8051 Special Function Registers (SFRs)

8051 Special Function Registers (SFRs)
8051 Special Function Registers (SFRs)

8051 Special Function Registers (SFRs)  setup Ports and Timers. Different bit addresses are listed in the instructions. SFR Memory addresses are accessed by assembly languages. While SFRs are typically used to configure ports and timers, they are crucial to DAC and ADC conversions. In DAC and ADC conversions, configuration bits are SFR configuration bits. These configuration bits help the module and external devices communicate with the micro controller. Module operations are configured in specific SFR bits. SFR pins can be turned from LOW to HIGH by setting port-related bits to high. SFR bits are toggled to finish the procedure. Similar things apply to other modules.

SFRs are strange to 8051 microcontroller beginners. What’s up? Join us as we explain 8051 Special Function Registers. This beginner’s guide will explain SFRs, why they important, and how to utilize them to maximize your 8051’s capabilities in 100 words or fewer. The essential SFRs will be explained in simple language with examples for beginners. Each register—ports, timers, serial controllers—and how to configure them can help your application run smoothly. Connect your dev board and configure! This tutorial is for you.

An Introduction to 8051 Microcontrollers

Popular embedded system microcontrollers include the 8051. Despite its simplistic design and limited instruction set, it has a lot of helpful features. For microcontroller beginners, the 8051 is ideal.
An entire family of 1980s Intel microcontrollers was named 8051. An 8-bit data bus, built-in memory, input/output ports, timers, interrupts, and more characterize the 8051 design. You can quickly design complicated embedded systems without external components using these features.

The CPU core of the 8051 includes:

  • 128 bytes of random access memory (RAM) for data storage
  • 4K bytes of on-chip read-only memory (ROM) for program storage
  • Two 16-bit timers/counters
  • Four 8-bit input/output ports
  • Serial interface
  • Six interrupt sources with two priority levels

With a basic architecture and few instructions, the 8051 is straightforward to master. It processes 8 bits of data per instruction cycle on an 8-bit data bus. Although it slows the 8051 to 12 MHz, it makes it easier to use. The development board has the microcontroller chip, buttons, LEDs, and an LCD display. The compiler lets you write 8051 applications in C or assembly. You upload compiled code to the microcontroller with the programmer.

With the right tools, you can build simple projects to flash LEDs, read button presses, show LCD messages, and more. Although the 8051 is an outdated architecture, it’s still a good platform for learning embedded programming and microcontroller engineering.

What Are Special Function Registers (SFRs)?

SFRs are in the 8051 microcontroller. These registers control the microcontroller and peripherals. Serial communication, interrupts, timers, and I/O ports are controlled by SFRs. SFRs take up the first 128 bytes of 8051 RAM. This indicates SFR addresses are 0x80–0xFF. These registers can be accessed like RAM using direct, indirect, or indexed addressing.

Important SFRs in the 8051 include:

Program Status Word (PSW): Carry, Auxiliary Carry, Parity, etc. To indicate ALU operations.
ACC or Accumulator: Stores 8-bit data and performs arithmetic and logic.
Some instructions use B as an 8-bit scratchpad register.
SP: Points to the next empty stack position. Used for PUSH/POP.
Data Pointer: Accesses external memory. Used in MOVX code.
TMOD or Timer Mode Register: Configures timers as Timer, Counter, PWM, etc.
TCON controls timers, generates interrupts, starts/stops them, etc.
Serial Control Register (SCON): Configures serial port communication types such UART, SPI, etc.
The 8051’s PCON register controls power control modes like Idle and Power Down.
The 8051’s SFRs allow variable resource and peripheral control, making it a versatile microcontroller. Configuring the SFRs unlocks the 8051’s power!

Key 8051 SFRs and Their Functions

Special function registers (SFRs) regulate serial communication, interrupts, timers, counters, and more in the 8051 microcontroller. Here are some of the most important SFRs and their functions.

PSW or Program Status Word

SFR status bits include carry, auxiliary carry, parity, and overflow flags. The interrupt enable flag enables or disables interrupts.

SP or Stack Pointer

The stack pointer contains the top address. It stores the return address during interrupts, subroutine calls, and returns.

ACC or Accumulator

This is an 8051 main register. All arithmetic and logic use the accumulator. It also stores temporary data during operations.

B Register

This is the second primary register utilized alongside the accumulator for some operations. It also stores temporary data during operations.

DPTR or Data Pointer

The DPTR holds data addresses during read/write operations. It helps access external memory.

TMOD or Timer/Counter Mode Control

SFR regulates timers and counters. It controls timer mode and comprises a bit to enable or disable timer 1.

TH0/TL0 or Timer 0 High/Low

This pair of SFRs initializes timer 0. The starting value’s low and high order bytes are in TL0 and TH0, respectively.

TCON or Timer Control

This SFR controls timers 0 and 1. It has overflow and interrupt bits and timer start/stop bits like TR0 and TR1.

The 8051 has various useful SFRs for ports, interrupts, serial connection, etc. Knowing these SFRs will help you program the 8051 microcontroller.

How to Use SFRs in Your 8051 Projects

Special Function Registers (SFRs) control 8051 microcontroller functionalities. Understanding how to modify these registers lets you use the 8051’s amazing features.

SFRs take up the first 128 bytes of the 8051’s 128-byte RAM. These control I/O ports, serial communication, timers, and interrupts. Each SFR has a purpose and address. The P0 register at 80h controls I/O port 0, while the TMOD register at 89h regulates timers. For example, write to P0:

Move P0, #data
Send the port value #data.

Some other useful SFRs include:

  • P1 – Controls I/O port 1
  • TCON – Timer control register
  • TMOD – Timer mode register
  • SCON – Serial control register
  • IE – Interrupt enable register
  • IP – Interrupt priority register

The 8051 is configured by setting bit values in these SFRs. Set bits 0 and 1 in TMOD to enable timer 0 in mode 1 (16-bit timer):

Move TMOD, #00000010b
Binary numbers end in b.

To use the 8051 microcontroller, you must master SFR manipulation. To learn about each SFR, consult your 8051 variant’s datasheet. You’ll master timers, serial communication, interrupts, and I/O ports with practice!


The 8051 SFR beginner’s guide is complete! We’ve barely scratched the surface of using these registers to control 8051 microcontroller nuts and bolts. With a solid foundation, you can experiment on your own. Use a basic dev board to experiment with blinking LEDs. Try making music using a little speaker. SFRs are your gateway to magic. Sometimes embedded initiatives fail, but that’s part of the fun. So grab your soldering iron and start tinkering! This is yours

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