Minima - Multi-Button Funciton

In previous revisions of my Minima Controller Code, I had used a "table" to lookup and decode the Buttons being pressed.

The buttons are connected together via  identical resistors to form a resistive ladder (see previous post). It was a simple matter of pressing a button, reading the resulting decoded value at the analog port of the ATMEGA328, and then inserting the correct value in the table for that button (Note: some of the current coding details have been left out, see source if necessary).

// ###############################################################################
int btnDown(){
  int val;
  
  val = analogRead(FN_PIN);

  if (val>1000) return 0;
 
  // 47K Pull-up, and 4.7K switch resistors,
  // Val should be approximately = 1024*btnN*4700/(47000+(btnN*4700))
  // N = 0 to Number_of_button - 1

  if (val > 350) return 7;
  if (val > 300) return 6;
  if (val > 250) return 5;
  if (val > 200) return 4;
  if (val > 150) return 3;
  if (val >  50) return 2;
  return 1;

}

That became a little boring, so I decided to replace the table with an algorithm.

// ###############################################################################
int btnDown(){
  int val = 0;

  val = analogRead(FN_PIN);

  if (val>1000) return 0;

  // 47K Pull-up, and 4.7K switch resistors,
  // Val should be approximately = 1024*btnN*4700/(47000+(btnN*4700))
  // N = 0 to Number_of_button - 1

  // 1024L*b*4700L/(47000L+(b*4700L))   >>>  1024*b/(10+b);

  for(int b = MAX_BUTTONS - 1; b >= 0; b--) {
      if(val + 15 > 1024*b/(10+b)) return b+1;
  }
  return 1;
}

The NEW algorithm is smaller, faster and more flexible than the "table", only MAX_BUTTONS need to be change to add/remove new buttons. There is probably a limit to the number of buttons that can reasonably be added to the Resistive/Switch chain, currently I have used eight.

Note: According to ohm's law, with 47K ohm Bias Resistor and 4.7K ohm Switch Resistors, (see previous post) the value read by the ATMEGA should be:

val = 1024 * b * 4700 / (47000 + (b * 4700));    Where "b" is the button pressed
As long as the Bias and Switch Resistors have a fixed ratio (in this case 10:1), they can be factored-out to leave a much smaller function:

val = 1024 * b / (10 + b);
The biggest disadvantage of using an algorithm is; it can NOT easily be tweaked to solve a resistive tolerance problems - the moral of the story, use good resistors and pay attention to the Bias source for the network and the ATMEGA contained ADC's.


-- Home Page: https://WA0UWH.blogspot.com