The 8051 microcontroller has the following key features: - 4K bytes of ROM, 128 bytes of RAM, four 8-bit I/O ports, and two 16-bit timers. It has pins for serial communication and can address up to 64K bytes each of external code and data memory. The 40-pin device uses 32 pins for I/O, with some pins having dual purposes. It uses a crystal oscillator connected to pins 18 and 19 to generate the clock signal. The 8051 has separate internal memory spaces for code and data and can access additional external memory. Special function registers like the Program Status Word and Accumulator control various functions.

1. The 8051 MicroController

2. Important 8051 Features
• 4K bytes ROM
• 128 bytes RAM
• Four 8-bit I/O ports
• Two 16-bit timers
• Serial interface
• 64K external code memory space
• 64K data memory space

3. Pin Layout
• The 8051 is a 40 pin device, but out of these
40 pins, 32 are used for I/O.
• 24 of these are dual purpose, i.e. they can
operate as I/O or a control line or as part of
address or data bus.

4. 8051 pinouts

5. Port 0 and Port 1
• Port 0 is a dual purpose port, it is located
from pin 32 to pin 39 (8 pins) and is labeled
as AD0 to AD7.
• Port 1 is a dedicated I/O port from pin 1 to
pin 8. It is generally used for interfacing to
external device thus if you need to connect
to switches or LEDs, you could make use of
these 8 pins.

6. Port 2 and Port 3
• Like port 0, port 2 is a dual-purpose port. It can be
used for general I/O or as the high byte of the
address bus for designs with external code
memory.
• Port 3 is also dual purpose but designers generally
avoid using this port unnecessarily for I/O because
the pins have alternate functions which are related
to special features of the 8051. Indiscriminate use
of these pins may interfere with the normal
operation of the 8051.

7. Oscillator ( clock) Input
• The 8051 is typically driven by a crystal
oscillator connected to pin 18 and 19.
• The words XTAL is short for crysTAL.

8. Driving the 8051 from a TTL oscillator

9. Machine Cycle and Clock Cycle
• 12 clock cycles make one machine cycle.

10. Relationship between oscillator clock cycles, states, and the machine cycle

11. Memory Structure
• While most microprocessors implement a shared
memory space for data and code (programs),
microcontrollers has limited memory and the
program is usually stored in ROM.
• In the 8051, both code and data may be internal
but they are stored in separate memories, namely
the internal ROM and RAM. Expandable to a
max of 64K using external memory.
• The next page shows the 8031 which has no
internal ROM.

12. Summary of the 8031 memory spaces

13. Summary of the 8051 on chip data memory

14. Register Banks
• 4 Register Banks — Bank0, Bank1, Bank2
and Bank3
• Each Bank consists of — R0, R1, R2, R3,
R4, R5, R6, R7
• Bank 0 is the default upon power up of the
microcontroller
• Other banks can be selected by
programming PSW register.

15. General Purpose RAM
• The general purpose RAM area is from
address 30H to 7FH. The locations from
address 20H to 2FH can also be used as
general purpose RAM although these
addresses have very specific role.

16. Bit-Addressable RAM
• The 8051 contains 210 bit-addressable
locations of which 128 are at byte address
20H through 2FH.
• This is the powerful feature of most
microcontroller because individual bits can
be set, cleared, ANDed, ORed etc. with a
single instruction instead of having to read a
byte and modify

17. • Example
– we could issue a simple instruction
SETB 67H
This would set the bit at address 67H to logic HIGH.
Bit 67H is bit 7(most significant bit) of byte location
2CH
In order to achieve the same result, a microprocessor
would need to do this:
MOV A, 2CH
ORL A,#10000000B
MOV 2CH,A

18. Special Function Registers
• Above 7FH, there is another block of
memory 80H to 0FFH in all the version of
MCS51 uP
• this 128 bytes of memory are reserved for
Special Function Register (SFR). There are
21 SFRs.

19. SFRs
• SFR are usually addressed by name
• Memory location 0F0H is given a name called
Register B, similarly 80H is called P0.
• Not all memory location has a name
– memory location 35H has no name
• Some locations between the SFRs have no names
as well e.g. 91H. Such locations should not be
used to store any data. If you do it then your data
may be lost.

20. Program Status Word (PSW)
• This is a very important register because it
contains status bits which indicates the current
state of the cpu.
• PSW.7 CarrY(CY)
• PSW.6 Aux Carry (AC)
• PSW.5 Flag 0 (F0)
• PSW.4 Register Bank Select 1
• PSW.3 Register Bank Select 0
• PSW.2 Overflow(OV)
• PSW.1 reserved
• PSW.0 Even Parity Flag (P)

21. Commonly used SFRs
• Accumulator, it has two names, A and
ACC. Many instruction make use of the
accumulator, eg: mov A,R0, push acc
• SP, always pointing to the top of the stack,
increasing by 1 before write to stack,
decreasing by 1 after read from stack

22. _____
• PSEN (Program Store Enable)
– This is a dedicated control line on pin 29 and is
used to enable external program (code)
memory. This pin usually connects to an
EPROM’s Output Enable (OE) pin.
– This is a logic low pin as represented by the bar
above the word PSEN, this means that during a
fetch stage involving an instruction stored in
external memory, the pin will be pulsed
‘LOW’.

23. • ALE ( Address Latch Enable)
– This pin is used to demultiplex the address and
data lines.
– Remember that port 0 has 2 functions. As the
low byte of the address bus and as the data bus.
In designs with external memory, port 0 is
connected to both the address and data lines of
the external RAM thus during the part of the
fetch cycle where the address is supplied, the
ALE is pulsed to enable the G (gate) control pin
of the latch IC thus the data goes to RAM and
is interpreted as an address.

24. ___
• EA (External Access)
– If you need to connect to external ROM then
this pin must be tied LOW (0V).
– This pin must be tied high (+5V) if the
programs executes from internal ROM.

25. RST (Reset)
• This is pin 9 of the IC and is used as the
master reset for the 8051. In order for the
8051 to recognize that a reset has occurred,
this pin must be brought HIGH for at least
two machine cycles. During normal
operation, this pin must be at logic LOW.

26. • Power Connections
– The 8051 requires a +5V input on its Vcc input
(pin 40) and Vss connection is on page 20.

27. Accessing External Code Memory
• If the design involves external code
memory, both P0 and P2 should not be used
as general purpose I/O since P2 is now the
Higher address bus while P0 is the
multiplexed Lower address bus and the data
bus.
_____
• As stated earlier, the PSEN pin must be
used.

28. FIGURE 2–8 Multiplexing the address bus (low-byte) and data bus

29. FIGURE 2–9 Accessing external code memory

30. An opcode fetch for 2-byte instruction
• Fig.2-10 shows what happens during an
opcode fetch for a 2 byte instruction that
has a time of 1 machine cycle.

31. Accessing External RAM
• For designs with external RAM, a typical
connection is shown in fig.2-12. Note the
control lines that must be used.
• MOVX instruction is used to indicate that
the external RAM is involved.
• e.g. MOVX A, @dptr ( a read operation)
• e.g. MOVX @dptr, A (a write operation)

32. • The timing diagram for a read operation is
shown in fig 2-11.

33. FIGURE 2–12 Interface to 1K RAM