/*created  2020.
  by Fathia Hamed.
  FabLab Libya Team
 ****
 This example code is used to filter the input signal as Low Pass Filter at fc=10Hz & sample rate=500.
 read analoge signal from (a1)  and write it at (DAC1).
 you can see the input signal at DAC0.
 this signal can be generated & tested by (MyDAQ) using NI Elvis software. 
 Note: this code running only on ARDUINO DUE.
 Note: we must add DC offset to appear full signal.
 neither that the ARDUINO will cut the negative edge.
  ****
 Reference: http://musicdsp.org/
  */
int  led = 13;
float wc,wc2,wc3,wc4,k,k2,k3,k4,sq_tmp1,sq_tmp2,sq_tmp4,a_tmp;
float b1, b2, b3, b4, a0,a1,a2,a3,a4;
float input,output,pi=3.14285714285714,sqrt2;
int fc=10, srate=500;

float xm1,xm2,xm3,xm4,ym1,ym2,ym3,ym4;
void setup() {
  Serial.begin(9600);
//Blink led 13 to check code. 
pinMode(led, OUTPUT);
digitalWrite(led, HIGH); // For test
delay(3000);
digitalWrite(led, LOW);
delay(1000);
/////////////
wc=2*pi*fc;
wc2=wc*wc;
wc3=wc2*wc;
wc4=wc2*wc2;
k=wc/tan(pi*fc/srate);
k2=k*k;
k3=k2*k;
k4=k2*k2;
sqrt2=sqrt(2);
sq_tmp1=sqrt2*wc3*k;
sq_tmp2=sqrt2*wc*k3;
a_tmp=4*wc2*k2+2*sq_tmp1+k4+2*sq_tmp2+wc4;

b1=(4*(wc4+sq_tmp1-k4-sq_tmp2))/a_tmp;
b2=(6*wc4-8*wc2*k2+6*k4)/a_tmp;
b3=(4*(wc4-sq_tmp1+sq_tmp2-k4))/a_tmp;
b4=(k4-2*sq_tmp1+wc4-2*sq_tmp2+4*wc2*k2)/a_tmp;
///////////////
a0=wc4/a_tmp;
a1=4*wc4/a_tmp;
a2=6*wc4/a_tmp;
a3=a1;
a4=a0;
 }

void loop() {
  
// analogWriteResolution(12);
//analogReadResolution(12);

///////////////
input = analogRead(1);
// analogWrite(DAC0,input);
output=a0*input+a1*xm1+a2*xm2+a3*xm3+a4*xm4-b1*ym1-b2*ym2-b3*ym3-b4*ym4;
xm4=xm3;
xm3=xm2;
xm2=xm1;
xm1=input;
ym4=ym3;
ym3=ym2;
ym2=ym1;
ym1=output;
//analogWrite(DAC1,output);
Serial.print(input);
Serial.print(" "); 
Serial.println(output);
delayMicroseconds(2000);
}