Introduction to Microcontrollers

• Microcontrollers have become integral to electronic gadgets
• They have simplified/advanced embedded system designs
• Microcontrollers are single chips which consist of CPU, RAM, EPROM/PROM/ROM, I/O (serial and parallel), timers, and an interrupt controller
• Intel 8051 is an 8-bit microcontroller.
• Intel 8096 is a 16-bit microcontroller.

Types of Microcontrollers

• 4-bit microcontrollers are small, have a minimum pin count, and are low cost
• 4-bit microcontrollers have applications like LED & LCD display drivers and portable battery chargers
• The power consumption of 4-bit microcontrollers is low
• Renasa M34501 is a 20 pin DIP chip with 4kB of ROM, 256 Bytes of RAM, 2 Counters and 14 I/O Pins and is a popular 4-bit controller
• ATAM862 series from ATMEL also a 4-bit microcontroller
• 8-bit microcontrollers are widely used and popular
• 55% of all CPUs sold in the world are 8-bit microcontrollers
• 8-bit microcontrollers have an 8-bit internal bus and the ALU performs arithmetic/logical operations on a byte instruction
• The 8051 was designed by Intel in 1980 for use in embedded systems and is a well-known 8-bit microcontroller
• Other 8-bit microcontrollers include Intel 8031/8052, Motorola MC68HC11, AVR Microcontrollers, and Microchip's PIC Microcontrollers like 12C5XX, 16C5X and 16C505
• 16-bit microcontrollers perform 16-bit arithmetic and logical operations at an instruction with an internal bus width of 16-bits
• 16-bit microcontrollers have increased memory size and speed of operation compared to 8-bit microcontrollers
• They are most suitable for programming in high-level languages like C or C++
• 32-bit microcontrollers are used in high-end applications like automotive control, communication networks, robotics, cell phones, GPRS & PDAs
• PIC32, ARM 7, ARM9, SHARP LH79520, ATMEL 32 (AVR), and Texas Instrument's TMS320F2802x/2803x are some popular 32-bit microcontrollers

8051 Microcontroller

• Intel introduced the 8051 as a popular 8-bit microcontroller in 1981
• It has become an academic standard
• The 8051 is based on an 8-bit CISC core

Features of 8051 Microcontroller

• 4 KB on chip program memory (ROM or EPROM)
• 128 bytes on chip data memory (RAM)
• 8-bit data bus
• 16-bit address bus
• 32 general purpose registers each of 8 bits
• One 16-bit program counter and One 16-bit DPTR (data pointer)
• One 8-bit stack pointer
• One Microsecond instruction cycle with 12 MHz Crystal
• One full duplex serial communication port

Architecture of the 8051 Microcontroller

• It has Harvard architecture with RISC (Reduced Instruction Set Computer) concept
• 8-bit ALU
• One 8-bit PSW(Program Status Register)
• A and B registers
• One 16-bit Program counter
• One 16-bit Data pointer register(DPTR)
• 128 bytes of RAM and 4kB of ROM
• Four parallel I/O ports each of 8-bit width

8 Bit ALU

• 8051 has 8-bit ALU, which can perform all the 8-bit arithmetic and logical operations in one machine cycle
• The ALU is associated with two registers A & B

A and B Registers

• The A and B registers are special function registers which hold the results of many arithmetic and logical operations of 8051
• The A register is also called the Accumulator
• The Accumulator is used as a general register to accumulate the results of a large number of instructions
• The Accumulator is used for all mathematical operations and data transfer operations between CPU and any external memory

Program Counter (PC)

• 8051 has a 16-bit program counter that always points to the address of the next instruction
• After execution of one instruction, the program counter is incremented to point to the address of the next instruction to be executed
• Contents of the PC are placed on the address bus to find and fetch the desired instruction
• 8051 can access program addresses from 0000H to FFFFH, a total of 6kB of code because the PC is 16-bit wide

Stack Pointer Register (SP)

• It is an 8-bit register which stores the address of the stack top
• The Stack Pointer is used to indicate where the next value to be removed from the stack should be taken from
• When a value is pushed onto the stack, the 8051 first increments the value of SP and then stores the value at the resulting memory location
• When a value is popped off the stack, the 8051 returns the value from the memory location indicated by SP, and then decrements the value of SP
• The SP is only 8-bit wide it is incremented or decremented by two
• SP is modified directly by the 8051 by six instructions

Data Pointer Register (DPTR)

• It is a 16-bit register which is the only user-accessible
• DPTR is used to point to data
• It is used by a number of commands which allow the 8051 to access external memory
• When the 8051 accesses external memory it will access external memory at the address indicated by DPTR
• The DPTR can also be used as two 8-registers DPH and DPL

Program Status Register (PSW)

• The 8051 has a 8-bit PSW register which is also known as Flag register
• Only 6-bits in the 8-bit register are used by 8051, the two unused bits are user definable bits
• In the 6-bits, four are conditional flags: Carry -CY, Auxiliary Carry-AC, Parity P, and Overflow-OV
• These flag bits indicate some conditions that resulted after an instruction was executed

Memory Organization

• The 8051 microcontroller has 128 bytes of Internal RAM and 4kB of on chip ROM
• The RAM is also known as Data memory and the ROM is known as program memory
• The program memory is also known as Code memory
• The Code memory holds the actual 8051 program that is to be executed
• In 8051 this memory is limited to 64K
• Code memory may be found on-chip, as ROM or EPROM and can be stored completely off-chip in an external ROM, or more commonly, an external EPROM

Internal RAM

• Internal RAM is found on-chip on the 8051, which makes it the fastest RAM available
• Internal RAM is the most flexible in terms of reading, writing, and modifying its contents
• Internal RAM is volatile, so when the 8051 is reset this memory is cleared
• The 128 bytes of internal RAM is organized as four register banks, 16 bytes of bit addressable area and 80 bytes of general-purpose area
• Four register banks (Bank0, Bank1, Bank2 and Bank3) each of 8 bits (total 32 bytes)
• The default bank register is Bank0, remaining banks are selected with the help of RSO and RS1 bits of PSW Register.
• 80 bytes of general purpose area (Scratch pad memory) is utilized by the microcontroller as a storage area for the operating stack

Internal ROM

• The 8051 microcontroller has 4kB of on chip ROM but it can be extended up to 64kB
• This ROM is also called program memory or code memory
• The CODE segment is accessed using the program counter (PC) for opcode fetches and by DPTR for data