Using CD74HC4067 16-Channel Analog Multiplexer Demultiplexer is not working as expected

Question

I am working on a project that needs multiple digital signals and I am considering the CD74HC4067 (on a break-board) to work as a multiplexer. More specifically i am trying to build a keypad with multiple ttp123 break-boards. Each ttp123 signal is connected to one of the 0-15 pins of the CD74HC4067. The S0-S3 pins are connected to digital Pins 7-4 on a pro mini. Signal is on A0 and EN is on A1. A1 and EN have a 4K7 resistor to ground. I power the setup with a separate 5v and the arduino is connected on a pc usb port.

I expect when a ttp123 is pressed to trigger a HIGH and this event to be passed to SIG pin. Here is my code:

//Using CD74HC4067 16-Channel Analog Multiplexer
//Mux control pins

int s0 = 6;
int s1 = 7;
int s2 = 8;
int s3 = 9;


//Mux in "SIG" pin
int SIG_pin = A0;
int EN_pin  = A1;

void setup(){
  pinMode(s0, OUTPUT); 
  pinMode(s1, OUTPUT); 
  pinMode(s2, OUTPUT); 
  pinMode(s3, OUTPUT); 
  pinMode(EN_pin, OUTPUT);

  digitalWrite(s0, LOW);
  digitalWrite(s1, LOW);
  digitalWrite(s2, LOW);
  digitalWrite(s3, LOW);
  digitalWrite(EN_pin, LOW);

  Serial.begin(9600);
  Serial.println("keypad interfacing_CD74HC4067 starting...");
}


void loop(){
  String a;
  //digitalWrite(EN_pin, HIGH);
  Serial.println(" press any key to continue ...");
  //digitalWrite(EN_pin, LOW);
  while (!Serial.available());
  while(Serial.available()) {
    
  //Loop through and read all 16 values
  for(int i = 0; i < 16; i ++){
    //Serial.print("Value at channel ");
    //Serial.print(i);
    //Serial.print("is : ");
    //Serial.println(readMux(i));
    
      float volts = readMux(i);
          
      Serial.print("Value at channel ");
      Serial.print(i);
      Serial.print(" is : ");
      Serial.print(volts);
      Serial.println();
    
    //delay(1000);
  }
  a= Serial.readString();// read the incoming data as string
}
}
float 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 j = 0; j < 4; j ++){
    digitalWrite(controlPin[j], muxChannel[channel][j]);
  }
  //read the value at the SIG pin
  int val = analogRead(SIG_pin);

  //return the value
  float voltage = (val * 5) / 1024.0;
  return voltage;
}

Just for simplicity, I have connected only one ttp123 on the system, ttp123 signal to channel12 on 74HC4067. With ttp123 touched, I would expect to get a high on SIG when I write {0,0,1,1}, to S0-S3. All other channels should be LOW. However here is what I get:

On my first run after boot:

 press any key to continue ...
Value at channel 0 is : 2.53
Value at channel 1 is : 2.58
Value at channel 2 is : 2.53
Value at channel 3 is : 2.60
Value at channel 4 is : 2.51
Value at channel 5 is : 2.55
Value at channel 6 is : 2.50
Value at channel 7 is : 2.52
Value at channel 8 is : 2.42
Value at channel 9 is : 2.47
Value at channel 10 is : 2.43
Value at channel 11 is : 2.50
Value at channel 12 is : 2.40
Value at channel 13 is : 2.43
Value at channel 14 is : 2.38
Value at channel 15 is : 2.44

and then all channels seem to lower...

 press any key to continue ...
Value at channel 0 is : 1.70
Value at channel 1 is : 1.76
Value at channel 2 is : 1.71
Value at channel 3 is : 1.78
Value at channel 4 is : 1.70
Value at channel 5 is : 1.76
Value at channel 6 is : 1.72
Value at channel 7 is : 1.77
Value at channel 8 is : 1.67
Value at channel 9 is : 1.72
Value at channel 10 is : 1.69
Value at channel 11 is : 1.76
Value at channel 12 is : 1.69
Value at channel 13 is : 1.73
Value at channel 14 is : 1.67
Value at channel 15 is : 1.74

and I get no HIGH on channel 12 as I should.

Is there a need for a pullup or pulldown resistors in this setup? Should I connect a resistor to each S0-S3 with ground? Why is this not working?

Debugging wiring is off-topic here. You said you have S0-S3 on pins 7-4, but your code has them on pins 6-9. And you didn't mention if you even have grounds connected. — gre_gor, Jan 21 '22 at 15:56
@gre_gor thank you for your comment. You are right about the pins. The description was wrong. The true setup is in the code , that is s0=D6,s1=d7,s2=8,s3=9. The grounds between the independent power supply to the 74HC4067 and the pro mini are connected. Is wiring an off-topic here? Where should I post it if it is a wiring issue? — thermike, Jan 21 '22 at 18:05
Stack Overflow is specifically only about programming. An appropriate site would be [arduino.se]. — gre_gor, Jan 21 '22 at 19:18
Quote from ATmega328P datasheet: "A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (ADEN in ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry". I think you read the ADC prematurely, what happen if you add a slight delay after the switching of the multiplier? One more thing, ATmega328p is expecting the input impedance of an ADC not higher than 10k... — hcheung, Jan 24 '22 at 02:37
@hcheung as gre_gor suggested I posted the question here [link](https://arduino.stackexchange.com/questions/88484/using-cd74hc4067-16-channel-analog-multiplexer-demultiplexer-is-not-working-as-e) as well. I will try the delay. Could you please elaborate or reference something about measuring the impedance of the mux? — thermike, Jan 24 '22 at 09:14

score 0 · Answer 1 · answered Feb 06 '22 at 08:38

Since there was an elementary failure in the code as well I would like to post the solution in here:

I had failed to initialize the A0 pin as input:

pinMode(SIG_pin, INPUT);

Here is the working set of code:

// interfacing mux with tp223

//Mux control pins

const int s0 = 6;
const int s1 = 7;
const int s2 = 8;
const int s3 = 9;

//Mux in "SIG" pin
int SIG_pin = A0;
int EN_pin  = 10;

void setup(){
  pinMode(s0, OUTPUT); 
  pinMode(s1, OUTPUT); 
  pinMode(s2, OUTPUT); 
  pinMode(s3, OUTPUT); 
  pinMode(SIG_pin, INPUT);
  pinMode(EN_pin, OUTPUT);
  
  digitalWrite(s0, LOW);
  digitalWrite(s1, LOW);
  digitalWrite(s2, LOW);
  digitalWrite(s3, LOW);
  digitalWrite(EN_pin, LOW);

  Serial.begin(115200);
  Serial.println("keypad interfacing_CD74HC4067 starting...");
}

void loop(){
    
  //Loop through and read all 16 values
  for(int i = 0; i < 16; i ++){
      int val = readMux(i);
      
      if (val > 0) {
        Serial.print(i);
        Serial.print(" pressed ");
        Serial.print(val);
        Serial.println();
      }
  }
  
}
float 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 j = 0; j < 4; j ++){
    digitalWrite(controlPin[j], muxChannel[channel][j]);
  }
  //read the value at the SIG pin
  int val = digitalRead(SIG_pin);
  //return the value
  return val;
}

I would like to mention that there was a cabling issue as well. I pulled down the signal pin with a 10k resistor.

Using CD74HC4067 16-Channel Analog Multiplexer Demultiplexer is not working as expected

1 Answers1