Nuts and Volts - April 2014

The main purpose of our first experiment is just to test the hardware setup, so we don’t need to get into the details of the program. (If you’re interested, you may want to reread the December 2012 column.) At this point, just download the temp9700test.bas program to your breadboard setup.

You should see the three values (ADCval, tempC, and tempF) updating every five seconds in the terminal window. (Don’t forget, even though there’s a total of 10 seconds wait time in the main loop, we’re running at 8 MHz, so the actual loop time is about five seconds.)

If you gently squeeze the 9700A between two fingers, you should also see the readings increase a bit, and slowly decrease when you remove your fingers from the sensor. When you’re sure your hardware setup is functioning correctly, we’re ready to move on to our next experiment, which will send the ADC data from the 08M2 to the Pi. However, before we do that, let’s take a little break and play with a few of the features of the Python print() function.

Playing with Print()

The remainder of our experiments this month will all include the use of the Python3 print() function, so that we can visually check that the Pi is receiving the correct data from the

08M2. The print() function includes many advanced features that make it suitable for printing various reports that require precise formatting. We definitely won’t be covering the majority of these features, but there are a couple of them we do need to discuss, and I think it will be easier if we do that before we actually tackle the remaining experiments this month.

It may be tempting to compare the print() function with serial output such as the PICAXE serout and sertxd statements. However, there are important differences between the two operations. Serial output is frequently used to send data from one processor to another. In that situation, we only want the data transmitted (without any additional characters, such as spaces). In PICAXE BASIC, if we write serout txPin, T9600_ 8, (hiByte,loByte), we expect to be sending exactly two bytes, and that’s — in fact — what happens.

However, if we’re transmitting something that’s going to be read by a human (e.g., a sertxd transmission to the terminal window), we need to insert spaces (and punctuation characters) to make the output readable. In that case, we need to write something like sertxd (tempC,” “,tempF) so that the two values are separated by a single space, and therefore easily readable.

On the other hand, the output of a Python3 print() function is almost always intended to be read by a human. So, by default, Python3 automatically includes a blank space between each argument in the print() function. In addition, it also prints a “new line” character at the end of the printed line, so that whatever we print subsequently appears on a separate line.

In order to clarify this, let’s take a look at a few examples. Open the printPlay.py program in idle3. You will see two variable definitions (tempC

and tempF) and a numbered series of print() functions — all of which have been commented out. Each example includes a final “empty” print function, which is one way we can print a blank line between each example. As you read each of the following explanations, un-comment the corresponding functions in printPlay.py, and run the program to see the printed result:

1. The output is printed on one line with a space between each argument. However, sometimes we don’t want a space between arguments. For example, the standard degree symbol is character 176 in the extended ASCII table. If we wanted to get fancy and use the symbol rather than the word “degrees,” we might try #2.

2. Of course, the phrase is again printed on one line, with a space between each argument. That’s definitely not what we want. There shouldn’t be a space between the number and the degree symbol, so we need a way to override the default behavior of the print() function. Actually, Python3 provides multiple ways to accomplish our goal, but I’m just going to mention the two solutions that I have found to be the simplest to understand and use.

3. First, the print() function includes two optional arguments: sep (the “separator” character) and end (the “end” character). If we omit either or both of these arguments (which is what we have done so far), sep defaults to a “space” character, and end defaults to the “newline” character.