How to properly read values from analog sensors trough multiplexer?

Question

I need to read several analog sensors on a Arduino Mega ADK. I want to use a multiplexer for this (CD74HC4067E), see the schematics.

The output however from the multiplexer channels is not consistent with the output which i read directly from on the analog input:

Through the Mux x: 333   y: 276  z: 323      Direct analog readings x: 328 | y: 334 | z: 285
Through the Mux x: 333   y: 276  z: 321      Direct analog readings x: 328 | y: 335 | z: 277
Through the Mux x: 334   y: 276  z: 322      Direct analog readings x: 329 | y: 335 | z: 277
Through the Mux x: 333   y: 276  z: 324      Direct analog readings x: 328 | y: 334 | z: 283
Through the Mux x: 333   y: 276  z: 299      Direct analog readings x: 329 | y: 335 | z: 282

ALthough it might seem that there is a wiring problem (simply switch X and Z), my setup is correct (triple checked!).

When i turn the sensor 90 degrees clockwise so that Y is up, i get the following:

Through the Mux x: 334   y: 344  z: 270      Direct analog readings x: 266 | y: 334 | z: 344
Through the Mux x: 334   y: 345  z: 269      Direct analog readings x: 265 | y: 334 | z: 344
Through the Mux x: 333   y: 343  z: 271      Direct analog readings x: 264 | y: 333 | z: 343
Through the Mux x: 335   y: 344  z: 270      Direct analog readings x: 265 | y: 334 | z: 344

so it seems that the X and Z pins should be switched.

I can i improve this?

And my arduino code:

//to hold direct read from the analog output of the ADXL335
int xAnaRead;
int yAnaRead;
int zAnaRead;

//to hold readings from the mux:
int xMuxRead;
int yMuxRead;
int zMuxRead;

//mux pins
int s0 = 8;
int s1 = 9;
int s2 = 10;
int s3 = 11;

//The pin on which the Mux outputs
int SIG_pin = A0;

//Analog read pins
const int xPin = A8;
const int yPin = A9;
const int zPin = A10;

void setup(){
  Serial.begin(9600);
}

void loop(){

  //read value on channel 0 of Mux
  xMuxRead = readMux(0);  
  //read analog value
  int xAnaRead = analogRead(xPin);
  delay(100); //to let the capacitator discharge

  //read value on channel 1 of Mux
  yMuxRead = readMux(1);  
  //read analog value
  int yAnaRead = analogRead(yPin);
  delay(100); //to let the capacitator discharge

  //read value on channel 2 of Mux
  zMuxRead = readMux(2);  
  //read analog value
  int zAnaRead = analogRead(zPin);
  delay(100); //to let the capacitator discharge


  //Output the readings
  Serial.print("Through the Mux x: ");
  Serial.print(xMuxRead);  
  Serial.print("\t y: ");
  Serial.print(yMuxRead);  
  Serial.print("\t z: ");
  Serial.print(zMuxRead);  

  Serial.print("\t\t Direct analog readings x: ");
  Serial.print(xAnaRead);
  Serial.print(" | y: ");
  Serial.print(yAnaRead);
  Serial.print(" | z: ");
  Serial.print(zAnaRead);
  Serial.println("");
  delay(100);//just here to slow down the serial output - Easier to read
}


//this is verbose but it works, and is more readable (i need that :)
int readMux(int channel){

  int controlPin[] = {
    s0, s1, s2, s3        };
  int muxChannel[16][4]={
    {
      0,0,0,0    }
    , //channel 0
    {
      1,0,0,0                }
    , //channel 1
    {
      0,1,0,0                }
    , //channel 2
    {
      1,1,0,0                }
    , //channel 3
    {
      0,0,1,0                }
    , //channel 4
    {
      1,0,1,0                }
    , //channel 5
    {
      0,1,1,0                }
    , //channel 6
    {
      1,1,1,0                }
    , //channel 7
    {
      0,0,0,1                }
    , //channel 8
    {
      1,0,0,1                }
    , //channel 9
    {
      0,1,0,1                }
    , //channel 10
    {
      1,1,0,1                }
    , //channel 11
    {
      0,0,1,1                }
    , //channel 12
    {
      1,0,1,1                }
    , //channel 13
    {
      0,1,1,1                }
    , //channel 14
    {
      1,1,1,1                }  //channel 15
  };

  //loop through the 4 sig
  for(int i = 0; i < 4; i ++){
    digitalWrite(controlPin[i], muxChannel[channel][i]);
  }
  //read the value at the SIG pin
  int val = analogRead(SIG_pin);

  //return the value
  return val;
}

Answer 1

You simply have switched Y and Z on the multiplexer.

Answer 2

For starters, I would make your wiring follow a more logical ordering. For example, upon looking at your schematic, you have 8 going to S1, 9 going to S0, 10 going to S3, and 11 going to S2. The truth table for your chip goes from S0-S3, LSB first, so you have 8 and 9 swapped, and 10 and 11 swapped. Also, you have A8 going to Y and A9 going to X, but your code says that xPin is 8, and yPin is 9. I'd fix these issues first and then try again.

To summarize, the value you're reading to check your multiplexer is the wrong one (for Y and X), and your MUX mapping is also wrong, so when you loop over the channels, you're not switching to the input that you want.

Answer 3

i didnt set the pinmode in the setup:

pinMode(S0, OUTPUT);
pinMode(S1, OUTPUT);
...

when the pins are not initialized in the setup they float between 0 and 1. The readings i got where more luck than wisdom.

After adding initialisation, i also didnt need the capacitator any more.