Things
u need
- A MCU (atmega16 or atmega8)
- A led and/or motor and/or dc fan.
Difficulty
Level
Easy
from application point of view, a bit typical from programming point of view.
Time
Required
If
you are using the code given my me, then 1/2 or 1 hour.
Concept
PWM means Pulse
Width Modulation.
Before
lettting you what does it mean, let mew ask you a simple question, How to regulate a motor's speed?
You
may answer that using a variable resistance, but what if I need to do it by
using programming. Here PWM comes in play. One more famous example is How to show the dimming effect
in a led. Again PWM the answer.
What
PWM does is that instead of sending a
continuous supply of voltage to load (led, motor, dc fan, etc) it sends the
supply in PULSE , that’s why the process is called PULSE
WIDTH MODULATION.
Now
if we have to control the speed of motor or intensity of led then we have to control the WIDTH of the
PULSE.
There
are two things to learn to use PWM,
- Timers
- OCR register
There
are two internal Timers in atmega8, T0 (pin6, D4) and T1 (pin11, D5) and in
atmega16 also T0 (pin1, B0) and T1 (pin2, B1).
These
timers once activated run continuously from 0 to 255 (8 bit) as long as the
program runs.
There
are 3 OCR registers in both atmega8 as well as atmega16.
In
atmega8 pin15 is OC1A, pin16 is OC1B, pin17 is OC2.
In
atmega16 pin18,19,21 are OC1A,OC1B and OC2 respectively.
The
PULSE WIDTH is set by these OCR registers
only.
The
time upto which MCU will send supply depends on what is value of OCR register.
Suppose
the value of OC1A is 128 then while the timer which is running is between 0-128
the MCU will send supply to pin19 (atmega16) or pin15 (atmega8) and when its between 129-255 the MCU will
not send supply to that pin, so the bottom line is that for exactly half time
pin is getting the supply and for exactly half time the supply is off, so the
effective voltage reduces from 5v to 2.5v. The motor will run slow , the led
will dimmed.
|
Below
is the code shown for using PWM
This
will make the LED to glow from minimum intensity (OCR1A=0 ) and then the
increases upto highest intensity
(OCR1A=255) . You can add delay to increase the time required to complete one
cycle.
To
get full code download the link.
Download this link which has the
required Header Files (timer0.h,timer1.h,timer0.c,timer1.c).
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