Nuts and Volts - February 2016

GPS Receiver and Decoder

For the time base, I chose the Garmin GPS receiver shown in Figure 6, mainly because I already had it left over from a previous project. I could have used an RTC (Real Time Clock) but then I would have had to add more buttons to set the time. Besides, in my junk box I also had a Parallax SX28 microcontroller programmed to parse the sentences and output the time and date over a 9600 baud five volt line. I bought a bunch of SX28 protoboards when Parallax had an End-Of-Life sale several years ago; they are handy for small projects like this one.

The time and date are parsed from the GPS RMC sentence and the number of satellites in use is taken from the GGA sentence. As you’ll notice in Figure 4, I display the number of satellites on the 2x16 LCD, only because I am awed by the whole GPS concept and love to see it working. The Garmin unit works fine inside my single-story house, but your reception may vary.

For some obscure reason, I decided to write my own Arduino routine to convert UTC time to local time/date, instead of adapting a Library routine. Big mistake! It looked simple enough, but then I realized that I would have to include time zones, leap years, and daylight savings. Garf! After many hours of debugging, I got it working and confirmed that it converts the UTC data to the correct time and date for all 50 states. Just enter your Time Zone (TZ) in the Setup Section of the code before you load it. If you are not in the US, simply adapt the code to fit your location.

Program Flow

The basic program flow is as follows:

1. In the background, the Arduino measures wind speed at 2.250 intervals, and the SX28 reads the GPS time/date and number of satellites.

2. At the end of each minute, the SX28 sends a pulse to the Arduino.

3. The Arduino then: a. Determines the peak velocity for the past minute and adds the value to the daily array (1440 points). b. If the daily maximum velocity has increased, it moves the red fid mark and value to a new point, and sounds an audio signal. c. It serially transmits updated config and data array files to Gnuplot in the RPi.

4. The RPi sends a new HDMI plot to the LCD monitor, but in a small window.

5. The RPi expands the window to fill the LCD screen.

6. Steps 3, 4, and 5 are repeated at the end of each minute, 1,440 times per day.

7. Everything resets at midnight.

8. The GPS GMT time/date from the SX28 is converted to local time as it comes in.

The biggest problem I had with the programming was juggling the code for the three different processors. The SX28 used assembly language, the Arduino used C, and the RPi required a combination of command-line programming and Python. I had a heck of a time keeping track of which language I was using at any one time. Plus, it was my first experience with Python, so there was a learning curve. Fortunately, the Internet has a cornucopia of helpful hints and I was able to work my way through the problems. I also bought several books and tapped my knowledgeable friends.

This project was also my first experience with the fantastic free Gnuplot plotting program. It was fun to customize the display, but it was almost impossible to stop tweaking it. (You know how it is.) I chose to locate the main Gnuplot configuration file in the Arduino because I needed to update many of the displayed numbers 1,440 times a day. Normally, the config file would be permanently located in the RPi main directory. Instead, I send an updated config file to the RPi at the end of each minute, at 115200 baud.

The source code for the Parallax SX28, Arduino Mega, and RPi is available at the article link. Also available is a document explaining how to install the necessary support applications/tools for the RPi.