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Chapter 2

Overview

The 8051 Microcontroller

Lê Chí Thông

Ref. I. Scott Mackenzie, The 8051 Microcontroller

Chapter 2: Overview

• Hardware Summary

• Address Decoder Summary

• First Program

• Simple Programs

• Instruction Set Summary

Features of 8051

• 4KB ROM
• 128 bytes RAM
• Four 8-bit I/O ports
• Two 16-bit timers
• Serial interface
• 64KB external code memory space Ref. I. Scott Mackenzie •^ 64KB external data memory spaceLê Chí Thông^4

Comparison of MCS-51 Family

FIGURE 2– 2 8051 pinouts

PORT 0

• IO Port
• Address/Data Bus (A0 – A7) / (D0 – D7)

PORT 1: IO Port

PORT 2

• IO Port
• Address Bus (A8 – A15)

PORT 3

• IO Port
• Control signals

Around The Pins

• Port 0 : dual purpose

o I/O port o Multiplexed address and data bus (AD0-AD7)

• Port 1 : I/O port
• Port 2 : dual purpose

o I/O port o High-byte address bus (A8-A15)

• /PSEN (Program Store Enable)

o Connects to /OE (Output Enable) of EPROM to permit reading of program bytes /PSEN (8051)  /OE (ROM)

• ALE (Address Latch Enable)

o ALE (8051)  G (74373) o Use to demultiplex the address and data bus PORT 0  D0 – D7 and A0 – A

• /EA (External Access) It is tied to +5V or ground.

o /EA = +5V : executes programs from internal ROM o /EA = 0V : executes programs from external ROM

• RST (Reset):

A machine cycle is 12 oscillator periods. Use 12 MHz crystal: a machine cycle is 1 μs

Machine Cycle

Internal clock fCLK = fCrystal / 12

• Xtal 12 MHz  MC = 1 μs
• Xtal 6 MHz  MC =? μs

 MC = 2 μs

• Xtal 24 MHz  MC =? μs

 MC = 0.5 μs

Machine cycle MC = 12 / fCrystal

Don’t need Bus-driver

I/O Port Structure

• Port 1, 2, and 3 have internal pull-up resistors.
• Port 0 has no internal pull-up resistors.
• Instructions that input a port bit require the ‘1’ port latch.

Memory Organization

• Memory

o ROM: for program (code)  Code memory  External: maximum 64K  Internal (on-chip): depend on chips o RAM: for data  Data memory  External: maximum 64K  Internal (on-chip): depend on chips