Ex-03: Edge Detection and Debouncing¶

In this exercise, you will learn about the side effects of electromechanical components and how to deal with them in software. You will also optimize your solution from the previous exercises and implement a simple latch for the joystick center button.

Description¶

Your first LED application has sampled the input state of a “GPIO” pin connected to the USER button “B1” (low active) and set the inverse value on another “GPIO” pin connected to the LED “LD2” (high active). This simple task was repeated in a super loop after initialization. However, you may have updated the output pin regardless of whether the button state changed or not. Since this operation is only needed on a change, a lot of unnecessary operations were performed by the CPU. Now the output “GPIO” connected to the RGB LED should only be set or reset if the input connected to the button changes.

Tasks¶

Write a simple program to detect a “rising edge” event on the joystick center button and toggle a channel of the RGB LED. Wait, do you know what an “edge” actually is?

Hint

An edge describes a digital signal going from one steady state or level to another. In software we can call such a transition an event of a signal.

• Rising Edge Event ER

  Transition from a low state (likely GND) to a high state (likely VDD)

• Falling Edge Event EF

  Transition from a high state (likely VDD) to a low state (likely GND)

Implementation¶

Your first implementation is a pure combinatorial logic, so the output follows the value (state) of the current input. A next evolution of this program should implement a behavior that does a state transaction only on an edge (push and release sequence). So it behaves like a switch instead of a pushbutton. Each push of the button should change the state, toggling the output pin (turning the LED on/off). To implement such a behavior, “sequential logic” is needed. Sequential logic differs from combinatorial logic in that it depends on both the current and previous state(s).

Debouncing¶

While testing your implementation, you may have encountered some unexpected behavior. Pressing the button once may have toggled the LED twice or more. This is a side effect of the button. Because the button is a mechanical device, it tends to oscillate when it changes state.

Hint

However, the duration of the oscillation is usually quite short. A simple solution to this problem would be to accept a new input level only after a certain period of time. Thus, only if N samples after a transition have the same value, the edge is adopted. Such an implementation, which is a kind of digital filter, is called software debouncing. It is also possible to have a hardware debouncing circuit. See the NULCEO-64 MB1136 schematic for an example of a hardware debouncing circuit (USER button B1). For low-cost electronic designs, hardware debouncing may not be an option, so software debouncing solves the problem of side effects due to mechanical bouncing.

Example Software Debouncing

if (sample != currentState){
   ++counter;
} else {
   counter = 0;
}

if (counter == AR){
   counter = 0;
   currentState != currentState;
}

if (currentState != formerState){
  if (currentState) {
      risingEdgeCallback();
  } else {
      fallingEdgeCallback();
  }
}

formerState = currentState;