Input/Output Interfacing – I/O Basics, Serial & Parallel, Interrupts, DMA, and Real‑World Applications

Unit 9: Input/Output Interfacing

9.1 Overview of I/O Interfacing

Input/Output (I/O) interfacing is the bridge that allows a microprocessor to exchange data with the outside world. It is realized through parallel and serial buses, each with distinct characteristics. Parallel buses transmit multiple bits simultaneously over several wires, offering high throughput but consuming many pins. Serial buses transmit one bit at a time over a few wires, trading speed for simplicity and lower pin usage.

Parallel I/O is still used in high‑speed memory interfaces (e.g., DDR), while serial I/O dominates modern embedded systems due to its compactness and robustness.

9.2 Types of I/O Devices

I/O devices are classified by their data transfer mode and interface protocol:

9.3 Parallel I/O

Parallel I/O uses a set of data lines, a clock line, and control signals. The classic parallel port (DB‑25) used in printers is a 8‑bit data bus with separate read/write strobe lines. Data is transferred in a single cycle, enabling high throughput.

Worked Example – Parallel Port Data Transfer
A microcontroller writes the byte 0xA5 to a printer:

The entire byte is transferred in one clock cycle, but the need for 10 pins (8 data + 2 control) limits scalability.

9.4 Serial I/O

Serial communication transmits data bit‑by‑bit over one or two wires. Three widely used serial protocols are UART, SPI, and I²C.

9.4.1 UART (Universal Asynchronous Receiver/Transmitter)

UART is asynchronous; it does not share a clock line. Each frame starts with a start bit, followed by data bits, optional parity, and stop bits. The baud rate determines the bit duration.

Mermaid State Diagram – UART Transmission

stateDiagram-v2
    [*] --> Idle
    Idle --> Start : Start bit detected
    Start --> Data : 8 data bits transmitted
    Data --> Parity : Parity bit (if enabled)
    Parity --> Stop : Stop bit(s)
    Stop --> Idle : Frame complete

9.4.2 SPI (Serial Peripheral Interface)

SPI is synchronous and uses four lines: MISO (Master In Slave Out), MOSI (Master Out Slave In), SCLK (Serial Clock), and CS (Chip Select). The master generates the clock and selects the slave.

9.4.3 I²C (Inter‑Integrated Circuit)

I²C uses two wires: SCL (clock) and SDA (data). It supports multiple masters and slaves, with each slave identified by a 7‑bit address.

9.5 I/O Control Mechanisms

9.5.1 Polling

The CPU repeatedly checks a status register to see if data is ready. Polling is simple but wastes CPU cycles.

9.5.2 Interrupts

An interrupt signal notifies the CPU that an event has occurred. The CPU saves its context, executes an interrupt service routine (ISR), and resumes normal execution.

Mermaid Flowchart – I/O Interfacing Process

flowchart TD
    A["Start"] --> B["Check I/O type"]
    B -->|"Parallel"| C["Configure parallel pins"]
    B -->|"Serial"| D["Configure serial registers"]
    C --> E["Enable interrupts"]
    D --> E
    E --> F["Wait for interrupt"]
    F --> G["Execute ISR"]
    G --> H["Process data"]
    H --> I["Return to main"]

9.5.3 Direct Memory Access (DMA)

DMA allows peripherals to transfer data directly to/from memory without CPU intervention, freeing the CPU for other tasks.

9.6 I/O Addressing and Port I/O

I/O devices are accessed via memory‑mapped I/O or port I/O. In memory‑mapped I/O, device registers occupy specific memory addresses; the CPU uses normal load/store instructions. In port I/O, dedicated instructions (e.g., IN, OUT on x86) access I/O ports.

Example – Memory‑Mapped I/O
A UART data register at address 0x4000_0000:

#define UART_DATA  (*(volatile uint8_t*)0x40000000)
UART_DATA = 0x55;   // Send byte

Example – Port I/O
On x86, port 0x3F8 is the COM1 data register:

mov al, 0x55
out 0x3F8, al

9.7 Practical Design Example: Temperature Sensor Interface

A common embedded application is reading a temperature sensor (e.g., TMP102) via I²C and displaying the value on an LCD.

System Diagram

classDiagram
    class MCU {
        +GPIO
        +I2C
        +LCD
    }
    class TMP102 {
        +TempReg
    }
    class LCD {
        +DataBus
    }
    MCU --> TMP102 : I2C
    MCU --> LCD : Parallel

Step‑by‑Step Trace

1. MCU initiates I²C start condition.
2. Sends sensor address 0x48 with R/W=1.
3. Sensor acknowledges.
4. MCU reads 16‑bit temperature value.
5. MCU converts raw value to Celsius.
6. MCU writes the formatted string to the LCD via parallel port.

Worked Example – Raw Data 0x1A2B

• Raw value = 0x1A2B = 0001101000101011₂.
• Temperature (in 0.0625°C units) = 0x1A2B * 0.0625 = 26.8125°C.
• MCU formats as "26.8°C" and sends to LCD.

9.8 In the Real World

9.9 Exam Tip

• Understand the difference between parallel and serial I/O: pin count, speed, and typical use cases.
• Know the frame formats of UART, SPI, and I²C; be able to draw them from memory.
• Explain interrupt vs. polling vs. DMA: when to use each and their impact on CPU load.
• Be able to design a simple I/O circuit: choose the appropriate bus, set up control registers, and write a short ISR.
• Practice tracing data through an I²C transaction or UART frame to reinforce timing and bit ordering.

Real‑World Images

Figure – UART, SPI, and I²C Frame Formats