---
title: "Control methods for interface communication"
subject: "Embedded System"
chapter: 5
key: "semester_6/embedded_system:interfacing-control-methods"
source: https://www.bctengineer.com/q/semester_6/embedded_system/interfacing-control-methods
site: BCT Engineering
---

# Control methods for interface communication

Embedded System · Chapter 5: Interfacing

- Asked in: 2078 Chaitra
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/interfacing-control-methods
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-5

## Question

Explain the control methods used for communication in interfacing.

## Short answer

Programmed I/O (polling) uses a busy-wait loop, wasting CPU time but simple. Interrupt-driven I/O lets the peripheral signal readiness via an interrupt, causing the CPU to run an ISR and then resume, improving efficiency. DMA transfers blocks directly between peripheral and memory with minimal CPU involvement, providing high speed for large transfers.

## Answer

Interfacing connects a processor to a peripheral, and the control method decides how the transfer is controlled. The main methods are:

#### 1. Programmed I/O (polling)
The processor repeatedly reads the status register of the peripheral (busy-wait loop) and transfers data when the device is ready. Simple, but wastes CPU time while polling.

#### 2. Interrupt-driven I/O
The peripheral sends an interrupt request when it is ready; the processor suspends the main program, runs the ISR for the transfer and then resumes. Uses the CPU efficiently. Two forms: **fixed ISR location** and **vectored interrupt** (the device supplies the vector).

#### 3. Direct memory access (DMA)
A DMA controller transfers blocks of data between the peripheral and memory without CPU involvement for every byte. Best for large, fast transfers; the CPU only sets up the transfer and handles completion.

#### Handshaking and timing control
The transfer is also controlled by protocol: **strobe** (data valid pulse) and **handshake** (Req/Ack).

#### Bus access control
When multiple masters exist, **arbitration** (priority or daisy chain) decides who controls the bus.

| Method | CPU involvement | Speed | Use |
|---|---|---|---|
| Polling | Full (busy wait) | Low | Simple devices |
| Interrupt | During ISR only | Medium | Keyboard, serial |
| DMA | Setup and end only | High | Disk, network |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does programmed I/O (polling) work and what is its drawback?
   A. The CPU sleeps; it loses data
   B. The device interrupts the CPU; it needs a vector
   C. A DMA controller moves data; it is slow
   D. The CPU repeatedly reads the status register; it wastes CPU time
   Answer: D. Polling is a busy-wait loop on the status register: simple but wasteful. (Key point: Polling)

2. What happens in interrupt-driven I/O?
   A. The peripheral requests an interrupt when ready; CPU suspends the main program, runs ISR, then resumes
   B. The CPU loops until the device is ready
   C. The DMA controller runs the ISR
   D. The CPU is not involved at all
   Answer: A. The CPU is used efficiently since it is involved only during the ISR. (Key point: Interrupt-driven I/O)

3. For which transfers is DMA best and what does the CPU do?
   A. Large fast block transfers; CPU only sets up the transfer and handles completion
   B. Single byte transfers; CPU transfers each byte
   C. Keyboard input; CPU polls
   D. Arbitration; CPU grants the bus
   Answer: A. DMA moves blocks between peripheral and memory without per-byte CPU involvement. (Key point: DMA)

## Related in Embedded System

- [Advanced communication principles](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aadvanced-communication-principles) — Embedded System, chapter 5
- [Metrics for control objectives](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Acontrol-metrics-objectives) — Embedded System, chapter 7
- [Control evaluation steps; PD and PI design](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Acontrol-evaluation-steps-pd-pi) — Embedded System, chapter 7
- [Arbitration methods and I2C significance](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aarbitration-types-i2c) — Embedded System, chapter 5

## Related in other subjects

- [8255 at A0A0H: interface and control words](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-a0a0h-control-words) — Instrumentation II, chapter 2
- [Flow control mechanisms in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-mechanisms) — Telecommunications, chapter 8
- [Flow control in data communication network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-network) — Telecommunications, chapter 8
- [Memory capacity from range and interfacing](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Amemory-range-capacity-interfacing) — Microprocessor, chapter 4