Arduino Serial Begin: Why do you need it? Find out here exactly what it does and what its for. Find out why you should really use a different number than 9600...and what does that number mean anyway? Find out here!

The Arduino serial begin statement is piece of initialisation code that communicates with the built in serial module within your Arduino. You use it to set up a speed of operation that you want the serial hardware to use.
There are other parameters that are hidden from you that setup the hardware in a particular way (explored more below). For now lets just look at the piece of code that is essential to get the serial port initialised namely Arduino Serial begin which is written as follows:

    Serial.begin(9600);

The value 9600 specifies the Baud rate. Baud actually refers to symbols per second but as there is no special encoding, the Baud rate is the same as the number of bits per second i.e. 9600 bits per second or 9600Hz.

When you use the serial interface data is transmitted in both directions and for the serial link the method is to use two wires - one carries data to the PC, and one carries data from the PC.

When you send a character from the Arduino, the Arduino serial transmit hardware module transmits the byte on the TX line. Similarly when the Arduino accepts a byte receives it on the RX line into the serial receive hardware module. These two things are entirely separate pieces of hardware (other than the baud rate clock which is applied to both).

The reason for separate Rx and Tx modules is that either end could start a transmission at any time - so you can't use one wire. It also means there are two serial units in the hardware serial module or two hardware buffers; one for transmit and one for receive.

When you set the Baud rate to 9600 you are telling the hardware module to set a bit speed of 9600Hz so that each bit sent out from the transmit buffer is sent at 104.16us (1/9600). So every 104.16us, a bit is sent from the buffer until it is empty. Similarly, the receiver looks for data bits separated in time by 104.16us.

Arduino Serial Begin: Why 9600Baud?

The serial hardware module in the Arduino (or USART - Universal Synchronous Asynchronous Receiver Transmitter) was really designed for RS232 which is a physical protocol that allows transmission of data over longish distances.

It's called a USART because you can use it for different serial modes. You can change the number of bits, add a parity check, change the number of stop bits, use a synchronous clock etc.

A real RS232 setup uses a converter chip that takes the output of the USART and sets up different voltage levels (logic 1 is -25V and logic 0 is +25V) so that a long cable can be driven. This 25V level is not mandatory so a common level used is ±12V as it easier for logic chips to create. The higher voltage is used to overcome cable resistance (the receivers can accept levels of ±3V).

However the longer the cable, the more capacitance it has, so rise and fall times are affected. The RS232 protocol uses an asynchronous clock, meaning that the clock is determined from the transitions of the signal.  Eventually the protocol stops working as the rise and fall times become so slow you can't tell where a bit starts i.e it jitters around causing false data.

However by reducing the speed of the RS232 signal the rise and fall time can be mitigated i.e. the period of the bit will be large compared to the rise/fall time of the signal.

So in a real system you use the maximum speed you can get away with for the cable length/type you are going to use!

Arduino Serial Begin: Why change from 9600Baud

When you use an Arduino these limitations do not apply and you can actually get away with far higher Baud rates because there is no long cable between the PC and the Arduino. 9600Baud is just the accepted default rate used in most example programs - I usually change it to 115200 Baud in my programs! - If you have a lot of data to send to the PC it makes program output quicker!

The Baud rate of 115200 is the one actually used by the IDE to upload your program to the Arduino Board! You can see this enabling "Show verbose output during upload" from Menu > File > Preferences. So  the IDE always uses 115200 for its own operation!

Actually you can go far faster - my older 3dprinter uses 250000Baud (0.25MBaud) (using an Arduino Mega3260 as the Arduino processor chip - and it never had communications problems). That's a bit period of 4us! - so 10 bits takes 40us to transmit.

Arduino Serial Begin: Numbers matter

One thing you must make sure of is that both ends; the Arudino and the PC are setup the same. In terms of Arduino Software that means both ends must use the same Baud rate because all other parameters are fixed in Arduino software i.e.. number of bits, stop bit etc. If the parameters don't match then one end will see rubbish - or nothing at all.

Ensure that the Baud rates are the same as shown in the following picture. If you change your program's baud rate then also change the PC Baud rate.

Say you set the Arduino to 115200Baud and the PC to 9600Baud:

So in your Arudino program you use:

    Serial.begin(115200);

...and in the IDE you leave it at the default of 9600.

For the Arduino it sends data at a period of 8.6us (1/115200). If there are 10 bits transmitted then its complete in 86us. The ide will not even see the bits as its receive period is 104us.  Well, it samples in the middle of the bit so may output some noise but it will be rubbish.

This is one of the big problems with the RS232 protocol as there is no way for one end to interrogate the other end to find out its settings! It is entirely up to the programmer to set both ends identical.

For me this happened on a new 3dprinter Ender 3 - with the old printer the serial link worked fine but on the new one there was no output. This is not too good if you are setting up a new system as you won't know where the problem is. In fact the Ender3 used a different baud rate in the Arduino code on the machine.

Arduino Serial Begin: Why is there a serial module?

The serial hardware module can run at very high speed (up to 2Mbaud - but that is pushing it) but even a modest speed is too fast for the capabilities of a program in the microcontroller sampling pins (you can just about bit-bang a 9600Baud serial link but will struggle with higher speeds).

The Serial hardware module does it all for you and takes the pressure off the main processor - only interrupting the processor if a complete byte is received at the serial input. So all the processor needs to to is read a byte from the hardware module.

Arduino Serial Begin: Default hidden parameters

Deep within the vaults of Arduino code you will find the parameters for the serial interface that are fixed and hidden from you. Ok it makes the interface easy to use, as all you do is set is the Baud rate - the rest is done for you. 

There is one relevant class:

Class name	Location	Use
HardwareSerial	HardwareSerial.h	Hardware specific

HardwareSerial inherits the Steam class, so you can used methods defined in the Stream class.

In the class HardwareSerial we have:

    void begin(unsigned long baud) { begin(baud, SERIAL_8N1); }
    void begin(unsigned long, uint8_t);

Your code invokes the top method when you write Serial.begin(baudrate) and the body of that code invokes the method below using two parameters. That second parameter sets up the internal hardware using the value SERIAL_8N1 (the actual value of this definition controls the serial hardware directly). These are the fixed hidden parameters and they - say:

   8 - The hardware serial buffers use 8 bits - get or send 8 bits at a time.
   N - There is no parity bit (would be Odd or Even if used).
   1 - They allow one stop bit.

Note that a start bit is always used as this is how start of a data transmission is detected. So the total number of bits transmitted or received is  (S)+8+1 = 10. The stop bit is actually an idle time but must be accounted for to determine the total tx/rx time.

Arduino Serial Begin: Multiple Serial Ports

For Arduino boards with more than one serial port the HardwareSerial class will define numbered Serial objects;

For a single Serial port (Arduino Uno/Nano) you write:

    Serial.begin(115200);

For multiple Serial ports you write

    Serial.begin(115200);
    Serial1.begin(9600);

When there are multiple ports Serial1 is the next label - provision is made for 4 ports in total (Arduino Mega has 4).

Arduino Serial Begin: Conclusion

Arduino Serial begin is an essential piece of code for initialising the internal serial hardware. You need the serial hardware so that the processor is not overloaded. You can also increase the baud rate with no effect on your code (except to make it operate faster!).

The two other functions that for a basic part of Serial code are:

     Serial.available()
     Serial.read()

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