Nuts and Volts - December 2010

www.nutsvolts.com/index.php?/magazine/article/december2010_PICAXEPrimer

PICAXE PRIMER

automatic switching among program threads will occur so rapidly that it will create the illusion of multitasking.

EXPERIMENTING WITH THE 18M2 TOUCH SENSORS

We’ll explore many of the 18M2’s new hardware and software features in upcoming installments of the Primer. This month, we’re going to start with what I consider to be the most exciting new feature: the capacitive touch-sensitive inputs. In order to do so, we’ll conduct a series of simple experiments to familiarize ourselves with the capabilities and characteristics of the new touch inputs.

Before we get to our experiments, however, let’s take a quick look at the 18M2 pin-out (see Figure 2). The first thing you may notice is that the locations of the power, ground, sin, and sout pins don’t follow the pattern of the older M-class processors; the pin arrangement is much more in keeping with that of the 18X processor (which, along with the 18M, has been superceded by the 18X2). As a result, some care is necessary when setting up a basic 18M2 breadboard circuit, especially when connecting the + and – power pins. Secondly, the pin function that is closest to each pin name is that pin’s default function. As you can see, almost all the pins default to general-purpose inputs – this is a safety feature that’s designed to prevent outputs from being improperly initialized at startup. If you have worked with the 20X2 processor, you’re already familiar with this concept.

Another 18M2 feature has been adopted from the X2 processors: the “port dot pin” naming convention; for example, the name B. 7 identifies that pin as portB, pin7. This same pin-naming convention will also be used on all the subsequent M2-class processors. As a result, the way that a program

■ FIGURE 2. 18M2 pin-out.

refers to a pin (e.g., high B. 7) will be standardized across the entire line of PICAXE processors which greatly simplifies the process of writing programs for different processors.

If you don’t already have an 18M2 on hand, now would be a good time to order a couple. I ordered my first 18M2 processors directly from RevEd in England on the day that they were announced, and then I waited impatiently for 10 days or so to receive them in the mail. Fortunately, the 18M2 chips are now available here in the US from Peter Anderson ( www.phanderson.com/ picaxe/ index.html), and shipping only takes about three or four days or so.

pins. (You can see my sophisticated experimental setup in Figure 3.) The software for my first experiment is just as simple as the hardware:

; === TouchTest1.bas ===
; Tests a touch sensor.

; === Directives ===
  #com 5
  #picaxe 18M2
  #no_data
  #terminal 4800

; == Begin Main ===
do
  touch B.1, b1
  sertxd (#b1,cr,lf)
  pause 250
loop

EXPERIMENT 1: THE “I CAN’T WAIT!” EXPERIMENT

When I finally got my hands on the 18M2, the first thing I wanted to experiment with was the new touch-sensitive inputs. I was in such a hurry that I didn’t even want to take the time to make a simple touch pad, so I just cut a six inch piece of jumper wire, stripped about 3/8” of the insulation from one end, and inserted it into a basic 18M2 breadboard circuit so that the wire connected with pin B.1 — which is one of the 18M2’s 10 touch input As you can see, the touch command can be very simple, but you may also want to read the documentation in Section 2 of the PICAXE manual for some of the more complicated details. The complete syntax is: touch channel, variable. Channel is the pin that is being used as the touch input (pin B.1, in this case), and variable is the variable that receives the byte value of the touch reading (variable b1, in this case). Of course, I could also have defined meaningful names for both of the