Nuts and Volts - February 2011

The most valuable piece of information that I learned from reading AN1104 was just how limiting a “matrix mindset” can be when implementing touch inputs. For example, I have always assumed that you need eight input pins in order to implement a keypad with 16 keys (four columns and four rows, of course). However, AN1104 clearly explains how it’s possible to scan as many as 21 touch inputs using only six input pins. Even more amazing — at least to me — is the fact that eight input pins can scan up to 36 touch inputs!

It’s all accomplished by designing the touch input pads on a PCB in such a way that they become what are called paired sensor elements. Figure 6 (adapted from Figure 4 in AN1104) shows a traditional single sensor element on the left side; it’s simply a rectangular copper area that that can be connected to a single input pin and used as a touch sensor. The element on the right is called a paired sensor element because each of its two (separate) copper areas can be connected to a different input pin. Whenever a finger touches the (insulated) surface of a paired sensor element, touch values of two different touch inputs are affected.

As a simple example of how these two types of sensing elements can be combined, let’s consider how many sensors we can implement using only four input pins. First, each pin can be directly connected to its own single sensor element (that’s four sensors so far). Also, each input pin can be connected to three different paired sensor elements, each of which is connected to one of the remaining three input pins. At first glance, you might think that would be 4* 3 or 12 additional sensors, but it’s actually half that number. For example, connecting pin 2 and pin 3 to one paired sensor element is the same as connecting pin 3 and pin 2 to a second paired sensor element. In order to eliminate all those duplications, we need to divide by two. Therefore, four input pins can be connected to four single sensor elements, and ( 4* 3)/2 or six

■ FIGURE 6. Single sensor element vs. paired sensor element.

paired sensor elements for a total of 10 sensors.

If any of that isn’t clear, take a look at Figure 7 which presents the same information visually. The bottom row of Figure 7 displays the four single sensor elements. The next row up shows the three possible paired sensors that include input 4. The third row from the bottom shows the two possible paired sensors that include input 3. (Don’t forget, the 4 and 3 combination has already been accounted for in the previous row.) Finally, the top row contains the one remaining paired sensor combination that includes input 2. (Again, the other two possible combinations for input 2 have already been accounted for.) So, that gives us a total of 10 sensors for four touch inputs. Of course, on a PCB the keys aren’t restricted to the triangular arrangement of Figure 7; they can be placed in any arrangement you want — even a matrix!

In general, N input pins can be connected to N*(N-1)/2 + N sensors. If you do a little algebraic simplification, that formula is equivalent to N*(N+1)/2; so, with four touch input pins, we again arrive at 4*( 5)/2 = 10 sensors. (And you thought algebra was boring!) If you do the math, you will see how six input pins can be used to scan as many as 21 touch sensors.

If you don’t like doing math (or algebra), you can refer to Figure 8 which presents the maximum number of sensors vs. the number of touch input pins (from 2 to 10). As you can see, the 10 touch inputs on the 18M2 make it theoretically possible for the chip to interface with 55 touch sensors. (However, you might have to hold your finger on a touch sensor a little longer than usual to give the chip enough time to process all that data!)

PCBS TO THE RESCUE!

A couple of months ago, I decided to use the information discussed in AN1104 to design a PCB for use as a touch-sensitive keypad. Figure 9 is a photo of my first prototype PCB installed in an 18M2 breadboard circuit. (The piezo provides an audible key-click for each key-press.) Because of the rectangular key layout, it may look like a matrix keypad, but it isn’t – I have seen the light, and I am free!