Microcontoller - 8051 Architecture and Block Diagram

Microcontroller is a small computer on a single integrated circuit generally used in control applications. It contains all essential components such as CPU, RAM, ROM / EPROM, I/O lines etc. These devices used to control products like automobile engine control systems, medical devices, office machines, remote controls, power tools and appliances, toys and in other variety of products developed using embedded systems.

8051 Microcontroller Architecture and Block Diagram:

8051 microcontroller is a second generation 8 bit microcontroller used for a variety of applications with limited calculations and control strategies. Microcontrollers are used for industrial and commercial control applications, instrumentations, appliance control etc.

8051 microcontroller contains Boolean processor, serial port (full duplex), power saving circuit along with 8 bit CPU, RAM, ROM, Timer and Counter, I/O lines.

Pin 1-8 – Port 1: 8-bit bidirectional I/O port with internal pull-up resisters. It is just an I/O port and does not perform any other task.

Pin 9 – RST: Reset input pin used to reset the microcontroller to its initial position.

Pin 10-17 – Port 3: 8-bit bidirectional I/O port with internal pull-up resisters. It also performs special functions listed below:

Port 3 Pins	Function	Description
P3.0	RxD	Serial Input
P3.1	TxD	Serial Output
P3.2	INT0	External Interrupt 0
P3.3	INT1	External Interrupt 1
P3.4	T0	Timer 0
P3.5	T1	Timer 1
P3.6	WR	External Memory Write
P3.7	RD	External Memory Read

 Pin 18-19: XTAL1 and XTAL1 pins respectively. These pins are used for connecting an external crystal to get the system clock.

Pin 20 – GND: Ground pin. It provides the power supply to the circuit.

Pin 21-28 – Port 2: A bidirectional I/O port. Higher order address bus signals are multiplexed with this bidirectional port.

Pin 29 – PSEN: Program Enable Pin. Using this PSEN pin external program memory can be read.

Pin 30 – ALE/PROG: Address Latch Enable pin. Using this pin, external address can be separated from data.

Pin 31 – EA/VPP: Named as external Access Enable Pin (EA). It is used to enable or disable the external memory interfacing.

Pin 32-39 – Port 0: A bidirectional I/O pins but without any internal pull-ups. Hence, it requires external pins in order to use port 0 pins as I/O port. Lower order data and address bus signals are multiplexed with this port.

Pin 40 – VCC: This pin is used to supply power to the circuit.

8051 Interrupts:

Interrupt is a process of creating a temporary halt in the main program execution and then pass the control to the external sources in order to execute their related tasks and then passes the control to the main program where it is halted.

There are 5 Interrupt signals in 8051 microcontrollers, one from serial port, two from timers, two from external interrupts which are listed below:

• INT0
• TF0
• INT1
• TF1
• RI / TI

Note: The no. of interrupt sources differs from version to version. It varies from 5 to 15.

By setting and clearing a bit in the special function register Interrupt Enable – IE each of the interrupts can individually be enabled and disabled. The IE register also contains a global disable bit, which disables all the interrupts.

By setting and clearing bits in the special function register Interrupt Priority register – IP each interrupt can also be programmed to one of the priority level scheme.

Note that a low priority interrupt can be interrupted by a high priority interrupt, but it cannot be interrupted by another low priority interrupt. And also a high priority interrupt can't be interrupted by a low priority interrupt.