After the long time I'm not updating my blog because have a lot thing to do with my assignment and laboratory. Now, I am here again... :) Today, I will try my best to complete my project schedule during all of this time.. :)
*********************************************************************************
In the
research that I mention earlier in week 2, my supervisor has given me a task to
study and it is about RC Car. In that circuit, they are use PIC 16F877A as the
main part of their project and I will use it in my project too. :) This week, I
would like to brief you a little bit about PIC16F877A.
PIC16F877A
 |
Figure : PIC16F877A Pin Connections
|
|
PIC16F877A belongs to Mid-Range PIC16 device a
class of 8-bit microcontrollers. It offers a wide range of package option, as
well as low-to-high levels of peripheral integration. These PIC16 devices
feature various serial analogue and digital peripherals, such as USB, SPI
12CTM, USART, LCD and A/D converter. Its general structure is shown in Figure above.
- Program
memory (FLASH) – for storing a written program. Since memory made
in FLASH technology can be programmed and cleared more than once, it makes this
microcontroller suitable for device development.
- EEPROM – data
memory that needs to be saved when there is no supply. It is usually used for
storing important data that must not be lost if power supply suddenly stops.
For instance, one such data is an assigned temperature in temperature
regulators. If during a loss of power supply this data was lost, then it would
have to make the adjustment once again upon return of supply. Thus the device
looses on self-reliance.
- RAM – data
memory used by a program during its execution. In RAM are stored all
inter-results or temporary data during run-time. PORTA and PORTB are physical
connections between the microcontroller and the outside world. PORTA has five,
and PORTB has eight pins.
- Free-Run
Timer is an 8-bit register inside a microcontroller that works independently
of the program. On every fourth clock of the oscillator it increments its value
until it reaches the maximum (255), and then it starts counting over again from
zero. The exact timing between each two increments of the timer contents, timer
can be used for measuring time which is very useful with some devices.
- Central
Processing Unit has a role of connective element between other
blocks in the microcontroller. It coordinates the work of other blocks and
executes the user program.
Basic
Circuit Of PIC16F877A
(1) Power Supply
To start up your PIC16F877A, there are 7 pins you
should connect correctly. Likes others electronic component, the supply pin is
the most important. The ideal voltage for PIC16F877A is 5V (Direct Current). It
should not be higher then 5.5V because it going to blow up. It also should not
be less then 2V because it not going to be operate. You will have various type
of supply such as battery and DC adapter. The problem is, if you use battery,
it hard to fine normal battery in 5V. Yes you can use 3 normal 1.5V battery
that connected in series. But I don’t think it will be efficient enough. So how
if I use 9V battery? Yes I can use it but I need to step it down to 5V. How?
You can use voltage regulator. LM7805 the part number of the voltage regulator
that I used. The two number at the end of the part number is 05 which is mean
it will step down the larger input voltage to 5V. Let say if use LM7809 it will
step down the higher voltage then 9V to 9V. Here is the datasheet. This is the basic connection to
step down 9V to 5V by using LM7805.
Using LM7805 is not only limited to battery supply
source. But you can also use it at your rectifier circuit. Picture below show
how the connection from the output of rectifier to the LM7805 and to PIC16F877.
VCC
(+5V)
|
GND
|
PIN11
|
PIN12
|
PIN32
|
PIN31
|
2) Reset Pin
So, how to give power supply to PIC is already
covered. As I said before, there are 7 pins should be connect in order to let
your PIC operate. 4 pins is already connected which is 2 pins for 5V and 2 pin
for ground (negative). It 3 more pins left. The other important pin is Reset
pin (MCLR – Master Clear Reset at Pin number 1). If PIC read 0V at MCLR pin, it
will reset the program, so if you not connect the MCLR pin with 5V, PIC will
remain reset and your program will not execute. You can directly connect the
MCLR with 5V (series with resistor likes picture below) but you will unable to
reset if your system goes wrong.
If you want to use the reset function, you need to
create logic condition which is 1 and 0 to the reset pin. 1 is mean the reset
pin get 5V. If the reset pin logic is 1, then the program in your PIC will
execute, but if the reset pin logic condition is 0 (which is 0V) then your PIC
will not execute the program.
Logic
Condition
|
Voltage
|
Program
Status
|
0
|
0V
|
Not Execute
|
1
|
5V
|
Execute
|
So, how you can get this condition? The answer is
simple, you just need a switch. But you cannot simply add a switch between the
5V and the reset pin. You will need the Pull Up Resistor. What is that thing?
See the picture below, that is the correct way how to add up a switch in order
to create the logic condition.
When the switch is not pushed, current will flow
trough 10K resistor and MCLR Pin. As a result, MCLR Pin receive 5V and PIC read
it as logic 1. But when switch is push, current will flow through 10K resistor,
switch and directly to ground. There is no voltage will receive at MCLR Pin.
This give 0 logic at MCLR Pin.
(3)
The Oscillator
Five from seven of important pins already covered,
now there are only 2 pin left which is pin number 13 and 14. Those pin was
named as OSC1 and OSC2. You can connect the crystal oscillator from various
frequency. Pulse generated from the oscillator will some time have the noise.
To reduce the noise, two capacitors in pico farad value is needed. The value of
capacitor is depend on the speed of oscillator that you use. Here is the way
how to connect the the crystal oscillator and capacitor value table.
 |
| Table from PIC16F87XA Datasheet |
