8051 employs Harvard architecture. It
has some peripherals such as 32 bit digital I/O, Timers and Serial I/O. The
basic architecture of 8051 is given below:
|
|
Basic 8051
Architecture
|
Various features
of 8051 microcontroller are given as follows :
|
- 8-bit CPU
- 16-bit Program
Counter
- 8-bit Processor
Status Word (PSW)
- 8-bit Stack
Pointer
- Internal RAM of 128bytes
- Special Function
Registers (SFRs) of 128 bytes
- 32 I/O pins
arranged as four 8-bit ports (P0 - P3)
- Two 16-bit
timer/counters : T0 and T1
- Two external and
three internal vectored interrupts
- One full duplex
serial I/O
|
Pinouts
Pins 1-8: Port 1 Each of
these pins can be configured as an input or an output.
Pin 9: RS A logic one on
this pin disables the microcontroller and clears the contents of most
registers. In other words, the positive voltage on this pin resets the
microcontroller. By applying logic zero to this pin, the program starts
execution from the beginning.
Pins10-17: Port 3 Similar to
port 1, each of these pins can serve as general input or output. Besides, all
of them have alternative functions:
Pin 10: RXD Serial
asynchronous communication input or Serial synchronous communication output.
Pin 11: TXD Serial
asynchronous communication output or Serial synchronous communication clock
output.
Pin 12: INT0 Interrupt 0
input.
Pin 13: INT1 Interrupt 1
input.
Pin 14: T0 Counter 0 clock input.
Pin 15: T1 Counter 1 clock
input.
Pin 16: WR Write to external
(additional) RAM.
Pin 17: RD Read from
external RAM.
Pin 18, 19: X2, X1 Internal
oscillator input and output. A quartz crystal which specifies operating
frequency is usually connected to these pins. Instead of it, miniature ceramics
resonators can also be used for frequency stability. Later versions of
microcontrollers operate at a frequency of 0 Hz up to over 50 Hz.
Pin 20: GND Ground.
Pin 21-28: Port 2 If there
is no intention to use external memory then these port pins are configured as
general inputs/outputs. In case external memory is used, the higher address
byte, i.e. addresses A8-A15 will appear on this port. Even though memory with
capacity of 64Kb is not used, which means that not all eight port bits are used
for its addressing, the rest of them are not available as inputs/outputs.
Pin 29: PSEN If external ROM
is used for storing program then a logic zero (0) appears on it every time the
microcontroller reads a byte from memory.
Pin 30: ALE Prior to reading
from external memory, the microcontroller puts the lower address byte (A0-A7)
on P0 and activates the ALE output. After receiving signal from the ALE pin,
the external register (usually 74HCT373 or 74HCT375 add-on chip) memorizes the
state of P0 and uses it as a memory chip address. Immediately after that, the
ALU pin is returned its previous logic state and P0 is now used as a Data Bus.
As seen, port data multiplexing is performed by means of only one additional
(and cheap) integrated circuit. In other words, this port is used for both data
and address transmission.
Pin 31: EA By applying logic
zero to this pin, P2 and P3 are used for data and address transmission with no
regard to whether there is internal memory or not. It means that even there is
a program written to the microcontroller, it will not be executed. Instead, the
program written to external ROM will be executed. By applying logic one to the
EA pin, the microcontroller will use both memories, first internal then external
(if exists).
Pin 32-39: Port 0 Similar to
P2, if external memory is not used, these pins can be used as general
inputs/outputs. Otherwise, P0 is configured as address output (A0-A7) when the
ALE pin is driven high (1) or as data output (Data Bus) when the ALE pin is
driven low (0).
Pin 40: VCC +5V power
supply.
