Micro 8051

 The 8051 is the original member of the MCS®-51 family,
and is the core for all MCS-51 devices. The features ofthe
 8051 core are:
 8-bit CPU optimized for control applications
 Extensive Boolean processing (single-bit logic) capabilities
 64K Program Memory address space
 64K Data Memory address space
 4K bytes of on-chip Program Memory
 128 bytes of on-chip Data RAM
 32 bidirectional and individually addressable I/O lines
 Two 16-bit timer/counters
 Full duplex UART
 6-source/5-vector interrupt structure with two priority
levels
 On-chip clock oscillator

 Block diagram of 8051 consist of
 CPU
 Two memory sections (Data and Program)
 Input/output ports
 Special function registers
 Control logic
These all elements communicates using 8 bit data bus, know as
internal data bus

 8051 consist of 40 pin
 Many pins of 8051 are used for more than one function
 The alternate functions are shown with green colour.

 It consists of 8 bit arithmetic and logic unit
 Associated with registers like A,B,PSW,SP.
 It consist of 16 bit program counter and data pointer
 It also contains set of special function registers
 ALU performs arithmetic as well as logic function on 8 bit
variables
 Arithmetic operations such as addition, subtraction,
multiplication and division
 Logic operations such as AND,OR,XOR, rotate etc.

 8051 consist of 128 byte
internal RAM
 It is organized into three
areas
Working Registers
Bit Addressable
General Purpose
 First 32 bytes from 00H to
1FH are working registers.
 They are divided into 4 banks
of 8 registers from R0 to R7
 At a time only 1 bank is in
use, bits RS0 and RS1 in PSW
determine the bank in
current use.
 When register bank is not
selected then can be used as
general purpose RAM.

 8051 provides 16 bytes of bit addressable area
 It occupies RAM byte addresses from 20H to 2FH forming 128
addressable bits.
 The RAM area from 30H to 7FH are called as general purpose
RAM.

 8051 has 4k byte internal
ROM from 0000H to 0FFFH
 It is inbuilt and programmed
by manufacturer
 It cannot be erased or
altered after fabrication
 If the program exceed above
0FFFH then 8051
automatically fetch code
bytes from external program
memory.

 8051 has 32 I/O pins as 4
eight bit parallel Port
(P0,P1,P2 and P3)
 All ports are bidirectional
 They consist of latch, output
driver and input buffer.
 Port 0 and 2 of output driver
and port 0 of input buffers
are used in access to external
memory
 Pins of port3 are
multifunctional.

 Register A (accumulator)
 It is an 8 bit register, which holds operand and receives result of
arithmetic instruction.
 It can also be source or destination for logical operations.
 Register B
 In addition to accumulator, an 8 bit B-register is available as
general purpose register.

 Stack pointer is used to hold
internal RAM address that is
called top of stack
 It is 8 bit wide
 It is incremented before data is
stored during PUSH and CALL
 It is decremented after data is
restored during POP and RET
instructions
 It can reside anywhere on chip
RAM
 Stack pointer is initialized 07 h
after reset.
 Stack location begin at location
08h

 It consists of High byte (DPH) and
low byte (DPL)
 Its function is to hold 16 bit
address
 It can be used as 16 bit data
register or two 8 bit registers
 It does not have single internal
address.

 8051 has 16 bit program counter
 Which holds address of memory location from which next
instructions is to be fetched
 It can address up to 64k of memory
 It automatically increments to point next instruction in program
sequence
 PC is only register that does not have internal address.

 Like other microprocessors Intel
family 8051 uses memory mapped
I/O through special function
registers that in implemented above
128 bytes of RAM
 The figure shows special fubction bit
addresses
 It shows all 4 I/O ports , CPU
registers, interrupt control registers,
time/counter etc
 The power control is performed
between 80H and FFH

 All internal operations are
synchronized with help of clock
signal.
 Clock signal is generated using
oscillator circuit.
 The minimum and maximum
operating frequencies for 8051 are
typically 1 mhz to 16 mhz.
 A machine cycle of 8051 consists of
6 states from s1 to s6,divided int
two phases p1 & p2.
 Thus a m/c cycle consist of 12
oscillator periods from s1p1 to s6p2
in 1µsec.
 During m/c arithmetic and logical
operations takes place in p1 and
internal register to register in p2.
 ALE is activated twice a m/c cycle
for user accessible as external
reference
 (ALE is activated during s1p2 and
s2p1, s4p2 and s5p1.

 Execution of one cycle instruction begins at s1p2, when instruction is
latched into instruction register.
 If it is two byte instruction second instruction is read during s4.
 If it is one byte instruction there is still fetch at s4 but byte read is
ignored and pc Is not incremented.
 Most instruction execute in one cycle, but mul, div are instructions that
take two cycles to complete.
 Normally two bytes are fetched during every m/c cycle, only for MOVX it
takes 1 byte , 2 cycle for external memory.

 8051 has internal data and code
memory with limited memory
 Memory may not be sufficient for
some applications
 So their is need to connect
external ROM/EPROM and RAM to
8051
 To increase capacity of memory
 ROM is used as program memory
 RAM is used as data memory

 In 8051 when EA pin connected
to Vcc program fetches
addresses from 0000h to 0FFFh
are directed to internal ROM
 From 1000h to FFFFh are
directed to external ROM/EPROM
 And if EA pin is grounded all
addresses fetched by program
from 0000h to FFFFh are
directed to external ROM/EPROM
 PSEN signal is used to activate
output enable signal of
ROM/EPROM
 Port 0 is used as multiplexed
address/bus
 Initial T –cycle gives lower order
8 bit address and later used as
data bus
 8bit lower address is latched
using ALE signal

• Port 2 provides higher order 8
bit address
• The lower part of program
stores the vector addresses for
various vectors interrupt routine
•Each interrupt is assigned with
fixed location
• from figure we can see
•When the interrupt does not
use the location it is used as
general purpose memory.
•Instruction to access external
ROM/Program memory
•MOVC A,@A +DTPR
•MOVC A,@A +PC

 8051 can address upto 64k bytes
of external memory
 MOVX instruction is used to
access external data memory.
 Internal memory is divided into
three blocks lower 128 bytes ,
upper 128bytes and SFRS
 Upper block and SFRS occupy
same address space 80h to FFh
 Upper is accessible by indirect
addressing only
 SFRs is accessed by direct
addressing
 Lower is accessed by direct as
well as indirect addressing

 Circuit diagram shows
connectivity with external data
memory.
 Multiplexed address/data bus is
provided by external port 0,
external latch and ALE
 Port 2 gives higher order sddress
bus
 RD and WR signals selects
memory read and memory write
operations

 8051 has 16 bit Timer/counter
registers: Timer 0 and Timer 1 both
can be used either as timers or
counters
 If it is used as timer it will be
incremented after each m/c cycle
and the count rate is 1/12 of
oscillator frequency.
 If frequency is 12 MHZ then timer
clock frequency will be 1 MHZ.
 If it is used as counter it will be
incremented in response to 1-0
transition its corresponding external
input it takes two m/c cycles so
count rate is 1/24
 Timers and counters are controlled
by TR!/0 bits in TCON
 Gate bits in TMOD and INT1/0 input
pins of 8051
 C/T bit register decides the
operation 0 for timer selection and
1 for counter

Micro 8051

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