Chapter 8

• RS232 is a standard developed by the Electronic Industries Association (EIA).
• It is one of the oldest and most widely used communication interfaces.
• A personal computer (PC) typically has two RS232 ports designated as COM1 and COM2.
• Most microcontrollers provide an on-chip UART (Universal Asynchronous Receiver/Transmitter) serial interface.
• Processor evaluation boards are commonly connected to a host computer using the RS232 interface.
• RS232 is used to connect a Data Terminal Equipment (DTE) to a Data Circuit-Terminating Equipment (DCE).
• DTE Examples: PC, serial printer, plotter.
• DCE Examples: Modem, mouse, digitizer, scanner.
• RS232 specifies only the physical layer interface for serial communication.
• It is a serial communication standard, where data bits are transmitted one after another.
• Communication is full duplex, allowing simultaneous data transmission in both directions.

RS232 Serial Connector

UART Serial Communication

• In RS232 communication, the transmitting device sends data character by character.
• The binary representation of each character is known as the data bits.
• A Start Bit is transmitted before the data bits to indicate the beginning of a character.
• One or two Stop Bits are transmitted after the data bits to indicate the end of the character.
• The receiving device uses the Start Bit and Stop Bit to correctly identify and decode the incoming data.
• RS232 uses asynchronous communication, meaning that no separate clock signal is transmitted between devices.
• An optional Parity Bit may be included between the data bits and stop bit(s).
• The Parity Bit provides a simple method of error detection at the receiving end.

RS232 Data Frame: Start Bit → Data Bits → Parity Bit → Stop Bit(s)

• Both communicating devices must be configured with identical communication parameters for successful RS232 communication.
• Data Rate (Baud Rate)
  • Determines the speed of data transmission.
  • Measured in bits per second (bps).
  • Common baud rates: 50, 150, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 bps.
• Data Bits
  • Represents the number of bits used for each transmitted character.
  • Possible values: 5, 6, 7, or 8 bits.
  • ASCII characters typically use 7 data bits.

RS232 devices must use identical communication settings.

• Start Bit
  • Prefixed before the data bits.
  • Identifies the beginning of a character.
• Stop Bits
  • Appended after the data bits.
  • Indicates the end of the character.
  • If data bits are 7 or 8 → 1 Stop Bit.
  • If data bits are 5 or 6 → 2 Stop Bits.
• Parity Bit
  • Used for error detection.
  • Can be Even, Odd, or None.
  • Example Data: 1010110
  • Even Parity → Parity Bit = 0
  • Odd Parity → Parity Bit = 1
  • The receiver recalculates the parity to detect transmission errors.

• DTE (Data Terminal Equipment) is the source or destination of data.
• Examples:
  • Desktop Computer
  • Laptop
  • Microcontroller Board
  • Serial Terminal
• DCE (Data Circuit-Terminating Equipment) provides the communication interface.
• Examples:
  • Modem
  • Scanner
  • Industrial Controller
  • Communication Device
• RS232 defines communication between a DTE and a DCE.

Typical RS232 Communication between DTE and DCE

• When one device transmits data faster than the receiving device can process it, Flow Control is required.
• Flow Control temporarily stops and resumes data transmission to prevent data loss.
• This mechanism is also known as Handshaking.
• Hardware Handshaking
  • Uses two control signals:
  • RTS (Request To Send)
  • CTS (Clear To Send)
  • The sender asserts RTS; the receiver responds with CTS when ready.
• Software Handshaking (XON/XOFF)
  • Ctrl + S (0x13) → Suspend transmission (XOFF).
  • Ctrl + Q (0x11) → Resume transmission (XON).

Hardware and Software Flow Control

• Unlike TTL logic (0V and +5V), RS232 uses positive and negative voltages to represent binary values.
• Logic 1 (MARK)
  • Voltage between -3V and -15V.
• Logic 0 (SPACE)
  • Voltage between +3V and +15V.
• Voltages between -3V and +3V are considered an undefined region.
• Because of these voltage levels, a level converter (e.g., MAX232) is required to connect microcontrollers to RS232 devices.

MAX232 converts TTL voltage levels to RS232 voltage levels.

• RS232 defines communication between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE).
• The standard uses D-subminiature (D-Sub) connectors for physical connections.
• The two most common connector types are:
  • DB9 (9-pin connector)
  • DB25 (25-pin connector)
• Both connectors are available in Male and Female versions.
• Each pin has a specific communication or control function, such as transmitting data, receiving data, handshaking, or grounding.
• Modern computers typically use the DB9 connector, while older systems often used the DB25 connector.

D-Sub Connectors: DB9 and DB25 (Male and Female)

• Processors work with parallel data, whereas RS232 transmits data in serial format.
• RS232 also uses higher positive and negative voltage levels, which are different from the low-voltage logic levels (3.3V/5V) used by most embedded systems.
• These differences are overcome using a UART (Universal Asynchronous Receiver/Transmitter).
• UART has two sections:
  • Receive Section (RX): Converts serial data into parallel data for the processor.
  • Transmit Section (TX): Converts parallel data from the processor into serial data.
• UART automatically generates the Start Bit, Parity Bit, and Stop Bit(s).
• Most modern microcontrollers include an on-chip UART.
• A level shifter (e.g., MAX232) is still required to convert UART voltage levels to RS232 voltage levels.

• Simple and inexpensive communication interface.
• Easy to implement using built-in UART peripherals.
• Reliable for short-distance communication.
• Supports full-duplex communication.
• No clock synchronization required (asynchronous communication).
• Widely supported by industrial equipment and laboratory instruments.
• Suitable for debugging, configuration, and firmware updates.

RS232 remains widely used in industrial systems.

• Limited communication distance (approximately 15 m).
• Lower data rate compared to USB, Ethernet, and CAN.
• Point-to-point communication only.
• Poor noise immunity over long cables.
• Large voltage levels require level-shifting circuits.
• Modern laptops usually do not include native RS232 ports.
• Not suitable for high-speed networking applications.

Level shifters are required for microcontroller interfaces.

• Communication between PCs and microcontrollers.
• Industrial automation and PLC programming.
• Serial console for embedded Linux systems.
• GPS receivers and navigation devices.
• Point-of-sale (POS) terminals.
• Medical and laboratory instruments.
• Network switches, routers, and embedded debugging.
• Firmware downloading and device configuration.

RS232 console cable used for configuring networking equipment.

• RS422 and RS485 are serial communication standards developed to overcome the distance and speed limitations of RS232.
• RS422
  • Uses differential signaling for improved noise immunity.
  • Supports one transmitter and multiple receivers (up to 10).
  • Communication distance up to 1200 m.
  • Data rate up to 10 Mbps (short distance).
• RS485
  • Uses differential signaling.
  • Supports multiple transmitters and multiple receivers (multi-drop network).
  • Allows up to 32 devices (or more with modern transceivers).
  • Widely used in industrial automation, PLCs, and Modbus networks.
• Advantages over RS232: Longer distance, higher speed, better noise immunity, and multi-device communication.

• Universal Serial Bus (USB) is one of the most widely used communication interfaces for connecting peripheral devices to computers.
• Common USB-enabled devices include:
  • Desktop and Laptop Computers
  • Printers and Scanners
  • Display Devices
  • Video Cameras
  • External Hard Disk Drives
  • Audio Equipment
• USB follows a Master/Slave architecture, where:
  • The Host (Master) is typically a PC.
  • Peripheral devices act as USB Devices (Slaves).
• The host contains a USB Host Controller (hardware + software) that manages communication with all connected USB devices.
• USB devices can be connected:
  • Directly to the host.
  • Through a USB Hub, which expands the number of available ports (typically 2–8 ports).
• USB devices may be:
  • Self-powered
  • Bus-powered (powered directly by the USB cable).
• A standard USB port can supply up to 500 mA of current to connected devices.

USB Hub expands the number of available USB ports

• USB uses a shielded 4-wire twisted copper cable for communication and power delivery.
• The four wires in a standard USB cable are:
  • VBUS – +5 V Power Supply
  • D+ – Positive Differential Data Line
  • D− – Negative Differential Data Line
  • GND – Ground
• Data is transmitted using a differential twisted pair consisting of the D+ and D− wires.
• Differential signalling provides:
  • Better noise immunity
  • Higher data integrity
  • Reliable communication over longer cables
• The outer shield minimizes electromagnetic interference (EMI).

USB Type-A Connector Pinout

• Data Rates
  • USB 1.1
    • 12 Mbps (Full Speed)
    • 1.5 Mbps (Low Speed)
  • USB 2.0
    • Up to 480 Mbps (High Speed)
• Plug and Play
  • Devices are automatically detected.
  • No manual hardware configuration required.
• Bus Enumeration
  • The host exchanges packets with the device.
  • Identifies the device and configures communication.
• Hot-Pluggable
  • Devices can be connected or removed while the computer is running.

USB Host automatically detects connected devices.

• Packet-Based Communication
  • Communication occurs through data packets.
  • The host obtains the device configuration and properties.
  • Each connected device is assigned a unique ID (1–127).
• Host Responsibilities
  • Detect newly connected devices.
  • Assign device addresses.
  • Exchange control and data packets.
  • Receive notification when a device is removed.
• USB Packets
  • Short packets are used for handshaking and acknowledgements.
  • Data packets can contain up to 1023 bytes.

USB devices are automatically identified and classified.

• Ethernet is one of the most widely used wired communication technologies for computer networks.
• It is available on almost every desktop and laptop computer.
• The availability of low-cost Ethernet controllers (chips) and the TCP/IP protocol stack makes Ethernet easy to implement in embedded systems.
• An embedded system with an Ethernet interface can be connected directly to a Local Area Network (LAN).
• Sensor readings and other data collected by the embedded system can be transferred to a server or database on the LAN.
• Ethernet provides the Physical Layer and Data Link Layer services of the OSI model.
• Higher-level communication is achieved using TCP/IP and Application Layer Protocols (e.g., HTTP, FTP, MQTT).

Ethernet operates at the Physical and Data Link Layers.

• The Physical Layer of Ethernet defines how devices are physically connected to a Local Area Network (LAN).
• Ethernet commonly uses an RJ45 connector (8P8C) for network connections.
• Transmission media include:
  • Unshielded Twisted Pair (UTP) cable
  • Coaxial cable (legacy Ethernet)
• Ethernet uses differential signalling:
  • One wire carries signals from 0 V to +2.5 V.
  • The second wire carries signals from −2.5 V to 0 V.
  • The voltage difference between the pair is approximately 5 V.
• Common Ethernet speeds supported:
  • 10 Mbps (10BASE-T)
  • 100 Mbps (100BASE-TX)

RJ45 Ethernet Connector

• The Data Link Layer is responsible for reliable communication between devices on the same Local Area Network (LAN).
• It consists of two sublayers:
  • Medium Access Control (MAC)
  • Logical Link Control (LLC)
• MAC Layer
  • Controls access to the shared communication medium.
  • Uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.
  • Detects collisions and retransmits data when necessary.
• LLC Layer
  • Establishes logical communication.
  • Provides flow control.
  • Performs error control.
  • Supports acknowledgements.
• Every Ethernet interface has a unique 48-bit MAC Address.

Ethernet Frame and MAC Address

CSMA/CD Medium Access Mechanism

• To make an embedded system network-enabled, the TCP/IP protocol stack must run above the Ethernet layer.
• The TCP/IP stack is integrated with:
  • Operating System (OS)
  • Firmware
  • Application Software
• Application protocols provide different network services:
• SMTP – Sending e-mails
• FTP – File transfer
• HTTP – Web server communication
• These protocols enable embedded devices to communicate over LANs and the Internet.

TCP/IP Protocol Stack

MODM7AE70 ARM Cortex M-7 Embedded IoT Development Kit

Topics Covered

• Introduction to Wireless Communication
• Infrared (IR) Communication
• IEEE 802.11 (Wi-Fi)
• Bluetooth Technology
• Comparison of Wireless Communication Technologies
• Applications in AI and Embedded Systems

Common Wireless Communication Technologies

• Wireless communication is the transmission of information between devices without using physical cables.
• Data is transmitted through electromagnetic waves such as:
  • Radio Waves
  • Infrared (IR)
  • Microwaves
• Wireless communication enables:
  • Mobility
  • Remote monitoring
  • Real-time data exchange
  • Internet of Things (IoT)
• It is widely used in embedded systems, AI devices, smart homes, healthcare, industrial automation, robotics, and autonomous vehicles.
• Common wireless communication technologies include:
  • Infrared (IR)
  • Wi-Fi (IEEE 802.11)
  • Bluetooth

Wireless communication between embedded devices

Wireless technologies use different regions of the electromagnetic spectrum.

• Infrared (IR) is a wireless communication technology that uses infrared light waves to transmit data between devices.
• IR communication is a line-of-sight (LOS) technology, meaning the transmitter and receiver must face each other.
• It is suitable for short-distance communication, typically ranging from a few centimeters to several meters.
• IR communication is immune to radio frequency (RF) interference but can be affected by obstacles and strong sunlight.
• Advantages
  • Low cost and low power consumption
  • Simple hardware implementation
  • Secure due to limited transmission range
• Limitations
  • Requires direct line of sight
  • Cannot penetrate walls or solid objects
  • Limited communication range

Infrared communication requires a clear line of sight.

TV remote controls commonly use Infrared (IR) communication.

• For communication through the Infrared (IrDA) Interface, the IrPHY and IrLAP layers are specified in the standard.
• Link Management is performed by IrLMP, above which the Application Layer Protocols execute.

• The IR Sensor Module is an electronic sensor used for obstacle detection and proximity sensing.
• It works by transmitting infrared light and detecting the reflected light from nearby objects.
• The module mainly consists of:
  • IR Transmitter (IR LED)
  • IR Receiver (Photodiode)
  • LM358 Comparator
  • Variable Resistor (Preset)
  • Status LED
• Provides a digital HIGH/LOW output for microcontrollers.

IR Sensor Module

• Operating Voltage: 5 V
• Compatible with 3.3 V and 5 V logic
• Detection Range: Up to 20 cm
• Adjustable sensing distance
• Built-in Ambient Light Filter
• Supply Current: 20 mA

IR Sensor Pinout

1. IR LED continuously emits infrared light.
2. When no object is present, the emitted light does not return to the receiver.
3. When an object comes within range, the infrared light is reflected back.
4. The photodiode receives the reflected light.
5. LM358 Comparator compares the received signal with a preset threshold.
6. The module output changes HIGH/LOW and the indicator LED turns ON.

Object Detection using Reflected IR Light

• Obstacle Detection
• Line Following Robot
• Industrial Safety
• Wheel Encoder
• Automatic Door Systems
• AI Robots

Components of an IR Sensor Module

IR LED Transmitter

• Emits infrared light (700 nm – 1 mm wavelength).
• Invisible to the human eye.
• Typical emission angle: 20°–60°.
• Detection range varies from a few centimeters to several feet.

Photodiode Receiver

• Operates in reverse bias.
• Produces current proportional to the received infrared light.
• Black coating absorbs infrared efficiently.

LM358 Comparator

• Compares the photodiode voltage with the preset threshold.
• Photodiode Voltage > Threshold → Output HIGH → LED ON.
• Photodiode Voltage < Threshold → Output LOW.

Variable Resistor (Preset)

• Adjusts the threshold voltage.
• Controls the object detection distance.

Working Principle of an IR Sensor Module

• IEEE 802.11 is a family of standards for Wireless Local Area Networks (WLANs) and Personal Area Networks (PANs).
• IEEE 802.11 defines the Physical (PHY) layer and Medium Access Control (MAC) layer of Wireless LANs.
• The Logical Link Control (LLC) layer is the same as that used in Ethernet LAN.
• Every wireless LAN device contains an antenna for transmitting and receiving radio waves.
• All wireless nodes using the same MAC protocol and sharing the same wireless medium form a Basic Service Set (BSS).
• A BSS connects to a wired backbone network through an Access Point (AP).
• Multiple BSSs can be interconnected through the backbone LAN to form larger wireless networks. Access Points are commonly called Wi-Fi Hotspots.

IEEE 802.11 WLAN Architecture

Wireless Access Point (Wi-Fi Hotspot)

• Centralized Control: All communication between stations (STAs) must pass through the Access Point (AP), even if two STAs sit next to each other.
• Wired Gateway: The AP links wireless STAs to the wired backbone infrastructure (Ethernet/Internet).
• Scalability: Easily allows multiple client devices to join and authenticate using a unified SSID and security credentials.

• No Central Infrastructure: STAs communicate directly over radio waves within mutual range.
• Temporary Setup: Ideal for quick file sharing, emergency operations, or vehicle-to-vehicle (V2V) communications.
• Distributed Coordination: Beacon generation and timing synchronization are shared among all participating STAs.

• BSSID: The unique identifier for a BSS, which is the 48-bit MAC address of the Access Point's radio interface.
• Coverage Area (Cell): The geographic area within which stations can maintain radio contact with the AP.
• SSID: The human-readable network name shared by all devices within the service set.

• Seamless Roaming: Stations can move between AP1 and AP2 without losing connection, as all APs share the same network SSID.
• Distribution Backbone: Wired Ethernet (802.3) or wireless mesh interconnects APs to gateway routers.
• Unified Network: Appears as a single continuous Subnet / LAN to the upper network layers (Layer 3 IP).

Stations, Clients and Access Point

Distribution System connecting multiple BSSs

• A Service Data Unit (SDU) is the data received by a protocol layer from the layer immediately above it.
• The SDU contains only the payload (user data) and has no protocol header or trailer added by the current layer.
• Each protocol layer treats the received SDU as input for further processing.
• The current layer adds its own control information (headers/trailers) before transmission.
• After encapsulation, the SDU becomes part of the Protocol Data Unit (PDU).

• A Protocol Data Unit (PDU) is the data unit transmitted between peer protocol layers.
• A PDU contains:
  • Protocol Header
  • User Data (Payload)
  • Optional Trailer
• Every protocol layer creates its own PDU by adding protocol-specific control information.
• The PDU produced by one layer becomes the SDU of the next lower layer.

• The SDU of one layer becomes the payload of the next lower layer.
• Each lower layer adds its own header (and sometimes a trailer).
• This process is called Encapsulation.
• At the receiver, headers are removed layer by layer in a process called Decapsulation.
• The original application data is finally delivered to the destination application.

IEEE 802.11 defines multiple physical layer technologies for wireless communication.

The Industrial, Scientific and Medical (ISM) band (2400–2483.5 MHz) is an unlicensed radio spectrum, allowing wireless devices to operate without government licensing.

• IEEE 802.11 uses the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol for medium access.
• Before transmitting, a station listens to the wireless channel.
• If the channel is idle for a Distributed Inter-Frame Space (DIFS), the station immediately transmits the frame.
• If the channel is busy, the station continues sensing the channel.
• Once the channel becomes free for a DIFS period, the station waits for a random back-off interval before transmitting.
• The random back-off reduces the probability of two stations transmitting simultaneously.

The 802.11 MAC layer adds a header to IP packets for wireless transmission. Wi-Fi frames use up to 4 MAC addresses to handle complex routing between access points.

• Fast Discovery: Uses 3 dedicated "Advertising" channels so devices can find each other instantly.
• Deep Sleep: Devices stay asleep >99% of the time and wake up in microseconds just to send a tiny burst of data.
• GATT Architecture: Think of data like a menu. Devices offer Services (e.g., Battery) containing Characteristics (e.g., 85%).

An RTOS guarantees that tasks finish within strict deadlines. For AI (like self-driving cars), getting the answer on time is just as important as getting it right.

Running AI on tiny microcontrollers requires specialized engines like TensorFlow Lite Micro that sit right on top of the RTOS kernel.

In an RTOS, every recurring AI task is defined by three simple variables:

• $C$ (Computation Time): How long the AI takes to run (e.g., 5ms).
• $T$ (Period): How often the AI needs to run (e.g., every 33ms for a 30 FPS camera).
• $D$ (Deadline): When the answer must be ready by.

Because neural networks take a long time to run, standard RTOS scheduling isn't enough. We use clever AI tricks to guarantee deadlines:

Textbooks

1. Raj Kamal, Embedded Systems: Architecture, Programming and Design, 3rd Edition, McGraw-Hill Education.
2. Frank Vahid & Tony Givargis, Embedded System Design: A Unified Hardware/Software Introduction, John Wiley & Sons.
3. Wayne Wolf, Computers as Components: Principles of Embedded Computing System Design, Morgan Kaufmann.
4. David E. Simon, An Embedded Software Primer, Addison-Wesley.
5. Behrouz A. Forouzan, Data Communications and Networking, 5th Edition, McGraw-Hill.
6. Andrew S. Tanenbaum & David J. Wetherall, Computer Networks, 5th Edition, Pearson.

Online References

1. IEEE Standards Association
   https://standards.ieee.org/
2. IEEE 802.11 Working Group
   https://www.ieee802.org/11/
3. USB Implementers Forum (USB-IF)
   https://www.usb.org/
4. Components101 – IR Sensor Module
   https://components101.com/sensors/ir-sensor-module
5. Codrey Electronics – RS232 Serial Communication
   https://www.codrey.com/embedded-systems/rs232-serial-communication/
6. Analog Devices (MAX232 Datasheet)
   https://www.analog.com/
7. Arduino Documentation
   https://docs.arduino.cc/
8. Espressif ESP32 Documentation
   https://docs.espressif.com/