Serial
Communication
Most
of the electronic designs which are based on integrated circuits are doing data communication with external peripherals connected in to the same design or with
another device. Any kind of embedded system which having a micro-controller or a
microprocessor are definitely capable and need data communication. Also, that
kind of designs are having several circuits combined together using a
communication protocol.
In order to perform a successful data
communication between any kind of two devices, they must have a common
communication protocol. Communication protocols can be divided into several
categories considering their properties.
Most common and widely used two categories are;
- · Serial Communication Protocols
- · Parallel Communication Protocols
In
parallel communication protocols, one entity (Transmitter) can send more than
one bit at a time. In other word communication can be done with several data
bits transmitting parallelly using several physical lines. Usually we can see bunch of wires which are
serving as data busses in parallel communication devices. Following figure
shows a typical 8-bit parallel communication port.
According
to the figure there are 8 data lines to communication and another wire to send
a clock signal. This clock signal is sending in order to having a
synchronization between two entities. Width of the data bus can be varied such
as 4-bits, 8bits, 16-bits,32-bits etc.
Serial Communication
Here
we are sending a stream of data as a single bit at a time. This communication
method needs only a pair of data lines to transmit data bits. But sometimes we
need to use another clock signal line also.
Above
figure shows a single direction serial communication model between two entities
with a clock signal at the bottom line. It sends a set of 8bits (one byte) serially
using a single line but using several clock cycles. That means spend more time
than sending parallelly.
Parallel vs. Serial
When
we compare these two categories of protocols, each have their own pros and
cons. Parallel communication is really faster than serial communication because
it sends multiple bits in a single clock cycle, while the serial protocol uses
multiple clock cycles to send the same data amount.
But
it saves number of physical wires of a design and reduce the hardware
complexity. On the other hand, upper layers of the communication model needed
to handle serial data by may be increasing the software complexity.
In
some projects number of output/input pins of a micro-controller or an ASIC will
be a critical thing rather than the complexity of the code. Then it will be a
great relief with having a serial communication protocol with you.
Synchronous and Asynchronous
Communication
What is synchronization?
When
there is a digital data communication between any kind of two electronic
devices or circuits, both of them should have some common things. Specially a
common rhythm between two entities. That means a common clock rate to determine
where the data bits or frames are starting and ending. So, maintaining a common
clock rate and common rising and falling edges of the clock signal between two
devices is called the synchronization.
There
are two different synchronization methods for data transmission which are
differ by the clock source. In Synchronous method an external clock signal is
supplied over the medium (ex- extra line for clock signal) while asynchronous
method has no need of external clock but, the receiver entity can recover clock
signal and synchronize using some specific data signal variations such as start
and stop bits.
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Comparison between Synchronous and AsynchronousAsynchronous communication
According
to the previous details, here we are transmitting data without using an
external clock signal. When we sending data using this method the data is sent
frames, with several synchronization symbols within each frame instead of as a
continuous data stream.
Normally
data is sent as one byte at a time and adding start bit at the beginning of the
byte and stop bit at the ending of the byte.
Asynchronous Serial
Here
data is sent through a serial interface and asynchronous method is used for
synchronization. When we are considering some serial interfaces USB (Universal
Serial Bus), Ethernet, I2C and SPI (Serial peripheral Interface) have
widely used in many applications. Among them, I2C and SPI are serial
synchronous protocols which are using extra line for clock signal other than
the communication lines.
An
asynchronous serial interface communicates without a support from an external
clock line by minimizing the number of needed wires or number of IO pins of a
device. On the other hand, we have to put some extra effort to synchronization
from software side. Bluetooth modules, GPS modules, wi-fi modules and most of
other modules we have used for our projects are based on this serial
communication.
Structure of an Asynchronous Serial data frame
In
asynchronous serial protocol, there are several things to consider other than
the set of data bits.
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Data bits: Each packet contain a set of data bits which has a
varying length (ex-one byte). But most of the times we are using 8bit as the
data bits and sometimes 7bits if we are transferring 7bit ASCII characters.
Both devices should agree with the length of the frames and what are the
endianness of the data bits set.
Synchronization bits: There two or three bits in every data frame which are
specially assigned as synchronization bits. Their job is to show the starting
point and ending point of a data frame. Therefore, they are called as start bit and stop bits. Normally start bit contains only one bit while one or
two bits for the stop bits.
Parity bits: Most of the data frames there is an error detection data bit which is based on a parity
detection mechanism handled by two entities. This parity detection method is
based on whether the sum of the data bits (number of ‘1’s in the data bits) are
even or odd. There also two parity methods called odd parity and even parity.
If the sum is odd and the even parity is used then parity bit is set to “1”.
Set
the parity bit to ‘1’ or ‘0’ as following,
Even parity ---“sum of Data bits +
Parity bit ---> is an even number”
Odd parity ---“sum of Data bits +
Parity bit ---> is an odd number”
Apart
from the data frame, there is another important thing that we need to consider
in asynchronous serial communication. That is the baud rate.
Baud Rate
In
telecommunication theory, baud rate is the number of symbols sent along a channel
per second. Here also it describes the same thing and the baud rate is measured
by how much bits per second the serial interface can communicate. It gives us a
sense how fast a serial link can send data. Normally we are using 1200, 2400,
4800, 9600,19200,38400,57600 and 115200 bits per second as standard baud rates.
Most common and simple applications are using 9600 bps while the speed critical
applications are using 115200 bps as the baud rate.
Whatever
the data rate, the two communication entities must communicate in the same data
rate in order to obtain a successful communication.
Serial Asynchronous Interface in Hardware
In
most of the applications we expect a duplex communication between two devices. That
means either of two devices capable receive and transmit data. There are two kinds
of duplex communication. One is Full-duplex which enable to receive and
transmit data at the same time. Other is the Half duplex which enable transmit
or receive data to one entity after other one is completed.
So,
these kinds of interfaces must have two lines to receive and transmit data and
another line for a common ground.
There
an important thing about wiring the two entities. RX and TX pins are working relative
to themselves and we must have wired them considering the system. Tx pin of one
device should be connected in the RX pin of the other device and vice versa.
Bluetooth
modules, GPS modules, RTC modules and Communication between two microcontrollers
are some common applications of duplex.
Sometimes
we are using one direction data transmission. That means we need only to transmit
data to someone or only to receive data from someone. Then we use a single line
to obtain the communication. This method is called as the Simplex communication.
References:https://en.wikipedia.org/wiki/Serial_communication
https://learn.sparkfun.com/tutorials/serial-communication/all
