---
title: "Why 8051 / its operational features"
subject: "Embedded System"
chapter: 9
key: "semester_6/embedded_system:why-8051-features"
source: https://www.bctengineer.com/q/semester_6/embedded_system/why-8051-features
site: BCT Engineering
---

# Why 8051 / its operational features

Embedded System · Chapter 9: Microcontrollers in Embedded Systems

- Asked in: 2069 Bhadra, 3 marks; 2073 Bhadra, 3 marks (asked 2 times)
- Video: [8051 Microcontroller Explained: Features, Components, and Applications](https://www.youtube.com/watch?v=roM2y9P_YFs) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/why-8051-features
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-9

## Question

Why is the 8051 microcontroller used? What are the important operational features of the 8051 microcontroller?

## Short answer

The 8051 is a complete, low-cost single-chip computer, easy to program, widely available, and suitable for control tasks. Key features: 8-bit CPU, 4 KB ROM, 128 B RAM, 32 I/O pins, two 16-bit timers, full-duplex UART, five interrupts with two priorities, bit-addressable operations, and a 12-clock machine cycle.

## Answer

#### Why the 8051 is used

- It is a complete single-chip computer: CPU, RAM, ROM, timers, serial port and I/O, so the circuit is small and cheap.
- It is simple, with an easy instruction set and strong bit-handling instructions, so it is easy to learn and program in assembly and C.
- Widely available, cheap and well supported: many tools, compilers (Keil, SDCC), books and manufacturers (Atmel/Microchip, Philips/NXP, Dallas), with many variants.
- Low power, reliable, and enough for control tasks such as displays, keypads, motors and sensors.
- Memory can be expanded to 64 KB code plus 64 KB data.

#### Operational features

| Feature | 8051 |
|---|---|
| Data bus / address bus | 8-bit / 16-bit |
| On-chip ROM, RAM | 4 KB, 128 B |
| I/O | 4 ports, 32 lines |
| Timers | Two 16-bit |
| Serial | One full-duplex UART |
| Interrupts | 5 sources (INT0, INT1, T0, T1, serial), 2 priority levels |
| Machine cycle | 12 clock periods (1 µs at 12 MHz) |
| Registers | A, B, 4 banks of R0-R7, PSW, SP, DPTR |
| Instruction set | 255 opcodes, bit-addressable operations |

## Check yourself

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

1. Why does the 8051 make the circuit small and cheap?
   A. It needs many external chips
   B. It is a complete single-chip computer with CPU, RAM, ROM, timers, serial port and I/O
   C. It has no memory
   D. It uses 32-bit CPU
   Answer: B. Integrating everything on one chip reduces components, size and cost. (Key point: Single-chip)

2. How long is one 8051 machine cycle at 12 MHz?
   A. 12 us
   B. 0.083 us
   C. 1 us
   D. 10 us
   Answer: C. A machine cycle is 12 clock periods, so 12/12 MHz = 1 us. (Key point: Machine cycle)

3. How many interrupt sources and priority levels does the 8051 have?
   A. 2 sources, 5 levels
   B. 8 sources, 4 levels
   C. 3 sources, 1 level
   D. 5 sources, 2 priority levels
   Answer: D. INT0, INT1, T0, T1 and serial give five sources with two priority levels. (Key point: Interrupts)

4. How much external memory can the 8051 address?
   A. 64 KB code plus 64 KB data
   B. 4 KB total
   C. 1 MB
   D. 256 B
   Answer: A. A 16-bit address bus gives 64 KB code and 64 KB data. (Key point: Memory expansion)

## Related in Embedded System

- [Microcontroller vs microprocessor; 8051 features](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Amicro-vs-microprocessor-8051-features) — Embedded System, chapter 9, asked 2×
- [Microprocessor vs microcontroller; 8051 addressing](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-addressing-modes-micro-vs-processor) — Embedded System, chapter 9
- [Addressing modes of 8051](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-addressing-modes) — Embedded System, chapter 9, asked 4×
- [Purpose of port 3 and port 2 of 8051](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-port3-port2-purpose) — Embedded System, chapter 9, asked 2×
- [8051 internal structure and family chart](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-internal-structure-family) — Embedded System, chapter 9
- [8051 data memory organization](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3A8051-data-memory-organization) — Embedded System, chapter 9