Arduino Serial Part 2: Serial Data [VERIFIED]

Arduino Serial Part 2: Serial Data [VERIFIED]

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When anything sends serial data to the Arduino it arrives into the Arduino input buffer at a speed set by the baud rate. At 9600 baud about 960 characters arrive per second which means there is a gap of just over 1 millisecond between characters. The Arduino can do a lot in 1 millisecond so the code that follows is designed not to waste time waiting when there is nothing in the input buffer even if all of the data has not yet arrived. Even at 115200 baud there is still 86 microseconds or 1376 Arduino instructions between characters.

And because data arrives relatively slowly it is easy for the Arduino to empty the serial input buffer even though all of the data has not yet arrived. Many newcomers make the mistake of assuming that something like while (Serial.available() > 0) { will pick up all the data that is sent. But it is far more likely that the WHILE will empty the buffer even though only part of the data has arrived.

It is important to notice that each time the function recvWithEndMarker() or recvWithStartEndMarker() is called it reads whatever characters may have arrived in the serial input buffer and places them in the array receivedChars.

Note that the 64 byte size of the Arduino serial input buffer does not limit the number of characters that you can receive because the code in the examples can empty the buffer faster than new data arrives.

In the previous post I went through the basics of using serial on an Arduino and ran through the different commands. In this post I want to talk about different types of serial data and some of the things you should consider before starting to create code. The type of communication you use or can use will depend largely on the project but there are things that can be considered before starting.

The type of communication will determine how complex parts of the project are. One way communication is generally easier to implement but you need to consider which way; sent from the Arduino or sent to the Arduino. The Arduino does not have high level data processing functions so if you have complex data patterns you will need to create code that parses it.

The next couple of posts go through methods for sending serial data from one Arduino to another. In all the examples I use physical wires to connect the Arduinos but the wires can be replaced by any wireless module that speaks UART such as Bluetooth modules. And although I give examples of Arduino to Arduino, the same techniques can be used when receiving serial data from any device.

The SoftwareSerial library allows serial communication on other digital pins of an Arduino board, using software to replicate the functionality (hence the name "SoftwareSerial"). It is possible to have multiple software serial ports with speeds up to 115200 bps. A parameter enables inverted signaling for devices which require that protocol.

Return a character that was received on the RX pin of the software serial port. Unlike read(), however, subsequent calls to this function will return the same character. Note that only one SoftwareSerial object can receive incoming data at a time (select which one with the listen() function).

At sometime or another you may run out of pins on your Arduino board and need to extend it with shift registers. This example is based on the 74HC595. The datasheet refers to the 74HC595 as an "8-bit serial-in, serial or parallel-out shift register with output latches; 3-state." In other words, you can use it to control 8 outputs at a time while only taking up a few pins on your microcontroller. You can link multiple registers together to extend your output even more. (Users may also wish to search for other driver chips with "595" or "596" in their part numbers, there are many. The STP16C596 for example will drive 16 LED's and eliminates the series resistors with built-in constant current sources.)

How this all works is through something called "synchronous serial communication," i.e. you can pulse one pin up and down thereby communicating a data byte to the register bit by bit. It's by pulsing second pin, the clock pin, that you delineate between bits. This is in contrast to using the "asynchronous serial communication" of the Serial.begin() function which relies on the sender and the receiver to be set independently to an agreed upon specified data rate. Once the whole byte is transmitted to the register the HIGH or LOW messages held in each bit get parceled out to each of the individual output pins. This is the "parallel output" part, having all the pins do what you want them to do all at once.

The "serial output" part of this component comes from its extra pin which can pass the serial information received from the microcontroller out again unchanged. This means you can transmit 16 bits in a row (2 bytes) and the first 8 will flow through the first register into the second register and be expressed there. You can learn to do that from the second example.

Two of these connections simply extend the same clock and latch signal from the Arduino to the second shift register (yellow and green wires). The blue wire is going from the serial out pin (pin 9) of the first shift register to the serial data input (pin 14) of the second register.

In this tutorial, we will take a quick look on how to enable this feature (works for practically any sketch that uses serial communication), how a sample sketch looks like, and how it is expected to work.

Serial communication is the process of sending one bit of data at a time, sequentially, from one place to another. For example, using serial data, you could send data from your Raspberry Pi to a connected Arduino, or vice versa.

When bits of data start streaming in from your computer, a piece of hardware on your Arduino called a universal asynchronous receiver/transmitter (which can be shortened, thankfully, to UART) will assemble each of the 8 bits into a byte. The UART will then store those bytes (up to 64 bytes, in fact) in the serial receive buffer.

First, we need a place to store the incoming bytes from the serial receive buffer. We can use a char array for that. Then we need to check if anything is even available in the serial receive buffer. We can use Serial.available to make that happen. Then we need to actually read in a byte. We have Serial.read() in our back pocket for that task.

Now we need to check if any bytes are available in the serial receive buffer. While there are, we need to read in the bytes and save them to a temporary variable. We can combine a while loop, Serial.available, and Serial.read() to make this happen.

Sometimes, one serial port just isn't enough! When trying to communicate with multiple serial enabled devices, while also sending info back to the main serial window, a few extra RX/TX ports can be a welcomed thing. This example makes use of one of Arduino Mega's 3 auxiliary serial ports, routing any incoming data read on that connection straight to the main TX line, and, in turn, to the main serial window for you to view.

After checking the data sheet of whatever serial enabled device you choose to use for this example, make sure that it is both properly wired and powered. Connect the RX pin and TX pins of your device to the TX1 and RX1 pins of your Mega, as shown in the schematic below.

Arduino boards have built in support for serial communication on pins 0 and 1, but what if you need more serial ports? The SoftwareSerial Library has been developed to allow serial communication to take place on the other digital pins of your boards, using software to replicate the functionality of the hardwired RX and TX lines. This can be extremely helpful when the need arises to communicate with two serial enabled devices, or to talk with just one device while leaving the main serial port open for debugging purpose.

In the example below, digital pins 8 and 10 on your Arduino board are used as virtual RX serial lines. Pins 9 and 11 are virtual TX lines. The board listens on one virtual port (portOne) until it has read all available data. After that, it does the same on the second virtual port (portTwo).

There is no circuit for this example. Make sure that your Arduino board is attached to your computer via USB to enable serial communication through the serial monitor window of the Arduino Software (IDE).

This is called our setup method. It's where we 'set up' our program. Here, we're using it to start serial communication from the Arduino to our computer at a baud rate of 9600. For now, all you need to now about baud rate is that (basically) it's the rate at which we're sending data to the computer, and if we're sending and receiving data at different rates, everything goes all gobbledy-gook and one side can't understand the other. This is bad.

After our setup() method, we need a method called loop(), which is going to repeat over and over as long as our program is running. For our first example, we'll just send the string 'Hello, world!' over the serial port, over and over (and over). Type the following in your Arduino sketch, below the code we already wrote:

That's all we need for the Arduino side of our first example. We're setting up serial communication from the Arduino and telling it to send data every 100 milliseconds. Your Arduino sketch should now look something like this:

You should now see a line like import processing.serial.*; at the top of your sketch. Magic! Underneath our import statement we need to declare some global variables. All this means is that these variables can used anywhere in our sketch. Add these two lines beneath the import statement:

In order to listen to any serial communication we have to get a Serial object (we call it myPort but you can it whatever you like), which lets us listen in on a serial port on our computer for any incoming data. We also need a variable to recieve the actual data coming in. In this case, since we're sending a String (the sequence of characters 'Hello, World!') from Arduino, we want to receive a String in Processing.Just like Arduino has setup() and loop(), Processing has setup() and draw() (instead of loop). 2b1af7f3a8