Serial Interface is one of the most commonly used interface in Embedded Systems, during development and in field also. Almost every machine have a serial port over it. So that it could be connected to some peripheral or another Embedded System.

Types of Serial Communication:

1)Synchronous: In Master/Slave interface, both master and slave are driven with same clock(master clock). So this interface is termed as synchronous.
2)Asynchronous: In Point to Point or peer interfaces clock need not to be synchronized between DTE(Data Terminal Equipment) and DCE(Data Communication Equipment). Most of the peer interfaces are asynchronous.

Why Serial???

As every Embedded System has to communicate with external world, it could be some peripheral device or another system for debugging purpose. And Serial interface serves as the best due to many reasons.

1)Small number of pins needed (2 or 3).
2)Availability of different protocols for different interfaces.
3)Good for long distance communication.
4)High Speed.

Protocols:

There are many different serial protocols for different interfaces. Most popular ones are,
RS232, SPI, I2C CAN. Depending upon the application we can select the most appropriate one.
RS232 is commonly used for debugging purpose. Max possible speed is 115.2 kbps.
SPI(Serial Peripheral Interface) is as its name suggest used to interface peripherals. It is a Mater/Slave Synchronous protocol.
I2C(Inter IC) is commonly used to interface, on board peripherals(IC). It is a multi master synchronous protocol.
CAN is used for Inter MCU(micro controller unit) communication in a System. For example it is used in vehicle where number of MCUs are used to serve different purposes.