Overview of the Serial Communication Protocol

Testing the Code

With the Arduino's serial monitor, you can enter data directly to your Arduino. After your sketch is running, open the Serial Monitor by clicking its icon. Be sure to select 9600 baud in the lower right corner (Figure 3); then, you can type L (uppercase) to turn the LED on and l (lowercase) to turn it off.

Figure 3: The Arduino serial monitor lets you communicate with your Arduino over its serial port; select the speed from the lower right drop-down box.

To move the servo, enter a number between 0 and 180 followed by a period. Because the Arduino doesn't know how many digits are in your number, the period lets it know that you're done sending. Sending a number and waiting five seconds also works, because Serial.setTimeout (Listing 1, line 13) makes sure the program doesn't stall.

Independent of the output, if the button or switch changes state, you'll get a message in the serial window. If a program were to receive this serial output, it could interpret the messages and take action on the basis of the values, as shown in the Python snippet in Listing 2.

Listing 2

Python Serial Transmitter

01 import serial
02 import time
03 ser = serial.Serial('/dev/ttyAMA0')  # open the Arduino serial port
05 while 1:
06         ser.write('L')     # turn the LED on
07         time.sleep ( 1 )   # wait a second
08         ser.write ( 'l' )  # turn the LED off
09         time.sleep (1)
10 ser.close()                # close port

Controlling the LED

As with any Python program, import brings in an external library. In this case, line 1 is the interface to serial ports [4], and line 2 is the time module, which provides the sleep function.

The serial.Serial function opens a serial port. The only required argument is the port name. In this case, /dev/ttyAMA0 is the Arduino. Without any other parameters, communication will default to 9600 baud, 8N1 (data, parity, stop bits).

The infinite loop (lines 5-9) sends an uppercase L out the serial port (line 6), waits one second (line 7), sends a lowercase l out the serial port (line 8), and waits one second (line 9). Although line 10 will never be reached in this example, ser.close shuts down a port. Python also takes care of the port if the program terminates.

Once you've programmed the Arduino, run the Python snippet, and your LED should start blinking. Of course, you can now change the timing or send other commands by modifying the desktop program instead of reprogramming the Arduino. This arrangement can be very handy if the Arduino is embedded in another piece of equipment (or suspended 30 feet in the air).

Listening to Inputs

The Python code in Listing 3 reports the changing states of the button and switch. The infinite loop that starts in line 4 gets a line from the serial port with ser.readline (line 5) and splits it at the colon (line 6). To the left of the colon, the string will either be Switch or Button. If it is Switch (line 7) and the status (right of the colon, statusParts[1]) is  , then the switch is ON (line 8). Otherwise the switch is off (line 9). Lines 10-12 work the same way, except it checks and reports the state of the button.

Listing 3

Python Serial Receiver

01 import serial
02 ser = serial.Serial('/dev/ttyAMA0')  # open the Arduino serial port
04 While 1:
05         status = ser.readline()
06         statusParts = status.split ( ":" )
07         If statusParts [ 0 ] == "Switch":
08                 If statusParts [ 1 ] == "0": print ( "The switch is ON" )
09                 Elif statusParts [ 1 ] == "1": print ( "The switch is OFF" )
10         Elif statusParts [ 0 ] == "Button":
11                 If statusParts [ 1 ] == "0": print ( "The button is pressed" )
12                 Elif statusParts [ 1 ] == "1": print ( "The button is released" )

When you run the snippet in Listing 3 and fiddle with the button and switch, you should see messages announcing the changes in state in the terminal. Although this example just shows a message, it is not hard to see how this could trigger other code.

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