The document discusses the 8051 microcontroller. It begins by describing the key features of the 8051 microcontroller, including that it has an 8-bit CPU, 4KB of internal program memory, 128 bytes of internal data memory, 32 I/O lines that can be used as ports, and two 16-bit timer counters. It then provides more details on the internal architecture, describing the ALU, registers, memory organization, and other components. It concludes by explaining features like the register banks and stack memory.

1. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
MICROCONTROLLER
Microprocessor Vs. Microcontroller:
Microprocessor
 CPU is stand-alone, RAM, ROM, I/O, timer are separate
 Designer can decide on the amount of ROM, RAM and I/O ports.
 Bulkier
 Expensive
 General-purpose
Microcontroller
 CPU, RAM, ROM, I/O and timer are all on a single chip
 Fix amount of on-chip ROM, RAM, I/O ports
 Bit addressable
 For applications in which cost, power and space are critical
 Single-purpose

2. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
Embedded Systems Examples
Salient features of 8051 microcontroller
8051 Microcontroller is designed by Intel in 1981.
A Microcontroller is a programmable digital processor with necessary peripherals.
 Eight bit CPU
 On chip clock oscillator
 4Kbytes of internal program memory (code memory) [ROM]
 128 bytes of internal data memory [RAM]
 64 Kbytes of external program memory address space.
 64 Kbytes of external data memory address space.
 32 bi directional I/O lines (can be used as four 8 bit ports or 32 individually
addressable I/O lines)
 Two 16 Bit Timer/Counter :T0, T1
 Full Duplex serial data receiver/transmitter
 Four Register banks with 8 registers in each bank.
 Sixteen bit Program counter (PC) and a data pointer (DPTR)
 8 Bit Program Status Word (PSW)
 8 Bit Stack Pointer
 Five vector interrupt structure (RESET not considered as an interrupt.)

3. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
 8051 CPU consists of 8 bit ALU with associated registers like accumulator ‘A’ , B
register,
 ALU can perform arithmetic and logic functions on 8 bit variables.
 8051 has 128 bytes of internal RAM which is divided into
1. Working registers [00 – 1F]
2. Bit addressable memory area [20 – 2F]
3. General purpose memory area (Scratch pad memory) [30-7F]
Pin Configuration
 Pins 1 to 8 − These pins are known as Port 1. Each of these pins can be configured as
an input or an output means bi-directional I/O port.
 Pin 9 − It is a RESET pin, which is used to reset the microcontroller to its initial
values.

4. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
 Pins 10 to 17 − These pins are known as Port 3. each of these pins can serve as
general input or output pins. Besides, all of them have alternative functions
o Pin 10 RXD. Serial asynchronous communication input or Serial
synchronous communication output.
o Pin 11 TXD. Serial asynchronous communication output or Serial
synchronous communication clock output.
o Pin 12 INT0.External Interrupt 0 input
o Pin 13 INT1. External Interrupt 1 input
o Pin 14 T0. Counter 0 clock input
o Pin 15 T1. Counter 1 clock input
o Pin 16 WR. Write to external (additional) RAM
o Pin 17 RD. Read from external RAM
 Pins 18 & 19 − These pins are used for interfacing an external crystal to get the
system clock.
 Pin 20 − This pin provides the power supply to the circuit.
 Pins 21 to 28 − These pins are known as Port 2. It serves as I/O port. Higher order
address bus signals are also multiplexed using this port.
 Pin 29 − This is PSEN pin which stands for Program Store Enable. It is used to read
a signal from the external program memory.
 Pin 30 − This is EA pin which stands for External Access input. It is used to
enable/disable the external memory interfacing.
 Pin 31 − This is ALE pin which stands for Address Latch Enable. It is used to
demultiplex the address-data signal of port.
 Pins 32 to 39 − These pins are known as Port 0. It serves as I/O port. Lower order
address and data bus signals are multiplexed using this port.
 Pin 40 − This pin is used to provide power supply to the circuit.

5. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
Block Diagram of 8051
ALU: It is 8 bit unit. It performs arithmetic operation as addition, subtraction, multiplication, division,
increment and decrement. It performs logical operations like AND, OR and EX-OR.
Accumulator(A-reg): It is 8 bit register. It is used to store 8 bit data and to hold one of operand of
ALU units during arithmetical and logical operations. Result of arithmetic & logic operations
performed by ALU is accumulated by this register. Therefore it is called accumulator register.
B-register: It is bit and byte accessible. It is used in conjunction with A register as I/P operand for
ALU. It is used as general purpose register to store 8 bit data.
A and B registers together is also called MATH registers
Stack Pointer (SP) –It is 8-bit register. It is used to hold the internal RAM memory location addresses
which are used as stack memory.
Data pointer register (DTPR): It is a 16 bit register used to hold address of external or internal RAM
where data is stored or result is to be stored.
It is used to store 16 bit data. It is divided into2- 8bit registers.
DPH-data pointer higher order (83H) and DPL-data pointer lower order (82H).
Each register can be used as general purpose register to store 8 bit data and can also be used as
memory location.
Program Counter (PC) – 16 bit PC contains the address of next instruction to be executed. On reset
PC will set to 0000. After fetching every instruction PC will increment by one.

6. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
Flag Bits and PSW Register
The 8051 has four math flags and three general-purpose user flags ,they are grouped inside the
program status word (PSW) and the power control (PCON) registers.
The math flags include carry (C), auxiliary carry (AC), overflow (OV), and parity (P).
User flags are named FO, GFO, and GF1; they are general-purpose flags that may be used by the
programmer to record some event in the program. that can be set to I or cleared to 0 by the
programmer as desired
Program Status Word : This is an 8 bit register which contains the arithmetic status of ALU and the
bank select bits of register banks.
Power control register (PCON): it is 8-bit register. Its bits are used to control mode of power saving
circuit, either idle or power down mode and also one bit is used to modify baud rate of serial
communication

7. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
Special function Registers(SFR): The 8051 microcontroller has 11 SFR divided in 4 groups:
Timer/Counter register: 8051 microcontroller has two-16 bit Timer/counter registers
8051 microcontroller has two 8-bit timer control register i.e. TMOD and TCON register.
 TMOD Register: it is 8-bit register. It used to select mode and control operation of time by
writing control word.
 TCON register: It is 8-bit register. It is used to control operation of timer/ counter and
external interrupt .
Serial data register: 8051 micro controller has 2 serial data registers SBUF and SCON.
 Serial buffer register (SBUF): it is 8-bit register. It is used to hold data which is to be
transferred serially.
 Serial control register (SCON): it is 8-bit register. The 8-bit loaded into this register controls
the operation of serial communication.
Interrupt register: 8051 µC has two 8-bit interrupt register.
 Interrupt enable register (IE): it is 8-bit register. it is used to enable and disable function of
interrupt.
 Interrupt priority register (IP): It is 8-bit register. it is used to select low or high level priority
of each individual interrupts.
Memory Organization 0f 8051
The memory organization of 80851 is classified into two types
1. Data Memory or RAM 2. Program Memory or ROM
1.Data Memory or RAM
(a) Internal RAM/Data Memory :
• There are 128 bytes of RAM in the 8051.
– Assigned addresses 00 to 7FH
• The 128 bytes are divided into 3 different groups as follows:
– A total of 32 bytes from locations 00 to 1F hex are set aside for register banks and
the stack.
– A total of 16 bytes from locations 20H to 2FH are set aside for bit-addressable
read/write memory.
– A total of 80 bytes from locations 30H to 7FH are used for read and write storage,
called scratch pad.

8. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
(b) External RAM /Data Memory: Access to external memory is slower than access to
internal data memory. There may be up to 64K Bytes of external data memory.

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Program Memory or Code Memory
EA'(bar) and PSEN' determines whether Internal ROM is accessed or External
ROM is accessed.
Internal ROM
8051 has 4 KB internal Read Only Memory.
 It mainly contains some executable functions and some data which may
never change.
 Address range is 0000H to 0FFFH.
 EA'=1 In this case, the microcontroller executes first the program from
built-in ROM, then the program stored in external memory.
External ROM
 EA'=0 In this case, the microcontroller completely ignores internal
program memory and executes only the program stored in external
memory.
 Address range is 0000H to FFFFH.
Register Banks in 8051
A total of 32 bytes of RAM are set aside for the register banks and the stack. These 32 bytes are
divided into four register banks in which each bank has 8 registers, R0–R7..

10. Microcontrollers T.Srikrishna, M.Sc , M.Tech, GVP
Default Register Bank
If RAM locations 00–1F are set aside for the four registers banks, which register bank of R0–R7 do we
have access to when the 8051 is powered up?
The answer is register bank 0
How to Switch Register Banks
D4 and D3 bits of the PSW are used to select the desired register bank, since they can be accessed by
the bit addressable instructions SETB and CLR.
For example, "SETB PSW.3" will set PSW.3 = 1 and select the bank register 1.
Stack in the 8051
The stack is a section of a RAM used by the CPU to store information such as data or memory
address on temporary basis.
How Stacks are Accessed
As the stack is a section of a RAM, the register used to access the stack is known as the stack pointer
register.
The stack pointer in the 8051 is 8-bits wide.
When the 8051 is initialized, the SP register contains the value 07H. This means that the RAM
location 08 is the first location used for the stack.
The storing operation of a CPU register in the stack is known as a PUSH, and getting the contents
from the stack back into a CPU register is called a POP.

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