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Programmable Interface
Controllers
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Since the first integrated
circuit or silicon chip
was developed in the late 1950's The trend has
been towards ever increasing complexity and an ever decreasing size of
circuit.Perhaps the most obvious example of this is the
computer. If we ignore the various peripheral devices, the basic
structure of a computer is similar to the one shown below.
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The two main elements of a
microcomputer are the CPU or
Central Processing Unit and its
Memory. The
interconnections are known as buses because they contain a large number
of parallel connecting wires. The three sets of buses connecting
these two blocks are:
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data bus |
| The address bus
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| The control bus
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The data bus carries data being processed
in both directions. The single direction address bus carries memory
addresses.
The control bus is used to make sure everything works in the
correct sequence by sending and receiving timing signals. The
input/output unit allows the computer to communicate with
peripheral devices. |
| Early computers were
huge devices sometimes occupying several rooms, over the years
the size of microcomputers has reduced as their power has
increased.
PIC's or Programmable Interface Controllers
are essentially microcomputers on a 'chip'. A typical PIC such
as the 16F84 contains all the elements shown above.
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The 16F84 PIC,
combines a microprocessor, ROM,
RAM and an Input/Output
Unit in a single chip.
They can be obtained in 8, 18 and 28 pin
configurations which provide a variety of output and digital and
analogue inputs.
The chips use reprogrammable 'flash memory'
which can
written and rewritten to with ease.
Building a working controller involves simply
connecting the chip to power, interfacing input and output components
and adding a capacitor, resonator and a reset switch.
The most commonly used PIC is the 16F84 shown. This is
an 18 pin device which has 8 outputs and 5 inputs.
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| Connecting Power to the
16F84 PIC
The pin out diagram for the 16F84 is shown on the
right. The 16F84 requires a 6V DC supply. This can be provided by 4 x AA
cells.
A 4MHz ceramic resonator must also be connected as
shown below. The 16F84 provides an internal clock pulse. The resonator is used
to regulate the speed of the clock pulse (4MHz).
Pin 4 (reset) must be connected via a 4k7 resistor to
+V.
A reset facility can also be added by adding a push to make between
pin 4 and 0V. |
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Interfacing Input and
Output Devices
Providing care is taken over matching
voltage and current levels, digital sensors or transducers can be
directly connected to the input and output ports.
The most common digital sensors are
switches.
| Micro-Switches, Reed Switches, Tilt Switches
and Push Switches can all be directly connected to any input pin
as shown right. The 10k resistor prevents a short circuit and
the 1k resistor protects the input pin. In these cases the
digital input will move from logic 0 to logic 1 when the switch
is pressed. |
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Although the 16F84 does not have analogue
input pins an analogue sensor can be used if the sensor is connected via
a potential divider and transistor as shown.
A phototransistor can be used to switch
the input directly
In both these cases the input will move
from logic 1 to logic 0 when the switching level is reached.
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An LED can be driven directly from any output |
| As can a Seven Segment
display.
Taking the required outputs high in the
correct sequence will display the numbers 0 - 9 |
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A Piezo sounder can be connected directly to
produce a range of sounds.
The sounds are produced by pulsing the
required output with a variety of frequencies generated by the
programmed instructions. |
Higher current devices which cannot be
driven directly will require a simple transistor switching circuit. A
device which is frequently used is the BCX 38B darlington driver.
In some situations it will be impossible
to match voltage and current levels of the PIC and the input and output
devices you want to use. In this instance you will need to use two
separate power sources and drive the output transducer from a
transistor.
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Although the PIC and the output device have
different supply voltages the 0V rail must be common to both.
This is essential for the correct operation of
the circuit. |
| Stepper motors are motors which do
not spin freely but turn in steps of 7.5 0, so that
48 steps will cause the motor to turn through 3600 or
one complete revolution. The two types of stepper motor
available are known as unipolar and
bipolar types. The unipolar
is the type explained below.
A suitable motor can be obtained from - Rapid
Electronics - code 37-0500
Unipolar stepper motors have four coils which
must be switched on and off in the correct sequence to make the
motor turn. The table below shows the correct sequence.
| Step |
Coil 1 |
Coil 2 |
Coil 3 |
Coil 4 |
| 1 |
1 |
0 |
1 |
0 |
| 2 |
1 |
0 |
0 |
1 |
| 3 |
0 |
1 |
0 |
1 |
| 4 |
0 |
1 |
1 |
0 |
| 5 |
1 |
0 |
1 |
0 |
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The ULN 2003A darlington driver IC is used to
drive the stepper motor as show below. |
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This project board uses PCB mounted push
switches as inputs and LED's as outputs. Both IC's should be mounted in
sockets. The PCB mounted switches could be wired to other sensors and
there is the facility for 2 pin pcb connectors to be connected in
parallel to the output LED's to connect other output devices. A 7805
regulator and Darlington driver array is used to allow most output
devices to be driven safely. Double Click the PCB
to open the PCB Wizard file.
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Most modern microcontrollers use what is
called 'flash' programming using
EEPROM (electrically erasable programmable read only memory). This means
that the device should be capable of being re-programmed over 10,000
times.
This is important because you will rarely
get the microcontroller program correct first time and you will
inevitably want to change it. If you do make a mistake, the PIC can
simply be re-programmed with new code.
The program will stay in the PIC's memory until it is
re-programmed.
Microcontrollers use instructions
provided in binary (a sequence of
0's and 1's) often called machine code.
This 'code' means very little to anyone but a skilled programmer.
In industry microcontrollers are
programmed using a 'compiler' or 'assembler'. This is a programming
language which is slightly easier to understand but unless you take
A-Level computing you will be unlikely to be able to use or understand
it.
For most applications in school, the
complexity of these languages makes their use impossible (or at least
unrealistic) for most applications.
In KS4 you may be taught a programming
language called BASIC, which the
majority of students cope with quite well. At KS3 however, even learning
BASIC is unrealistic in the time we have.
For most students a much simpler
and easier to understand programming technique is to use a programme
editor such as the one provided with Crocodile Technology.
Most programme editors use a flowchart
approach to design programs. The editing software then converts the
flowchart into BASIC or Assembly language before the microcontroller is
programmed. |
PIC based Microcontroller systems are provided by a number of
suppliers. They are all capable of
PIC Logicator for instance requires a programmer
into which
you insert the microcontroller for programming.
The PICAXE system uses special microcontrollers with a
small programme called a bootstrap programme permanently written
into its memory which allows you to programme the microcontroller while
it is installed in your project.
The Chip
factory system is a low cost PIC programming unit that works without
a PC. It uses its own basic programming language, and is portable.
PIC Logicator and Chip factory both support a range of
28 and 18 pin microcontrollers. The PICAXE system provides support for
their own 8, 18 and 28 pin chips.
All this makes it very difficult to to explain
accurately how every microcontroller could be used. |
A number of newer Microcontrollers have
recently been released. They are much cheaper than the older devices,
enabling additional features at a lower cost. For instance the 18 pin
16F627 is much cheaper than the old 16F84A, has two analogue inputs, and
two extra input pins as it also has an internal resonator.
The types most commonly used are:
| 8 PIN - 12F629 (2 input, 4 output)
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| 18 PIN - 16F627 (4 input, 2 analogue, 8 output, 1
sound)
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| 18 PIN - 16F84A (4 input, 8 output, 1 sound)
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| 18 PIN - 16F818 (2 input, 2 analogue, 8 output, 1
sound)
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| 28 PIN - 16F872 (8 input, 4 analogue, 8 output, 1
sound) |
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