Butterworth Sample

butterworth.c

fp gain = 0x80;
fp k1   = 0x2E00;
fp k2   = 0x6C00;

fp x_phi[MAX_COEF];
fp y_phi[MAX_COEF];

int32_t c_phi;

void butterworth_phi () {

	// Save the sign, then remove it
	int32_t sign = (c_phi >> 31);
	c_phi *= sign;

	// Shuffle older samples back 
	x_phi[0] = x_phi[1]; x_phi[1] = x_phi[2];
	y_phi[0] = y_phi[1]; y_phi[1] = y_phi[2];

	// Insert latest sample (gain is actually 1/gain)
	x_phi[2] = mul(INT_TO_FP(c_phi), gain);

	// Compute next output sample
	fp a = mul(k1, y_phi[0]);
	fp b = mul(k2, y_phi[1]);
	y_phi[2] = x_phi[0] + (x_phi[1] << 1) + x_phi[2] - a + b;

	// Update the global variable
	c_phi = FP_TO_INT(y_phi[2]);

	// Restore the sign
	c_phi *= sign;
}