Nuts and Volts - April 2014

the Pi can do the entire computation in a single step.

4. Python’s round() function takes two arguments: the value that we want to round; and the number of decimal places we want to use.

5. In this statement, we’re converting the temperature reading from C to F by doing the same two computations we just carried out to convert ADCval to Centigrade. However, this time, we’re doing them both in a single statement.

Python computes the value of tempF using the standard C to F conversion formula, and then rounds the result to one decimal place — all in one line of code.

6. These three print() functions use some of the features we demonstrated earlier with the printPlay.py program. The second and third print() functions demonstrate the two methods we discussed for formatting the data.

You may want to refer back to that discussion to see if you can predict how the printed output will be formatted.

When you’re ready to carry out Experiment 2, run the Python program first. The blocking read() function will cause it to wait until the PICAXE program begins to send the serial data.

Experiment 3: An Interrupt-Triggered Serial Transmission

In this experiment, we’re putting the Pi in charge of when a serial transmission occurs. The Pi will issue an interrupt signal and the 08M2 will respond by serially sending the raw temperature data (ADCval) to the Pi. We’re again transmitting two data bytes (hiByte and loByte) to the Pi, but the same approach can easily be used for any amount of data.

In other words, it would be a simple matter to serially connect a larger processor (such as the PICAXE- 20M2) to the Pi and have the 20M2 monitor several sensors, then send all the updated data whenever the Pi issues an interrupt. Also, we’re going to unblock the Pi’s serial reception so that we don’t run the risk of the Pi hanging because there has been a glitch in a serial transmission.

All current PICAXE processors support some form of interrupt capability, but we’re going to limit our discussion to the M2-class processors and the 20X2. If you’re interested in the more advanced features of the 20X2 and 40X2 processors, you may want to read the relevant documentation on the setint and setintflags commands in Section 2 of the PICAXE manual.

Also, we discussed interrupts way back in the February 2009 Primer, so I won’t repeat all the details here. If you want more information than we’re about to discuss, you may want to reread the 2009 article, and the setint documentation in the manual.

Let’s begin with a brief explanation of how an interrupt functions. First, we need to include a setint command at the beginning of our program to specify which pin(s) we want to use for the interrupt, and whether we want the interrupt to be triggered by a high or low pulse. (We’ll see exactly how to do that shortly.)

Secondly, we also need to include an interrupt subroutine which must begin with the label interrupt: and end with a return statement. The interrupt code specifies the action we want to take place in response to the interrupt signal. (In this case, the 08M2 will send the values of hiByte and lowByte to the Pi.)

When we run the PICAXE program, after each program line is executed (and continuously during any pause or wait command), the PICAXE compiler checks to see whether an interrupt condition exists. If it does, the interrupt subroutine is immediately executed, and then