I/O and Interface Technology | Generated by AI
Great! Here’s a comprehensive tutorial for Part 4: I/O and Interface Technology, based on your course outline. This tutorial is structured to explain concepts clearly, provide practical examples, and show how components like 8255A, 8253/8254, and 8251A work together in a microcomputer system.
📘 Part 4: I/O and Interface Technology
1. Basics of I/O Interfaces
✅ Port Addressing
I/O interfaces allow a microprocessor to communicate with external devices. There are two main addressing schemes:
- Memory-Mapped I/O:
- I/O devices are treated like memory locations.
- Uses the same instructions as memory access (e.g.,
MOV). - Pros: Larger address space, can use all CPU instructions.
- Cons: Consumes memory address space.
- Isolated I/O (Port-Mapped I/O):
- Special instructions like
INandOUT. - Limited address space (usually 256 ports).
- Separate address space from memory.
- Special instructions like
| Type | Instruction Set Used | Address Space |
|---|---|---|
| Memory-Mapped | MOV, etc. |
Part of memory |
| Isolated (I/O-Mapped) | IN, OUT |
Separate I/O space |
✅ Data Transfer Modes
- Program-Controlled I/O:
- CPU checks device status and reads/writes data directly.
- Simple but inefficient (busy waiting).
- Interrupt-Driven I/O:
- Device notifies CPU when it’s ready via an interrupt.
- CPU executes an Interrupt Service Routine (ISR).
- Improves efficiency.
- DMA (Direct Memory Access):
- Device transfers data directly to/from memory.
- Bypasses CPU for large/fast data transfer.
- Used for high-speed devices like disks.
2. Interrupt Systems
✅ Interrupt Vector Table
- Stores addresses of Interrupt Service Routines (ISRs).
- Each interrupt type has a unique vector (e.g., INT 0x08 for Timer).
- The CPU looks up the table to jump to the correct ISR.
✅ Priority Handling
- When multiple interrupts occur simultaneously, priority determines which gets handled first.
- Priority can be fixed or programmable.
✅ 8259A Programmable Interrupt Controller
- Manages multiple interrupt sources (up to 8).
- Can be cascaded for 64 interrupt inputs.
- Key functions:
- Interrupt masking.
- Priority setting.
- Sending interrupt vector to CPU.
Registers:
- IMR (Interrupt Mask Register)
- ISR (In-Service Register)
- IRR (Interrupt Request Register)
Example: Keyboard and Timer both trigger interrupts — 8259A prioritizes them based on configured priority.
3. Common Interface Chips
✅ 8255A Programmable Peripheral Interface (PPI)
Used to interface with external parallel devices like switches, LEDs, etc.
- Has 3 ports: Port A, Port B, and Port C.
- Controlled via Control Word.
Modes of Operation:
- Mode 0 – Simple I/O
- Each port can be input/output.
- Mode 1 – Handshaking I/O
- Supports synchronization with peripheral.
- Mode 2 – Bidirectional I/O (only for Port A)
- Two-way data transfer with handshaking.
Example:
- Port A: output to LED display
- Port B: input from DIP switches
- Port C: used for control signals
✅ 8253 / 8254 Programmable Interval Timer
Used for generating delays, baud rates, etc.
- Has 3 independent 16-bit counters.
- Each counter has modes (0–5), e.g.:
| Mode | Description |
|---|---|
| 0 | Interrupt on terminal count |
| 2 | Rate generator (e.g., for clock) |
| 3 | Square wave generator |
Applications:
- Delay generation
- Real-time clock
- Baud rate generation for serial ports
Typical Use:
- Counter 0: OS timer tick
- Counter 1: DRAM refresh
- Counter 2: Speaker tone
✅ 8251A USART (Universal Synchronous/Asynchronous Receiver Transmitter)
Used for serial communication.
- Converts between parallel and serial data.
- Supports both:
- Synchronous: requires clock signal
- Asynchronous: uses start/stop bits
Key Features:
- Baud rate control (via 8253/8254)
- Configurable character length, stop bits, parity
- Supports transmit and receive buffering
Application: Serial port communication with PC or microcontroller.
✅ Summary Table
| Chip | Function | Key Features |
|---|---|---|
| 8255A | Parallel I/O | 3 ports, 3 modes, handshaking |
| 8253/54 | Timing/counting | 3 counters, multiple modes |
| 8251A | Serial communication | Sync/Async, baud control |
| 8259A | Interrupt control | Priority, masking, ISR vectoring |
🧠 Real-World Example Scenario
You want to design a simple microcomputer system to:
- Control an LED matrix via Port A (8255A Mode 0)
- Receive keyboard input via interrupts (8259A)
- Generate a periodic system tick every 1ms (8253 Mode 2)
- Send sensor data to PC via serial (8251A in async mode)
Each chip would be configured and connected to the CPU through address decoding and control signals, with appropriate software written to manage communication.
Let me know if you want example code, simulation diagrams, or a quiz for this chapter!