3 + gyroADCprevious[axis] + 2) / 4;
            gyroADCprevious[axis] = gyroADC[axis];
        }
    } else {
        if (ACC) {
            ACC_getADC();
            getEstimatedAttitude();
            if (BARO)
                getEstimatedAltitude();
        }
        if (GYRO)
            Gyro_getADC();
        else
            WMP_getRawADC();
        for (axis = 0; axis < 3; axis++)
            gyroADCp[axis] = gyroADC[axis];
        timeInterleave = micros();
        annexCode();
        if ((micros() - timeInterleave) > 650) {
            annex650_overrun_count++;
        } else {
            while ((micros() - timeInterleave) < 650);  //empirical, interleaving delay between 2 consecutive reads
        }
        if (GYRO)
            Gyro_getADC();
        else
            WMP_getRawADC();
        for (axis = 0; axis < 3; axis++) {
            gyroADCinter[axis] = gyroADC[axis] + gyroADCp[axis];
            // empirical, we take a weighted value of the current and the previous values
            gyroData[axis] = (gyroADCinter[axis] + gyroADCprevious[axis] + 1) / 3;
            gyroADCprevious[axis] = gyroADCinter[axis] / 2;
            if (!ACC)
                accADC[axis] = 0;
        }
    }
#if defined(TRI)
    gyroData[YAW] = (gyroYawSmooth * 2 + gyroData[YAW] + 1) / 3;
    gyroYawSmooth = gyroData[YAW];
#endif
}

// **************************************************
// Simplified IMU based on "Complementary Filter"
// Inspired by http://starlino.com/imu_guide.html
//
// adapted by ziss_dm : http://www.multiwii.com/forum/viewtopic.php?f=8&t=198
//
// The following ideas was used in this project:
// 1) Rotation matrix: http://en.wikipedia.org/wiki/Rotation_matrix
// 2) Small-angle approximation: http://en.wikipedia.org/wiki/Small-angle_approximation
// 3) C. Hastings approximation for atan2()
// 4) Optimization tricks: http://www.hackersdelight.org/
//
// Currently Magnetometer uses separate CF which is used only
// for heading approximation.
//
// Modified: 19/04/2011  by ziss_dm
// Version: V1.1
//
// code size deduction and tmp vector intermediate step for vector rotation computation: October 2011 by Alex
// **************************************************

//******  advanced users settings *******************
/* Set the Low Pass Filter factor for ACC */
/* Increasing this value would reduce ACC noise (visible in GUI), but would increase ACC lag time*/
/* Comment this if  you do not want filter at all.*/
/* Default WMC value: 8*/
#define ACC_LPF_FACTOR 4

/* Set the Low Pass Filter factor for Magnetometer */
/* Increasing this value would reduce Magnetometer noise (not visible in GUI), but would increase Magnetometer lag time*/
/* Comment this if  you do not want filter at all.*/
/* Default WMC value: n/a*/
//#define MG_LPF_FACTOR 4

/* Set the Gyro Weight for Gyro/Acc complementary filter */
/* Increasing this value would reduce and delay Acc influence on the output of the filter*/
/* Default WMC value: 300*/
#define GYR_CMPF_FACTOR 310.0f

/* Set the Gyro Weight for Gyro/Magnetometer complementary filter */
/* Increasing this value would reduce and delay Magnetometer influence on the output of the filter*/
/* Default WMC value: n/a*/
#define GYR_CMPFM_FACTOR 200.0f

//****** end of advanced users settings *************

#define INV_GYR_CMPF_FACTOR   (1.0f / (GYR_CMPF_FACTOR  + 1.0f))
#define INV_GYR_CMPFM_FACTOR  (1.0f / (GYR_CMPFM_FACTOR + 1.0f))
#if GYRO
#define GYRO_SCALE ((2380 * PI)/((32767.0f / 4.0f ) * 180.0f * 1000000.0f))     //should be 2279.44 but 2380 gives better result
  // +-2000/sec deg scale
  //#define GYRO_SCALE ((200.0f * PI)/((32768.0f / 5.0f / 4.0f ) * 180.0f * 1000000.0f) * 1.5f)     
  // +- 200/sec deg scale
  // 1.5 is emperical, not sure what it means
  // should be in rad/sec
#else
#define GYRO_SCALE (1.0f/200e6f)
  // empirical, depends on WMP on IDG datasheet, tied of deg/ms sensibility
  // !!!!should be adjusted to the rad/sec
#endif
// Small angle approximation
#define ssin(val) (val)
#define scos(val) 1.0f

typedef struct fp_vector {
    float X;
    float Y;
    float Z;
} t_fp_vector_def;

typedef union {
    float A[3];
    t_fp_vector_def V;
} t_fp_vector;

int16_t _atan2(float y, float x)
{
#define fp_is_neg(val) ((((byte*)&val)[3] & 0x80) != 0)
    float z = y / x;
    int16_t zi = abs(int16_t(z * 100));
    int8_t y_neg = fp_is_neg(y);
    if (zi < 100) {
        if (zi > 10)
            z = z / (1.0f + 0.28f * z * z);
        if (fp_is_neg(x)) {
            if (y_neg)
                z -= PI;
            else
                z += PI;
        }
    } else {
        z = (PI / 2.0f) - z / (z * z + 0.28f);
        if (y_neg)
            z -= PI;
    }
    z *= (180.0f / PI * 10);
    return z;
}

// Rotate Estimated vector(s) with small angle approximation, according to the gyro data
void rotateV(struct fp_vector *v, float *delta)
{
    fp_vector v_tmp = *v;
    v->Z -= delta[ROLL] * v_tmp.X + delta[PITCH] * v_tmp.Y;
    v->X += delta[ROLL] * v_tmp.Z - delta[YAW] * v_tmp.Y;
    v->Y += delta[PITCH] * v_tmp.Z + delta[YAW] * v_tmp.X;
}

void getEstimatedAttitude()
{
    uint8_t axis;
    int32_t accMag = 0;
    static t_fp_vector EstG;
#if MAG
    static t_fp_vector EstM;
#endif
#if defined(MG_LPF_FACTOR)
    static int16_t mgSmooth[3];
#endif
#if defined(ACC_LPF_FACTOR)
    static int16_t accTemp[3];  //projection of smoothed and normalized magnetic vector on x/y/z axis, as measured by magnetometer
#endif
    static uint16_t previousT;
    uint16_t currentT = micros();
    float scale, deltaGyroAngle[3];

    scale = (currentT - previousT) * GYRO_SCALE;
    previousT = currentT;

    // Initialization
    for (axis = 0; axis < 3; axis++) {
        deltaGyroAngle[axis] = gyroADC[axis] * scale;
#if defined(ACC_LPF_FACTOR)
        accTemp[axis] = (accTemp[axis] - (accTemp[axis] >> ACC_LPF_FACTOR)) + accADC[axis];
        accSmooth[axis] = accTemp[axis] >> ACC_LPF_FACTOR;
#define ACC_VALUE accSmooth[axis]
#else
        accSmooth[axis] = accADC[axis];
#define ACC_VALUE accADC[axis]
#endif
//    accMag += (ACC_VALUE * 10 / (int16_t)acc_1G) * (ACC_VALUE * 10 / (int16_t)acc_1G);
        accMag += (int32_t) ACC_VALUE *ACC_VALUE;
#if MAG
#if defined(MG_LPF_FACTOR)
        mgSmooth[axis] = (mgSmooth[axis] * (MG_LPF_FACTOR - 1) + magADC[axis]) / MG_LPF_FACTOR; // LPF for Magnetometer values
#define MAG_VALUE mgSmooth[axis]
#else
#define MAG_VALUE magADC[axis]
#endif
#endif
    }
    accMag = accMag * 100 / ((int32_t) acc_1G * acc_1G);

    rotateV(&EstG.V, deltaGyroAngle);
#if MAG
    rotateV(&EstM.V, deltaGyroAngle);
#endif

    if (abs(accSmooth[ROLL]) < acc_25deg && abs(accSmooth[PITCH]) < acc_25deg && accSmooth[YAW] > 0)
        smallAngle25 = 1;
    else
        smallAngle25 = 0;

    // Apply complimentary filter (Gyro drift correction)
    // If accel magnitude >1.4G or <0.6G and ACC vector outside of the limit range => we neutralize the effect of accelerometers in the angle estimation.
    // To do that, we just skip filter, as EstV already rotated by Gyro
    if ((36 < accMag && accMag < 196) || smallAngle25)
        for (axis = 0; axis < 3; axis++) {
            int16_t acc = ACC_VALUE;
#if not defined(TRUSTED_ACCZ)
            if (smallAngle25 && axis == YAW)
                //We consider ACCZ = acc_1G when the acc on other axis is small.
                //It's a tweak to deal with some configs where ACC_Z tends to a value < acc_1G when high throttle is applied.
                //This tweak applies only when the multi is not in inverted position
                acc = acc_1G;
#endif
            EstG.A[axis] = (EstG.A[axis] * GYR_CMPF_FACTOR + acc) * INV_GYR_CMPF_FACTOR;
        }
#if MAG
    for (axis = 0; axis < 3; axis++)
        EstM.A[axis] = (EstM.A[axis] * GYR_CMPFM_FACTOR + MAG_VALUE) * INV_GYR_CMPFM_FACTOR;
#endif

    // Attitude of the estimated vector
    angle[ROLL] = _atan2(EstG.V.X, EstG.V.Z);
    angle[PITCH] = _atan2(EstG.V.Y, EstG.V.Z);
#if MAG
    // Attitude of the cross product vector GxM
    heading = _atan2(EstG.V.X * EstM.V.Z - EstG.V.Z * EstM.V.X, EstG.V.Z * EstM.V.Y - EstG.V.Y * EstM.V.Z) / 10;
#endif
}

float InvSqrt(float x)
{
    union {
        int32_t i;
        float f;
    } conv;
    conv.f = x;
    conv.i = 0x5f3759df - (conv.i >> 1);
    return 0.5f * conv.f * (3.0f - x * conv.f * conv.f);
}

int32_t isq(int32_t x)
{
    return x * x;
}

#define UPDATE_INTERVAL 25000   // 40hz update rate (20hz LPF on acc)
#define INIT_DELAY      4000000 // 4 sec initialization delay
#define Kp1 5.5f                // PI observer velocity gain
#define Kp2 10.0f               // PI observer position gain
#define Ki  0.01f               // PI observer integral gain (bias cancellation)
#define dt  (UPDATE_INTERVAL / 1000000.0f)

void getEstimatedAltitude()
{
    static uint8_t inited = 0;
    static int16_t AltErrorI = 0;
    static float AccScale;
    static uint32_t deadLine = INIT_DELAY;
    int16_t AltError;
    int16_t InstAcc;
    static int32_t tmpAlt;
    static int16_t EstVelocity = 0;
    static uint32_t velTimer;
    static int16_t lastAlt;

    if (currentTime < deadLine)
        return;
    deadLine = currentTime + UPDATE_INTERVAL;
    // Soft start

    if (!inited) {
        inited = 1;
        tmpAlt = BaroAlt * 10;
        AccScale = 100 * 9.80665f / acc_1G;
    }
    // Estimation Error
    AltError = BaroAlt - EstAlt;
    AltErrorI += AltError;
    AltErrorI = constrain(AltErrorI, -2500, +2500);
    // Gravity vector correction and projection to the local Z
    //InstAcc = (accADC[YAW] * (1 - acc_1G * InvSqrt(isq(accADC[ROLL]) + isq(accADC[PITCH]) + isq(accADC[YAW])))) * AccScale + (Ki) * AltErrorI;
#if defined(TRUSTED_ACCZ)
    InstAcc = (accADC[YAW] * (1 - acc_1G * InvSqrt(isq(accADC[ROLL]) + isq(accADC[PITCH]) + isq(accADC[YAW])))) * AccScale + AltErrorI / 100;
#else
    InstAcc = AltErrorI / 100;
#endif

    // Integrators
    tmpAlt += EstVelocity * (dt * dt) + (Kp2 * dt) * AltError;
    EstVelocity += InstAcc + Kp1 * AltError;
    EstVelocity = constrain(EstVelocity, -10000, +10000);

    EstAlt = tmpAlt / 10;

    if (currentTime < velTimer)
        return;
    velTimer = currentTime + 500000;
    zVelocity = tmpAlt - lastAlt;
    lastAlt = tmpAlt;

    debug4 = zVelocity;
}