• 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 date bus.
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
in the fig.2-2 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.
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
• 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.
• 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
• ALE ( Address Latch Enable)
– This pin is used to demultiplex the address and
– 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. (see fig 2-10 and 211).
• 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.
• This is pin 9 of the IC and is used as the
master reset for the 8051. In order for the
8051 to recognise 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.
This will be discussed in more detail later.
Oscillator ( clock) Input
• The 8051 is typically driven by a crystal
oscillator connected to pin 18 and 19 as
shown in fig.2-3.
• The words XTAL is short for crysTAL.
Driving the 8051 from a TTL oscillator
• Power Connections
– The 8051 requires a +5V input on its Vcc input
(pin 40) and Vss connection is on page 20.
I/O Port Structure
• The internal circuitry for the I/O port is shown in
• If you want to read in from a pin, you must first
give a logic ‘1’ to the port latch to turn off the
FET otherwise the data read in will always be
• When you write to the port you are actually
writing to the latch e.g. a logic 0 given to the latch
will be inverted and turn on the FET which cause
the port pin to be connected to gnd (logic 0).
Machine Cycle and Clock Cycle
• 12 clock cycles make one machine cycle as
shown in fig 2-5.
• E.g. if we use a 12 MHz oscillator, each
clock cycle will have a time period of
1/12MHz. Twelve of these make one
machine cycle so 12 x (1/12 MHz) = 1
microsecond. That’s the time of 1 machine
Relationship between oscillator clock cycles, states, and the machine cycle
• 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
Summary of the 8051 on chip data memory
• 4 Register Banks — Bank0, Bank1, Bank2
• Each Bank consists of — R0, R1, R2, R3,
R4, R5, R6, R7
• Bank 0 is the default upon power up of the
• Other banks can be selected by
programming PSW register.
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 given in
the next section.
• The 8051 contains 210 bit-addressable
locations of which 128 are at byte address
20H through 2FH as shown in fig 2-7.
• 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
– we could issue a simple instruction
This would set the bit at address 67H to logic HIGH.
Bit 67H is bit 7(most significant bit) of byte location
In order to achieve the same result, a microprocessor
would need to do this:
MOV A, 2CH
Special Function Registers
• Above 7FH, there is another block of
memory 80H to 0FFH in all the version of
• this 128 bytes of memory are reserved for
Special Function Register (SFR). There are
21 SFRs. Refer to fig 2-7.
• 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.
• Some important or commonly used SFRs will be
discussed while others will be explained when you
need to use them in your projects.
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)
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
Fig 3-0 B
• In the SFR, register P0,
P1, P2, P3 are
connected to the
physical pin on the uP
• Some of 0 Port pin
serve an alternative
addressed by name
P2, individual pin
P2.0 to P2.7
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
• As stated earlier, the PSEN pin must be
used. See fig 2-9
Multiplexing the address bus (low-byte) and data bus
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.
Read timing for external code memory
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)
• The timing diagram for a read operation is
shown in fig 2-11.
• To reset the 8051, the RST pin must be held
high for at least 2 machine cycles.
• This can be achieved upon power–up using
an RC network.
• Fig.2-16 shows 2 circuits for achieving this,
one is a manual reset, the other is a poweron reset.
• How does the 2 circuit works?
– Try to remember capacitor is open during
Two circuits for system reset. (a) Manual reset (b) Power-on reset.