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Basic details of 8051 Microcontroller
Microcontroller manufacturers have been competing for a long time for attracting choosy customers and
every couple of days a new chip with a higher operating frequency, more memory and upgraded A/D
converters appeared on the market
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What is all this about?
The whole story has its beginnings in the far 80s when Intel launched the first series of microcontrollers
called the MCS 051
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The main reason for their great success and popularity is a skillfully chosen configuration which satisfies
different needs of a large number of users allowing at the same time constant expansions (refers to the
new types of microcontrollers)
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This is
the reason for having a great number of various microcontrollers which basically are solely upgraded
versions of the 8051 family
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128b of RAM (including SFRs) satisfies the user's basic needs
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The whole configuration is obviously thought of as to satisfy the needs of most programmers working on
development of automation devices
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In other words, it is created exactly in accordance to the average user‘s taste and needs
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2
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Pin 9: RS A logic one on this pin disables the microcontroller and clears the contents of most registers
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By applying logic zero to this pin,
the program starts execution from the beginning
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Besides, all
of them have alternative functions:
Pin 10: RXD Serial asynchronous communication input or Serial synchronous communication output
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Pin 12: INT0 Interrupt 0 input
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Pin 14: T0 Counter 0 clock input
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Pin 16: WR Write to external (additional) RAM
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Pin 18, 19: X2, X1 Internal oscillator input and output
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Instead of it, miniature ceramics resonators can also be
used for frequency stability
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Pin 20: GND Ground
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In case external memory is used, the higher address byte, i
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addresses A8-A15
will appear on this port
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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
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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
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As seen, port data multiplexing is performed by means of only one additional (and cheap)
integrated circuit
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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
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Instead, the program written to external ROM will be executed
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Pin 32-39: Port 0 Similar to P2, if external memory is not used, these pins can be used as general
inputs/outputs
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Pin 40: VCC +5V power supply
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3 Input / Output Ports (I/O Ports)
All 8051 microcontrollers have 4 I/O ports each comprising 8 bits which can be configured as inputs or
outputs
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Pin configuration, i
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whether it is to be configured as an input (1) or an output (0), depends on its logic
state
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In this case, voltage level on appropriate pin will be 0
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In this case, voltage level on appropriate pin will be 5V (as is the case with any TTL
input)
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It all becomes clear after studying simple
electronic circuits connected to an I/O pin
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4 Memory Organization
The 8051 has two types of memory and these are Program Memory and Data Memory
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Depending on the model in use (we are still talking about the 8051 microcontroller family
in general) at most a few Kb of ROM and 128 or 256 bytes of RAM is used
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It is
neither a mistake nor a big ambition of engineers who were working on basic core development
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Program Memory
The first models of the 8051 microcontroller family did not have internal program memory
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These models are recognizable by their label beginning with 803 (for
example 8031 or 8032)
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Even though such an
amount of memory is sufficient for writing most of the programs, there are situations when it is necessary
to use additional memory as well
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They are used in cases
when equations describing some processes are too complicated or when there is no time for solving
them
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