Unit - II : 8051 microcontroller

Features of 8051 Microcontroller:

• 8-bit CPU: Processes 8 bits of data at a time, making it suitable for simple control tasks.

• Memory:

  • ROM (Program Memory): 4 KB on-chip ROM for storing programs.

  • RAM (Data Memory): 128 bytes on-chip RAM for data storage.

• I/O Ports: Four parallel 8-bit input/output ports (P0, P1, P2, P3), each can be used as input or output and are bit-addressable.

• Timers/Counters: Two 16-bit timers/counters for timing operations and event counting.

• Serial Communication: Built-in full duplex UART for serial data transfer, allowing communication with other devices.

• Interrupts: Five interrupt sources (two external, two from timers, one from serial port) to handle urgent tasks quickly.

• Bus System:

  • 8-bit Data Bus: For transferring data within the microcontroller.

  • 16-bit Address Bus: For accessing memory locations.

• Register Banks: Four register banks, each with eight 8-bit registers, for efficient data handling.

• Special Function Registers (SFRs): Control various operations and peripherals of the microcontroller.

• Oscillator and Clock Circuit: Built-in oscillator to provide the necessary clock signal for operation.

• External Memory Support: Can connect to up to 64 KB of external ROM and 64 KB of external RAM if needed.

• Power-Saving Modes: Includes idle and power-down modes to save energy during inactivity.

• Bit and Byte Addressable RAM: Allows flexible data manipulation for control applications.

• Other Features: Some versions include additional features like ADCs, DACs, and op-amps for advanced interfacing.

Block Diagram & Architecture of 8051:

The 8051 microcontroller is a single-chip microcomputer integrating a CPU, memory, I/O ports, timers, and serial communication on one chip. Its block diagram includes the following main components:

• CPU (Central Processing Unit): Controls all operations, including the ALU (Arithmetic Logic Unit), Program Counter, Stack Pointer, Accumulator, and general-purpose registers.

• RAM (128 bytes): Divided into register banks, bit-addressable memory, and scratchpad area.

• ROM (4 KB): Stores program code.

• I/O Ports: Four 8-bit parallel ports (P0, P1, P2, P3) for interfacing with external devices.

• Timers/Counters: Two 16-bit timers/counters for timing operations.

• Serial Port: For serial communication.

• Interrupt Control: Manages six interrupt sources.

• Oscillator & Clock Circuit: Provides clock pulses for operation.

• Bus Control: Manages data and address buses for internal and external communication.

Internal Memory Organization:

The 8051 has a unique memory structure:

1. Program Memory (ROM)

• Internal ROM: 4 KB for program storage.

• External ROM: Up to 64 KB can be interfaced if needed.

2. Data Memory (RAM)

• Internal RAM: 128 bytes, mapped from 00H to 7FH, split into:

  • Register Banks (00H–1FH): Four banks, each with eight registers (R0–R7).

  • Bit-Addressable Area (20H–2FH): 16 bytes, each bit can be addressed individually (128 bits).

  • Scratchpad RAM (30H–7FH): 80 bytes for general-purpose storage.

• External RAM: Up to 64 KB can be interfaced for data storage.

3. Special Function Registers (SFRs)

• Located from 80H to FFH (upper 128 bytes of address space).

• Control and monitor various functions (timers, serial port, I/O, etc.).

Special Function Registers (SFRs)

• There are 21 SFRs in the 8051, each 1 byte (8 bits) wide.

• Examples include:

  • Accumulator (A), B Register

  • Program Status Word (PSW)

  • Port Latches (P0, P1, P2, P3)

  • Timer Registers (TCON, TMOD, TH0, TL0, TH1, TL1)

  • Serial Control (SCON), Serial Buffer (SBUF)

  • Interrupt Enable (IE), Interrupt Priority (IP)

• Some SFRs are bit-addressable, allowing manipulation of individual bits.

PSW (Program Status Word) Register:

• 8-bit register reflecting the CPU status and control.

• Bit functions:

  • CY (Carry flag): Arithmetic carry/borrow.

  • AC (Auxiliary Carry): For BCD operations.

  • F0: User-defined flag.

  • RS1, RS0: Register bank select bits.

  • OV (Overflow): Arithmetic overflow.

  • P (Parity): Even/odd parity of the accumulator.

Main Functions of the PSW Register:

• Indicates the results of arithmetic and logical operations.

• Controls which register bank (out of four) is currently being used.

• Helps manage program flow based on the outcome of operations.

Description of Each Bit:

• Carry Flag (CY): Set when there is a carry out of the most significant bit during addition, or a borrow during subtraction.

• Auxiliary Carry Flag (AC): Set when there is a carry from bit 3 to bit 4 in an operation, mainly used in BCD arithmetic.

• Flag 0 (F0): A general-purpose flag that can be used by the programmer for temporary storage.

• Register Bank Select Bits (RS1 and RS0): These two bits select which of the four register banks (Bank 0, 1, 2, or 3) is active for use.

• Overflow Flag (OV): Set when the result of an arithmetic operation is too large to fit in 8 bits.

• Parity Flag (P): Automatically set or cleared to indicate if the number of 1’s in the accumulator is even (parity).

PSW is Uses :

• The PSW is automatically updated after arithmetic and logical instructions.

• Programmers can also set or clear some bits as needed for program control.

• The register bank select bits allow switching between different sets of general-purpose registers, which is useful in interrupt routines or multitasking.

Importance of PSW :

• It helps the microcontroller make decisions, such as whether to branch to a different part of the program based on the result of an operation.

• It enables efficient use of resources by allowing quick switching between register banks.

Pin functions of the 8051 microcontroller :

Port Pins

• Pins 1 to 8: These are Port 1 pins (P1.0 to P1.7). They are used for general-purpose input and output operations.

• Pins 10 to 17: These are Port 3 pins (P3.0 to P3.7). They work as input/output pins and also have special alternate functions like serial communication, external interrupts, timer inputs, and external memory control.

• Pins 21 to 28: These are Port 2 pins (P2.0 to P2.7). They serve as general-purpose input/output pins and also provide the high-order address byte when using external memory.

• Pins 32 to 39: These are Port 0 pins (P0.0 to P0.7). They can be used as input/output pins and also act as multiplexed address and data bus lines when connecting to external memory. External pull-up resistors are needed when used as I/O.

Control and Special Pins

• Pin 9: This is the RESET pin. Applying a high signal resets the microcontroller.

• Pins 18 and 19: These are XTAL1 and XTAL2 pins, which connect to an external crystal oscillator to provide the clock signal.

• Pin 20: This is the GND pin and should be connected to ground.

• Pin 29: This is the PSEN (Program Store Enable) pin. It is used to enable external program memory.

• Pin 30: This is the EA (External Access) pin. It selects between internal and external program memory.

• Pin 31: This is the ALE (Address Latch Enable) pin. It is used to latch the lower byte of the address during access to external memory.

• Pin 40: This is the VCC pin and should be connected to the +5V power supply.

Summary

• Ports 0, 1, 2, and 3 are used for general input/output and special functions.

• The RESET, XTAL1, XTAL2, PSEN, EA, and ALE pins handle control, clock, and memory interface tasks.

• VCC and GND are for power supply connections.

These pin functions allow the 8051 microcontroller to interact with external devices, memory, and perform its required operations efficiently.

Structure & Operation of I/O Ports:

• Four 8-bit ports (P0, P1, P2, P3):

  • Port 0: Dual-purpose (I/O or multiplexed address/data bus for external memory). Needs external pull-up resistors.

  • Port 1: General-purpose I/O, internal pull-ups.

  • Port 2: General-purpose I/O or high-order address bus for external memory.

  • Port 3: General-purpose I/O and alternate functions (interrupts, serial, timers, etc.).

• Operation: Each port can be programmed as input or output. Ports are bit- and byte-addressable, enabling flexible interfacing.

External Memory Interface:

• Program Memory: Up to 64 KB external ROM can be connected. If EA (pin 30) is low, the microcontroller fetches code from external memory.

• Data Memory: Up to 64 KB external RAM can be interfaced.

• Control Signals:

  • PSEN (Program Store Enable): Activates external program memory.

  • ALE (Address Latch Enable): Latches the lower byte of the address from Port 0.

  • Port 0 and Port 2: Used to send address/data to external memory.

In summary:

The 8051 microcontroller features a well-organized architecture with integrated CPU, memory, I/O, and peripherals. Its memory structure separates program and data spaces, uses SFRs for hardware control, and supports both internal and external memory. The four I/O ports are versatile, and pin functions are designed for efficient interfacing in embedded applications.