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For describing pin diagram and pinconfiguration of 8051, we are
taking into
consideration a 40 pin DIP (Dual inlinepackage). Now lets go
through pin configurationin detail.
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Pin-40 : Named as Vcc is the main powersource. Usually its +5V
DC.
Pins 32-39: Known as Port 0 (P0.0 to P0.7) In addition to
serving as I/O port, lowerorder address and data bus signals
aremultiplexed with this port (to serve the
purpose of external memory interfacing).
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Pin-31:- ALE aka Address Latch Enable isused to demultiplex the
address-data signalof port 0 (for external memory interfacing.)
Pin-30:- EA/ External Access input is used toenable or disallow
external memoryinterfacing. If there is no external memory
requirement, this pin is pulled high byconnecting it to Vcc.
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Pin- 29:- PSEN or Program Store Enable isused to read signal
from external programmemory.
Pins- 21-28:- Known as Port 2 (P 2.0 to P 2.7) in addition to
serving as I/O port, higherorder address bus signals are
multiplexed.
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Pin 20:- Named as Vss it represents ground (0V) connection.
Pins 18 and 19:- Used for interfacing anexternal crystal to
provide system clock.
Pins 10 17:- Known as Port 3. This port alsoserves some other
functions like interrupts,timer input, control signals for
externalmemory interfacing RD and WR , serialcommunication signals
RxD and TxD etc.
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Pin 9:- As explained before RESET pin is usedto set the 8051
microcontroller to its initialvalues, while the microcontroller is
working
or at the initial start of application.
Pins 1 8:- Known as Port 1. Unlike otherports, this port does
not serve any other
functions.
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There are 21 Special function registers(SFR) in 8051 micro
controller and thisincludes Register A, Register B, Program
Status Word (PSW), PCON etc etc.There are 21 unique locations
for these 21special function registers and each of theseregister is
of 1 byte size.
Some of these special function registers arebit addressable
(which means you can access8 individual bits inside a single byte),
whilesome others are only byte addressable .
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The Accumulator (sometimes referred to as
Register A also) holds the result of most of arithmetic and
logic operations.
ACC is usually accessed by direct addressingand its physical
address is E0H.
Accumulator is both byte and bit
addressable. You can understand this fromthe figure shown
below.
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To access the first bit (i.e bit 0) or to accessaccumulator as a
single byte (all 8 bits atonce), you may use the same
physicaladdress E0H. Now if you want to access thesecond bit (i.e
bit 1), you may use E1H andfor third bit E2H and so on.
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The major purpose of this register is inexecuting multiplication
and division. The8051 micro controller has a single instruction
for multiplication (MUL) and division (DIV).
Ex: MUL A,B When this instruction isexecuted, data inside A and
data inside B is
multiplied and answer is stored in A.
For MUL and DIV instructions, it is necessarythat the two
operands must be in A and B.
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4 I/O ports named P0, P1, P2 and P3 has gotfour corresponding
port registers with samename P0, P1, P2 and P3.Data must be written
into port registers firstto send it out to any other external
devicethrough ports.Similarly any data received through portsmust
be read from port registers forperforming any operation.All 4 port
registers are bit as well as byteaddressable. Take a look at the
figure belowfor a better understanding of port registers.
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SP, stack pointer represents a pointer to thesystem stack. Stack
pointer is an 8 bit
register, the direct address of SP is 81H andit is only byte
addressable, which means youcant access individual bits of stack
pointer.
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