Pin diagram of 8051 Microcontroller

Introduction :

The 8051 microcontroller is a popular 8-bit microcontroller widely used in embedded systems. It is a single-chip microcontroller with a Harvard architecture that includes a CPU, RAM, ROM, and several peripherals. The 8051 microcontroller has a 40-pin dual in-line package (DIP) that provides various inputs and outputs for communication with external devices. 

8051 microcontroller is a 40 pin Dual Inline Package (DIP). These 40 pins serve different functions like read, write, I/O operations, interrupts etc. 8051 has four I/O ports wherein each port has 8 pins which can be configured as input or output depending upon the logic state of the pins. Therefore, 32 out of these 40 pins are dedicated to I/O ports. The rest of the pins are dedicated to VCC, GND, XTAL1, XTAL2, RST, ALE, EA’ and PSEN’. Pin diagram of 8051 microprocessor is as given below : Description of the Pins :

• Pin 1 to Pin 8 (Port 1) – Pin 1 to Pin 8 are assigned to Port 1 for simple I/O operations. They can be configured as input or output pins depending on the logic control i.e. if logic zero (0) is applied to the I/O port it will act as an output pin and if logic one (1) is applied the pin will act as an input pin. These pins are also referred to as P1.0 to P1.7 (where P1 indicates that it is a pin in port 1 and the number after ‘.’ tells the pin number i.e. 0 indicates first pin of the port. So, P1.0 means first pin of port 1, P1.1 means second pin of the port 1 and so on). These pins are bidirectional pins.
• Pin 9 (RST) – Reset pin. It is an active-high, input pin. Therefore if the RST pin is high for a minimum of 2 machine cycles, the microcontroller will reset i.e. it will close and terminate all activities. It is often referred as “power-on-reset” pin because it is used to reset the microcontroller to it’s initial values when power is on (high).
• Pin 10 to Pin 17 (Port 3) – Pin 10 to pin 17 are port 3 pins which are also referred to as P3.0 to P3.7. These pins are similar to port 1 and can be used as universal input or output pins. These pins are bidirectional pins. These pins also have some additional functions which are as follows:
  • P3.0 (RXD) : 10th pin is RXD (serial data receive pin) which is for serial input. Through this input signal microcontroller receives data for serial communication.
  • P3.1 (TXD) : 11th pin is TXD (serial data transmit pin) which is serial output pin. Through this output signal microcontroller transmits data for serial communication.
  • P3.2 and P3.3 (INT0′, INT1′ ) : 12th and 13th pins are for External Hardware Interrupt 0 and Interrupt 1 respectively. When this interrupt is activated(i.e. when it is low), 8051 gets interrupted in whatever it is doing and jumps to the vector value of the interrupt (0003H for INT0 and 0013H for INT1) and starts performing Interrupt Service Routine (ISR) from that vector location.
  • P3.4 and P3.5 (T0 and T1) : 14th and 15th pin are for Timer 0 and Timer 1 external input. They can be connected with 16 bit timer/counter.
  • P3.6 (WR’) : 16th pin is for external memory write i.e. writing data to the external memory.
  • P3.7 (RD’) : 17th pin is for external memory read i.e. reading data from external memory.
• Pin 18 and Pin 19 (XTAL2 And XTAL1) – These pins are connected to an external oscillator which is generally a quartz crystal oscillator. They are used to provide an external clock frequency of 4MHz to 30MHz.
• Pin 20 (GND) – This pin is connected to the ground. It has to be provided with 0V power supply. Hence it is connected to the negative terminal of the power supply.
• Pin 21 to Pin 28 (Port 2) – Pin 21 to pin 28 are port 2 pins also referred to as P2.0 to P2.7. When additional external memory is interfaced with the 8051 microcontroller, pins of port 2 act as higher-order address bytes. These pins are bidirectional.
• Pin 29 (PSEN) – PSEN stands for Program Store Enable. It is output, active-low pin. This is used to read external memory. In 8031 based system where external ROM holds the program code, this pin is connected to the OE pin of the ROM.
• Pin 30 (ALE/ PROG) – ALE stands for Address Latch Enable. It is input, active-high pin. This pin is used to distinguish between memory chips when multiple memory chips are used. It is also used to de-multiplex the multiplexed address and data signals available at port 0. During flash programming i.e. Programming of EPROM, this pin acts as program pulse input (PROG).
• Pin 31 (EA/ VPP) – EA stands for External Access input. It is used to enable/disable external memory interfacing. In 8051, EA is connected to Vcc as it comes with on-chip ROM to store programs. For other family members such as 8031 and 8032 in which there is no on-chip ROM, the EA pin is connected to the GND.
• Pin 32 to Pin 39 (Port 0) – Pin 32 to pin 39 are port 0 pins also referred to as P0.0 to P0.7. They are bidirectional input/output pins. They don’t have any internal pull-ups. Hence, 10 K? pull-up registers are used as external pull-ups. Port 0 is also designated as AD0-AD7 because 8051 multiplexes address and data through port 0 to save pins.
• Pin 40 (VCC) – This pin provides power supply voltage i.e. +5 Volts to the circuit.

The pin diagram of the 8051 microcontroller is as follows:

1. VCC: This pin is connected to the power supply and provides a voltage of +5V to the microcontroller.
2. GND: This pin is connected to the ground and serves as the reference voltage for the microcontroller.
3. XTAL1 and XTAL2: These pins are used for connecting an external crystal oscillator that provides the clock signal for the microcontroller.
4. Reset: This pin is used for resetting the microcontroller. A high pulse on this pin resets the microcontroller to its initial state.
5. Port 1: This is an 8-bit bidirectional input/output port that can be used for interfacing with external devices.
6. Port 2: This is an 8-bit bidirectional input/output port that can be used for interfacing with external devices.
7. Port 3: This is an 8-bit bidirectional input/output port that can be used for interfacing with external devices.
8. Port 4: This is an 8-bit bidirectional input/output port that can be used for interfacing with external devices.
9. INT0: This is an external interrupt 0 input pin.
10. INT1: This is an external interrupt 1 input pin.
11. T0: This is an external timer 0 input pin.
12. T1: This is an external timer 1 input pin.
13. WR: This is the write signal for external memory.
14. RD: This is the read signal for external memory.
15. ALE: This is the address latch enable signal that is used to latch the address for external memory.
16. PSEN: This is the program store enable signal that is used for accessing the program memory.
17. 17-24. Address bus: These pins are used for transmitting the address information to the external memory.
18. 25-32. Data bus: These pins are used for transmitting the data between the microcontroller and external memory.
19. RST: This is the output signal that indicates the microcontroller is being reset.
20. EA: This is the external access enable signal that is used for selecting the program memory.
21. VPP: This pin is used for programming the microcontroller.
22. 36-39. XTAL: These pins are used for connecting an external crystal oscillator.
23. P1.0 (AD0) – P1.7 (AD7): These pins are used for interfacing with external analog devices.

Uses of pin diagram of the 8051 microcontroller :

The pin diagram of the 8051 microcontroller is used for various purposes in embedded systems. Some of the main uses of the pin diagram are:

1. Interfacing with external devices: The 8051 microcontroller has several input/output pins that can be used for interfacing with external devices such as sensors, actuators, displays, and communication modules. The pin diagram provides the information about the location of these pins, their functionalities, and their electrical characteristics.
2. Programming the microcontroller: The 8051 microcontroller can be programmed using various programming languages such as Assembly, C, and BASIC. The pin diagram provides the information about the pins that are used for programming the microcontroller, such as the PSEN pin and the ALE pin.
3. Debugging and testing: The pin diagram provides access to the internal signals of the microcontroller, such as the address and data buses, which can be used for debugging and testing the microcontroller. Special hardware tools such as logic analyzers and oscilloscopes can be connected to the pins to monitor the signals and diagnose any issues in the system.
4. Expansion and customization: The pin diagram provides the flexibility to expand and customize the functionality of the microcontroller by connecting external devices and peripherals. For example, additional memory can be added by connecting external RAM or ROM chips to the address and data buses.

Issues in pin diagram of the 8051 microcontroller :

1. Power supply voltage: The 8051 microcontroller requires a stable power supply voltage of 5V. If the voltage is not stable or drops below the required level, it can cause the microcontroller to malfunction.
2. Input/output current: Each pin of the 8051 microcontroller has a maximum current rating, and exceeding this rating can cause damage to the microcontroller or other components connected to it. It is important to ensure that the input/output current is within the specified limits.
3. Input/output voltage: The 8051 microcontroller has specific voltage requirements for its input/output pins. If the voltage is too high or too low, it can cause the microcontroller to malfunction or even damage it.
4. Interference: External interference such as electromagnetic interference (EMI) and radio frequency interference (RFI) can affect the performance of the 8051 microcontroller, particularly if the pins are not properly shielded.
5. Pin conflicts: When using multiple peripherals or devices with the 8051 microcontroller, there can be conflicts between the pins used by different devices. Careful planning and circuit design is required to avoid pin conflicts and ensure proper functionality of all devices.