The document provides an introduction to the 8051 microcontroller. It describes the 8051 as a single-chip computer containing a CPU, ROM, RAM, timers, serial port, and I/O ports. It details the 8051's features, block diagram, pin diagram, ports, important pins including PSEN, EA, ALE, and RST, and how signals like the crystal oscillator operate.

1. INTRODUCTION
TO 8051
MICROCONTROLLER

2. WHAT IS A MICROCONTROLLER?
• All of the components needed for a controller
were built right onto one chip.
• Thus A one chip computer, or microcontroller
was born.
• A microcontroller is a highly integrated chip
which includes, on one chip, all or most of the
parts needed for a controller.
• The microcontroller could be called a "one-
chip solution".

3. Features :
• The Intel 8051 is used in embedded
systems
– 8-bit CPU
– 4k bytes ROM for the program
– 128 BYTES of RAM for variables
– 32 I/O lines ( 4 PORTS WITH 8 EACH )
– 2 timers
– 1 Serial port
– 6 interrupt sources
– Low cost (25-30 Rs per chip)

4. Block Diagram
External Interrupts
Interrupt 4k 128 bytes Timer 1
Control ROM RAM Timer 2
CPU
OSC Bus
4 I/O Ports Serial
Control
P0 P2 P1 P3 TXD RXD
Addr/Data

5. 8051 – PIN DIAGRAM

6. 8051 – 40 PIN IC

7. • 8051 contains four I/O ports (P0 - P3)
• Each port can be used as input or output
(bi-direction)
Port 0 Port 1 Port 2 Port 3
P0（P0.0～P0.7） P1（P1.0～P1.7） P2（P2.0～P2.7） P3（P3.0～P3.7）
8-bit R/W -
8-bit R/W - General
General
General Purpose I/O
Purpose I/O
Purpose I/O Only 8-bit
R/W -
General Or high if not using
Or acts as a Purpose byte of the any of the
multiplexed
I/O address internal
low byte bus for peripherals
address and (timers) or
data bus for
external
external memory external
memory design interrupts.
design

8. IMPORTANT PINS
• PSEN (out): Program Store Enable, the read signal
for external program memory (active low).
• ALE (out): Address Latch Enable, to latch address
outputs at Port0 and Port2
• EA (in): External Access Enable, active low to access
external program memory locations 0 to 4K
• RXD,TXD: UART pins for serial I/O on Port 3
• XTAL1 & XTAL2: Crystal inputs for internal
oscillator.

9. SIGNALS - OPERATION
• Vcc（pin 40）：
– Vcc provides supply voltage to the chip.
– The voltage source is +5V.
• GND（pin 20）：ground
• XTAL1 and XTAL2（pins 19,18）：
– These 2 pins provide external clock.
– Way 1：using a quartz crystal oscillator
– Way 2：using a TTL oscillator

10. RST - RESET
• RST（pin 9）：reset
– input pin and active high
• The high pulse must be high at least 2 machine
cycles.
– power-on reset.
• Upon applying a high pulse to RST, the
microcontroller will reset and all values in
registers will be lost.

11. EA & PSEN
• EA（pin 31）：external access
– There is no on-chip ROM in 8031 and 8032 .
– The EA pin is connected to GND to indicate the
code is stored externally.
– PSEN ＆ ALE are used for external ROM.
– active low.
• PSEN（pin 29）：program store enable
– This is an output pin and is connected to the OE
pin of the ROM.

12. ALE
• ALE（pin 30）：address latch enable
It is an output pin and is active high.
8051 port 0 provides both address and data.
The ALE pin is used for de-multiplexing the
address and data by connecting to the G pin of
the 74LS373 latch.