The document describes the architecture of an 8051 microcontroller. It contains the following key components: CPU, ROM, RAM, timers, I/O ports, serial port and interrupt sources. The microcontroller has all these components integrated on a single chip. It has fixed amounts of on-chip ROM, RAM and I/O ports. Microcontrollers are used in applications where cost, power and space are critical as compared to microprocessors which have external memory and I/O devices.

1. Program Counter
ALU
ROM
External
Bus
interface
Logic
RAM
External
Display
Registers.
Peripheral
Controller
External
i/p’s

2. EXTERNAL
INTRRUPTS
INTERRUPT
CONTROL
ON-CHIP
ROM FOR
PROGRAM
CODE
TIMER0
ON-CHIP
RAM
TIMER1
COUNTER
INPUTS
CPU
OSC
BUS
CONTROL
4 I/O
PORTS
P0
P1 P2 P3
ADDRESS/DATA
SERIAL
PORT
TXD
RXD

3. 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.
•Expensive
•Versatility
•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
•For applications in which cost, power and space are critical
•Single-purpose

5. Overview of 8051 family
8051 is a 8-bit micro controller ,it is introduced by
Intel corporation.8051 is the original member of the 8051 family.
Other members of 8051 family
8052 and 8031 are the other family members of 8051.
The following table gives comparison of 8051 family members
Feature
8051
8052
8031
ROM(on-chip program space in bytes)
4K
8K
0K
RAM(bytes)
128
256
128
Timers
2
3
2
1/O pins
32
32
32
Serial Port
1
1
1
Interrupt sources
6
8
6

6. Various 8051 microcontrollers
8051 is available in different memory types,such as UV-EPROM ,
flash,and NV-RAM,all of which have different memory types.
AT89C51 from Atmel corporation:
This 8051 chip has on-chip ROM in the form of flash memory.
AT89C51 is used in place of the 8751 to eliminate the waiting
time needed to erase the chip and thereby speed up the
development time.

7. DS from Dallas Semiconductor
DS5000 is another chip from Dallas semiconductor.The onchip ROM for the DS5000 is in the form of NV-RAM.The
read/write capability of NV-RAM allows the program to be
loaded into the on-chip ROM while it is in the system.
1.Having on-chip back up battery
2.Having special circuit which connects RAM to battery
immediately after removing the supply voltage

8. P89C51RD2BN from Phillips:
It is another major producer of 8051 family from Phillips
Corporation
It has the following features
1.It has 64K bytes of on-chip ROM
2.Allows In system programming (ISP).Hence no need to use
PROM programmer

9. 8051 PIN description
P1.0
1
P1.1
2
P1.2
3
P1.3
40
Vcc
39
P0.0 (AD0)
38
P0.1 (AD1)
4
37
P0.2 (AD2)
P1.4
5
36
P0.3 (AD3)
P1.5
6
35
P1.6
7
34
P0.4 (AD4)
P0.5 (AD5)
P1.7
8
33
P0.6 (AD6)
RST
9
32
P0.7 (AD7)
31
EA/VPP
(RXD) P3.0
10
(TXD) P3.1
(INT0) P3.2
(INT1) P3.3
11
(T0) P3.4
12
13
14
(T1) P3.5
(WR) P3.6
15
16
(RD) P3.7
XTAL2
XTAL1
GND
8051
30
29
28
27
26
ALE/PROG
25
PSEN
P2.7 (A15)
P2.6 (A14)
P2.5 (A13)
P2.4 (A12)
17
24
P2.3 (A11)
18
19
23
22
P2.2 (A10)
P2.1 (A9)
20
21
P2.0 (A8)

10. Pin number
Description
Vcc(Pin 40)
Provides supply voltage to the chip .The voltage
source is +5V
GND(Pin 20)
It is the Ground
XTAL1(Pin19)
and
XTAL2(Pin18)
The 8051 needs external clock to run it.Most often
quartz crystal oscillator is connected to inputs
XTAL1(Pin 19) and XTAL(Pin 18)
RST(Pin 9)
It is the RESET pin.Upon applying high pulse to
this pin the micro controller will reset and terminate
all activities
EA (Pin 31)
The 8051 family members all come with on-chip
ROM to store programs.So EA connected to Vcc.
PSEN (Pin 29)
This is an output pin .PSEN stands for “Program
store enable”
ALE (Pin 30)
ALE(address latch enable) is an output pin and is
active high.This is used for demultiplexing the
address and data by connecting to the G pin of
74LS373 chip

11. Interfacing External program memory
P1
Instruction
P0
EA
ALE
P3
1
Address
Latch
P2
OE
PSEN
0
Microcontroller
External ROM

12. Interfacing External data memory
P1
Data
P0
EA
Vcc
ALE
1
P3
Latch
Address
P2
RD
WR
Microcontroller
I/O
Page bits
WE
OE
0
External data memory

14. CPU can work only in binary,it can do so at a very high
speed ,but its slow and tedious for humans to deal with 0’s
and 1’s I.e with machine language.
Assembly language were developed which provided
mnemonics for the machine code instructions, plus other
features which made programming faster and less prone to
error.
Assembler is a program which converts assembly
language into machine level language
Assembly language is referred to as a low level language
because it deals directly with the internal structure of the
CPU
High level languages are translated into machine code by
a program called a complier

15. - In the CPU ,registers are used to store information temporarily.
- In 8051 there is only one data type : 8 bits.
D7
D6
D5
D4
D3
D2
Most significant bit
B
R0
D0
Least significant bit
Registers
A
D1
R1
R2
R3
R4
R5
R6
R7
8- bit Registers of the 8051
DPTR
PC
DPH
DPL
PC(Program counter)
16-bit register
16-bit register

16. An assembly language program consists of series of
assembly language instructions .
An assembly language instruction consists of a
mnemonic , optionally followed by one or more operands. The
operands are the data items being manipulated , and the
mnemonics are the commands to the CPU, telling it what to
do with those items.
Example:
ORG 00h
; start at location 0
MOV R5,#25h; load 25h into R5
MOV A, #0
;load 0 into A
ADD A,R5
;add contents of R5 to A
; now A =A +R5
END
; end of the asm source file

17. The following diagram shows steps to create an executable
assembly language program
Editor Program
Myfile.asm
Assembler Program
Myfile.lst
Other obj files
Myfile.obj
Linker Program
Myfile.abs
OH Program
Myfile.hex

18. Data Type
8051 micro controller has only one data type.It is of 8-bits,and
the size of each register is also 8 bits.
DB(Define byte)
DB directive is used to define data, the numbers can be in
decimal,binary,hex,or ASCII formats.
Examples:
DATA1: DB 28
;Decimal(1C in hex)
DATA2: DB 00110101B
;Binary (35 in hex)
DATA3: DB 39h
;hex
ORG 510H
DATA4: DB “My name is MIC “
;ASCII characters

19. Following are the more widely used directives of the 8051
ORG(origin)
The ORG directive is used to indicate the beginning of the address
ORG 025h
;starts at the 25th address
EQU(equate)
This is used to define a constant without occupying a memory
location.The EQU directive does not set aside storage for data item but
associates a constant value with a data label so that when the label
appears in the program,its constant value will be substituted for the label.
COUNT EQU 25
MOV R3 , #COUNT
;R3 becomes 25 here
END Directive
This indicates to the assembler the end of the source(asm) file.Anything
after the END directive is ignored by the assembler.

20. The flag register in the 8051 is called the Program Status
Word (PSW) register.It is used to indicate arithmetic conditions
such as a carry but.
PSW(Program status word) Register:
• The PSW register is an 8-bit register but only 6 bits of it are used
by the 8051 .
• The two unused bits are user-definable flags.
• Four of the flags are conditional flags meaning that they indicate
some conditions that resulted after an instruction was executed.
These are CY,AC,P,OV.
• The bits PSW.3 and PSW.4 are designated as RS0 and RS1 ,and are
used to change the bank registers.
• The PSW.5 and PSW.1 bits are general-purpose status flag bits and
can be used by the programmer for any purpose

21. CY
AC
F0
RS1
RS0
OV
----
P
CY
PSW.7
Carry flag
AC
PSW.6
Auxiliary carry flag
--
PSW.5
Available to the user for general purpose
RS1
PSW.4
Register bank selector bit 1
RS0
PSW.3
Register bank selector bit 0
OV
PSW.2
Overflow flag
--
PSW.1
User definable bit
P
PSW.0
Parity flag.Set/Cleared by hardware each
instruction cycle to indicate an odd/even
number of bits in the accumulator

22. RS1
RS0
Register Bank
Address
0
0
0
00H - 07H
0
1
1
08H – 0FH
1
0
2
10H – 17H
1
1
3
18H – 1FH

23. Example: Show the status of the CY,AC and P flags after the
addition of 38H and 2Fh in the following instructions
MOV A,#38h
ADD A,#2FH
Solution:
38
00111000
+
2F
00101111
67
01100111
CY = 0 since there is no carry beyond D& bit
AC = 1 Since there is no carry from D3 to the D4 bit
P = 1 Since the accumulator has an odd number of 1’s

24. Instruction
CY
OV
AC
ADD
X
X
X
ADDC
X
X
X
SUBB
X
X
X
MUL
0
X
DIV
0
X
DA
X
RRC
X
RLC
X
SETB C
1
CLR C
0
CPL C
X
ANL C,bit
X
ANL C,/bit
X
CJNE
X

25. RAM Allocation in 8051
7F
Scratch pad RAM
30
2F
20
1F
Bit Addressable RAM
Register Bank 3
18
17
10
0F
Register Bank 2
Register Bank 1(stack)
08
07
00
Register Bank 0

26. 128 Bytes of Internal RAM
7F
General purpose RAM
30
7F
7E
7D
7C
7B
7A
79
78
23
1F
1E
1D
1C
1B
1A
19
18
22
17
16
15
14
13
12
11
10
21
0F
0E
0D
0C
0B
0A
09
08
20
07
06
05
04
03
02
01
00
2F
Bit
Addressable
Locations
.
.
.
.
.
1F
18
17
10
0F
08
07
00
Bank3
Bank2
Bank1
Default register bank for R0-R7

27. Special function registers
99
98
9A
99
98
SBUF
SCON
9F 9E 9D 9C 9B
90
97
93 92
91
90
96
95
94
Not bit addressable
8B
Not bit addressable
8A
Not bit addressable
TH1
TH0
TL1
TLO
Not bit addressable
Not bit addressable
8F 8E 8D 8C 8B 8A
Not bit addressable
89
TMOD
88 TCON
PCON
82
Not bit addressable
DPH
DPL
81
Not bit addressable
SP
87
86
85
84
83
82
81
80
P0
E5
B8
-- -- --
B6
F3
F2
F1 F0
D3 D2 D1 D0
BC BB BA B9 B8
B5
B
E3 E2 E1 E0 ACC
E4
D0 D7 D6 D5 D4
B0 B7
Not bit addressable
80
F0 F7 F6 F5 F4
E0 E7 E6
8D
8C
89
88
87
84
FF
P1
Not bit addressable
B4
B2
B1
A8 AF -- -- AC AB AA A9
B0
PSW
IP
P3
A8 IE
A0 A7 A6 A5 A4 A3 A2 A1 A0 P2

28. Push operation
MOV R6,#25h
PUSH 6
0B
0B
MOV R1,#12h
PUSH 1
0B
0A
0A
09
09
08
08
0B
0A
09
25
Start SP
07
SP
08
F3
0A
12
09
25
08
After PUSH 6
MOV R4,#0F3h
PUSH 4
After PUSH 1
SP
09
12
08
25
After PUSH 4
SP
0A

29. POP 3
; POP stack into R3
POP 5
; POP stack into R5
POP 2
; POP stack into R2
After POP 3
0B
54
0A
F9
09
76
08
6C
Start SP
0B
0B
SP
09
76
6C
08
0A
76
09
0B
0B
F9
0A
08
6C
0A
After POP 2
After POP 5
0A
SP
09
08
09
6C
SP
08