The original PIC was built to be used
with General Instruments' new 16-bit CPU, the CP1600. While generally a good
CPU, the CP1600 had poor I/O performance, and the 8-bit PIC was developed in
1975 to improve performance of the overall system by offloading I/O tasks from
the CPU. The PIC used simple microcode stored in ROM to perform its tasks, and
although the term was not used at the time, it shares some common features with
RISC designs.
In 1985, General Instruments spun off their
microelectronics division and the new ownership cancelled almost everything —
which by this time was mostly out of-date. The PIC, however, was upgraded with
internal EPROM to produce a programmable channel controller and today a huge
variety of PICs are available with various on-board peripherals (serial
communication modules, UARTs, motor control kernels, etc.) and program memory
from 256 words to 64k words and more (a "word" is one assembly
language instruction, varying from 12, 14 or 16 bits depending on the specific
PIC micro family).
PIC and PIC micro are registered trademarks of Microchip
Technology. It is generally thought that PIC stands for Peripheral Interface
Controller, although
General Instruments' original acronym for the initial
PIC1640 and PIC1650 devices was "Programmable Interface Controller".
The acronym was quickly replaced with “Programmable Intelligent Computer".
The PIC architecture is characterized by its multiple
attributes:
ü
Separate code and data spaces (Harvard architecture) for
devices other than
PIC32, which has Von Neumann architecture.
ü
A small number of fixed length instructions.
ü
Most instructions are single cycle execution (2 clock
cycles, or 4 clock cycles in
8-bit models), with one delay cycle on branches and
skips.
ü
All RAM locations function as registers as both source
and/or destination of math.
ü
A hardware stack for storing return addresses.
ü
A fairly small amount of addressable data space
(typically 256 bytes), extended through banking.
ü Data space
mapped CPU, port, and peripheral registers.
ü The program
counter is also mapped into the data space and writable (this is used to
implement indirect jumps).
Microcontrollers
Microcontrollers must contain at least two primary components
– random access memory (RAM), and an instruction set. RAM is a type of internal
logic unit that stores information temporarily. RAM contents disappear when the
power is turned off. While RAM is used to hold any kind of data, some RAM is
specialized, referred to as registers. The instruction set is a list of all
commands and their corresponding functions. During operation, the
microcontroller will step through a program. Each valid instruction set and the
matching internal hardware that differentiate one microcontroller from another.
Most
microcontrollers also contain read-only memory (ROM), programmable read-only
memory (PROM), or erasable programmable read-only memory (EPROM). Al1 of these
memories are permanent: they retain what is programmed into them even during
loss of power. They are used to store the firmware that tells the
microcontroller how to operate. They are also used to store permanent lookup
tables. Often these memories do not reside in the microcontroller; instead,
they are contained in external ICs, and the instructions are fetched as the
microcontroller runs. This enables quick and low-cost updates to the firmware
by replacing the ROM.
The
input/output (I/O) port pins is the way of communicating with the outside
world. The number of I/O pins per controllers varies greatly, plus each I/O pin
can be programmed as an input or output (or even switch during the running of a
program). The load (current draw) that each pin can drive is usually low. If
the output is expected to be a heavy load, then it is essential to use a driver
chip or transistor buffer.
Most
microcontrollers contain circuitry to generate the system clock. This square
wave is the heartbeat of the microcontroller and all operations are
synchronized to it. Obviously, it controls the speed at which the
microcontroller functions. All that needed to complete the clock circuit would
be the crystal or RC components. We can, therefore precisely select the
operating speed critical to many applications.
To
summarize, a microcontroller contains (in one chip) two or more of the
following elements in order of importance:
i.
Instruction set
ii.
RAM
iii.
ROM,PROM or EPROM
iv.
I/O ports
v.
Clock generator
vi.
Reset function
vii.
Watchdog timer
viii.
Serial port
ix.
Interrupts
x.
Timers
xi.
Analog-to-digital converters
xii.
Digital-to-analog converters
Microcontroller PIC 16F877A
The microcontroller chip that has been selected for the
purpose of controlling the Multi sensor blind stick and the speed of DC motor
is PIC16F877A manufactured by Microchip. This chip is selected based on several
reasons:
I.
Its size is small and equipped with sufficient output ports without having to
use a decoder or multiplexer.
ii.
Its portability and low current consumption.
iii.
It has PWM inside the chip itself which allow us to vary the duty cycle of DC
motor drive.
iv.
It is a very simple but powerful microcontroller. Users would only need to
learn 35 single word instructions in order to program the chip.
v.
It can be programmed and reprogrammed easily (up to 10,000,000 cycles) using
the universal programmer in robotics lab.
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