8051 Microcontroller

In this article we will look at 8051 microcontroller, its pin diagram and architecture along with different components.

 

Watch this video for introduction to microcontrollers:

 

Introduction to 8051 Microcontroller

The Intel 8051 microcontroller is one of the most popular general purpose microcontrollers in use today. The success of the Intel 8051 spawned a number of clones, which are collectively referred to as the MCS-51 family of microcontrollers, which includes chips from vendors such as Atmel, Philips, Infineon, and Texas Instruments.

 

Watch this video to learn about 8051 microcontroller and instruction set:

 

The Intel 8051 is an 8-bit microcontroller which means that most available operations are limited to 8 bits. Some of the features that have made the 8051 popular are:

• 4 KB on chip program memory
• 128 bytes on chip data memory (RAM)
• 4 register banks
• 128 user defined software flags
• 8-bit data bus
• 16-bit address bus
• 16-bit timers (usually 2, but may have more, or less)
• 3 internal and 2 external interrupts
• Bit as well as byte addressable RAM area of 16 bytes
• 16-bit program counter and data pointer

 

8051 Schematic Diagram

8051 Schematic Diagram is as shown in the below figure.

 

We can see 3 system inputs, 3 control signals and 4 ports (for external interfacing). A Vcc power supply and ground is also shown. System inputs are necessary to make the micro controller functional. So the first and most important of this is power, marked as Vcc with a GND  (ground potential). XTAL 1 and XTAL 2 are for the system clock inputs from crystal clock circuit.

 

RESET input is required to initialize microcontroller to default/desired values and to make a new start. There are 3 control signals EA, PSEN and ALE. These signals known as External Access (EA), Program Store Enable (PSEN), and Address Latch Enable (ALE) are used for external memory interfacing. There are four ports numbered 0,1,2,3 and called as Port 0, Port 1, Port 2 and Port 3 which are used for external interfacing of devices like DAC, ADC, 7 segment display, LED etc. Each port has 8 I/O lines and they all are bit programmable.

 

8051 Pin Diagram

8051 Pin Diagram is as shown in the figure below.

 

Pins 1-8

• Known as Port 1. Unlike other ports, this port does not serve any other functions
• Port 1 is an internally pulled up, quasi bi directional I/O port

 

Pin 9

• As explained before RESET pin is used to set the 8051 microcontroller to its initial values, while the microcontroller is working or at the initial start of application
• The RESET pin must be set high for 2 machine cycles

 

Pins 10-17

• Known as Port 3
• This port also serves some other functions like interrupts, timer input, control signals for external memory interfacing RD and WR , serial communication signals RxD and TxD etc
• This is a quasi bi directional port with internal pull up

 

Pins 18 and 19

• Used for interfacing an external crystal to provide system clock

 

Pin 20

• Named as Vss – it represents ground (0 V) connection

 

Pins 21-28

• Known as Port 2 (P 2.0 to P 2.7) – in addition to serving as I/O port, higher order address bus signals are multiplexed with this quasi bi directional port

 

Pin 29

• PSEN or Program Store Enable is used to read signal from external program memory

 

Pin 30

• ALE aka Address Latch Enable is used to demultiplex the address-data signal of port 0 (for external memory interfacing)
• 2 ALE pulses are available for each machine cycle

 

Pin 31

• EA/ External Access input is used to enable or disallow external memory interfacing
• If there is no external memory requirement, this pin is pulled high by connecting it to Vcc

 

Pins 32-39

• Known as Port 0 (P0.0 to P0.7) – In addition to serving as I/O port, lower order address and data bus signals are multiplexed with this port (to serve the purpose of external memory interfacing)
• This is a bi directional I/O port (the only one in 8051) and external pull up resistors are required to function this port as I/O

 

Pin 40

• Named as Vcc is the main power source
• Usually its +5V DC

 

8051 Architecture

The simplified internal architecture of 8051 is as illustrated below.

 

The system bus connects all the support devices with the central processing unit. 8051 system bus composes of an 8 bit data bus and a 16 bit address bus and bus control signals. RxD and TxD (serial port input and output) are interfaced with port 3.

 

8051 Instruction Set

The complete 8051 Instruction Set or all 8051 instructions are broadly classified in to four groups:

• Data moving/handling instructions
• Logical instructions
• Arithmetic instructions
• Branching instructions

 

Some examples of data moving/handling instructions are as shown in the table below.

Mnemonics	Description	Addressing Mode	No.of Bytes Occupied	No.of Cycles Used
Mov a, #num	Copy the immediate data num into acc	Immediate	2	1
Mov Rx, a	Copy the data from acc to Rx	Register	1	1
Mov a, add	Copy the data from direct address add to acc	Direct	2	1
Mov a, @Rp	Copy the data that is at address in Rp to acc	Indirect	2	2

 

 

Some examples of logical instructions are as shown in the table below.

Mnemonics	Description	Addressing Mode	No.of Bytes Occupied	No.of Cycles Used
Clr a	Clear each bit of acc	Direct	1	1
Cpl a	Complement each bit of acc	Direct	1	1
Rl a	Rotate acc one bit left	–	1	1
Rr a	Rotate acc one bit right	–	1	1

 

 

Some examples of arithmetic instructions are as shown in the table below.

Mnemonics	Description	Addressing Mode	No.of Bytes Occupied	No.of Cycles Used
Inc a	Add 1 to acc	Register	1	1
Inc add	Add 1 to the content of add	Direct	2	1
Addc a, #num	Add the immediate num with acc and carry, stores result in acc	Immediate	2	1
Dec a	Subtract 1 from acc	Register	1	1

 

 

Some examples of branching instructions are as shown in the table below.

Mnemonics	Description	No.of Bytes Occupied	No.of Cycles Used
Jc label	Jump to label if carry is set to 1	2	2
Jnc label	Jump to label if carry is cleared to 0	2	2
Jz label	Jump to label if content of acc is 0	2	2
Nop	Short form of no operation means do nothing and go to next instruction	1	1