Serial Communication: UART, SPI, and I2C Explained, Slides of Microprocessor and Assembly Language Programming

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Serial Communication (Asynchronous

UART and Synchronous SPI, I2C)

Chapter 11, 12

Definition of Serial Communication

Bit by bit transmission of information in series

A B

Travels in series

Data Transmission

Serial^ Parallel

Cost

Speed

Transmissio n Amount

Txn Lines

Txn Distance

Example

Cheap

Slow

Single bit

One line to transmit one to receive

Long distance

Modem

Expensive

Fast

8 bits (8 data lines) Transmitter & Receiver

8 lines for simultaneous transmission

Short distance (synchronization)

Printer Connection

Serial Communication Implementation

 Popular implementation found in older and some newer

computers is known as the RS-232 serial connection found

in microcomputers

 Newer type of serial connections

 Universal Serial Bus (USB)

 IEEE 1394 serial connection that is also known as the FireWire

connection

 Many popular serial communication standards exist—some

examples are:

 RS-232 (using UART)

 Serial peripheral interface (SPI)

 System management bus (SMBus)

 Serial ATA (SATA) (abbreviated from Serial AT Attachment )

Serial Cable

DB25 Connector

DB9 Connector

Source Black Box

Asynchronous Serial

Communication

 With asynchronous communication, the transmitter

and receiver do not share a common clock

Transmitter + Receiver

1 byte-wide Data

Dat a

1 byte-wide Data

The Receiver

 Extracts the data using its

own clock

 Converts the serial data

back to the parallel form

after stripping off the

start, stop and parity bits

The Transmitter

 Shifts the parallel data

onto the serial line using

its own clock

 Also adds the start, stop

and parity check bits

Add: Start, Stop, Parity Bits

Remove: Start, Stop, Parity Bits

Asynchronous Serial Communication

 (^) Asynchronous transmission is easy to implement but

less efficient as it requires an extra 2-3 control bits for every 8 data bits

 (^) This method is usually used for low volume transmission

Synchronous Serial Communication

 (^) In the synchronous mode, the transmitter and receiver share a common clock  (^) The transmitter typically provides the clock as a separate signal in addition to the serial data

Transmitter Receiver Data

Clock

The Receiver

 Extracts the data

using the clock

provided by the

transmitter

 Converts the serial

data back to the

parallel form

The Transmitter

 Shifts the data onto the

serial line using its own

clock

 Provides the clock as a

separate signal

 No start, stop, or parity

1 byte-wide Data

1 byte-wide Data

Tx and Rx

 The UART has a transmission engine, and also

a reception engine (they can operate

simultaneously)

 Software controls the UART’s operations by

accessing several registers, using the CPU’s

input and output instructions

 A little history is needed for understanding some

of the UART’s terminology

Serial data-transmission

0 1 1 0 0 0 0 1

The Transmitter Holding Register (8-bits)

0 1 1 0 0 0 0 1

The transmitter’s internal ‘shift’ register

clock

Software outputs a byte of data to the THR

The bits are immediately copied into an internal ‘shift’-register

The bits are shifted out, one-at-a-time, in sync with a clock-pulse

1 - 0-1-1-0-0-0-0-1 -

start bit

stop bit

data-bits

clock-pulses trigger bit-shifts

DCE and DTE

 Original purpose of the UART was for PCs to

communicate via the telephone network

 Telephones were for voice communication

(analog signals) whereas computers need so

exchange discrete data (digital signals)

 Special ‘communication equipment’ was needed

for doing the signal conversions (i.e. a

modulator/demodulator, or modem )

PC with a modem

computer terminal modem

serial

cable

phone wire

Data

Terminal

Equipment

(DTE)

Data

Communications

Equipment

(DCE)

Signal functions

 CD: Carrier Detect The modem asserts this

signal to indicate that it successfully made its

connection to a remote device

 RI: Ring Indicator The modem asserts this

signal to indicate that the phone is ringing at the

other end of its connection

 DSR: Data Set Ready Modem to PC

 DTR: Data Terminal Ready PC to Modem

Signal functions (continued)

 RTS: Request To Send PC is ready for the

modem to relay some received data

 CLS: Clear To Send Modem is ready for the PC

to begin transmitting some data