Step by Step: having fun with PWM and a LED (part 1)

Part 1 : controlling the brightness of a LED


In this "Step-by-Step" tutorial, we're going to (try to) have some fun with PWM. PWM stands for Pulse Width Modulation, and is quite weird when you first face this (this was at least my first feeling).


So, how does PWM look like ?...

PWM is about switching one pin (or more) high and low, at different frequencies and duty cycles. This is a on/off process. You can either vary:

• the frequency,
  
• or the duty cycle, that is the proportion where the pin will be high
  

Both have a 50% duty cycle (50% on, 50% off),
but the upper one's frequency is twice the bottom

Three different duty cycle (10%, 50% and 90%),
all at the same frequency

But what is PWM for ? What can we do with it ? Many things, like:

• producing variable voltage (to control DC motor speed, for instance)
  
• playing sounds: duty cycle is constant, frequency is variable
  
• playing PCM wave file (PCM is Pulse Code Modulation)
• ...

One PWM channel + one LED = fun

For now, and for this first part, we're going to see how to control the brightness of a LED. If simply connected to a pin, it will light at its max brightness, because the pin is "just" high (5V).

Now, if we connect this LED on a PWM pin, maybe we'll be able to control the brightness: as previously said, PWM can be used to produce variable voltages. If we provide half the value (2.5V), maybe the LED will be half its brightness (though I guess the relation between voltage and brightness is not linear...). Half the value of 5V. How to do this ? Simply configure the duty cycle to be 50% high, 50% low.

But we also said PWM is just about switching a pin on/off. That is, either the pin will be 0V, or 5V. So how will we be able to produce 2.5V ? Technically speaking, we won't be able to produce a real 2.5V, but if PWM frequency is high enough, then, on the average, and from the LED's context, it's as though the pin outputs 2.5V.


Building the whole

Enough theory, let's get our hands dirty. Connecting a LED to a PWM pin on a 16f88 is quite easy. This PIC has quite a nice feature about PWM, it's possible to select which pin, between RB0 and RB3, will carry the PWM signals. Since I use tinybootloader to upload my programs, and since tiny's fuses are configured to select the RB0 pin, I'll keep using this one (if you wonder why tinybootloader interferes here, read this post).




On a breadboard, this looks like this:

The connector brings +5V on the two bottom lines
(+5V on line A, ground on line B).

LED is connected to RB0

Writing the software

For this example, I took one of the 16f88's sample included in jallib distribution (16f88_pwm_led.jal), and just adapt it so it runs at 8MHz, using internal clock. It also select RB0 as the PWM pin.

So, step by step... First, as we said, we must select which pin will carry the PWM signals...

pragma target CCP1MUX      RB0            -- ccp1 pin on B0

and configure it as output

var volatile bit pin_ccp1_direction is pin_b0_direction
pin_ccp1_direction = output
-- (simply "pin_b0_direction = output" would do the trick too)

Then we include the PWM library.

include pwm_hardware

Few words here... This library is able to handle up to 5 PWM channels (PIC using CCP1, CCP2, CCP3, CCP4 and CCP5 registers). Using conditional compilation, it automatically selects the appropriate underlying PWM libraries, for the selected target PIC.

Since 16f88 has only one PWM channel, it just includes "pwm_ccp1" library. If we'd used a 16f877, which has two PWM channels, it would include "pwm_ccp1" and "pwm_ccp2" libraries. What is important is it's transparent to users (you).

OK, let's continue. We now need to configure the resolution. What's the resolution ? Given a frequency, the number of values you can have for the duty cycle can vary (you could have, say, 100 different values at one frequency, and 255 at another frequency). Have a look at the datasheet for more.

What we want here is to have the max number of values we can for the duty cycle, so we can select the exact brightness we want. We also want to have the max frequency we can have (ie. no pre-scaler).

pwm_max_resolution(1)

If you read the jalapi documentation for this, you'll see that the frequency will be 7.81kHz (we run at 8MHz).

PWM channels can be turned on/off independently, now we want to activate our channel:

pwm1_on()

Before we dive into the forever loop, I forgot to mention PWM can be used in low or high resolution. On low resolution, duty cycles values range from 0 to 255 (8 bits). On high resolution, values range from 0 to 1024 (10 bits). In this example, we'll use low resolution PWM. For high resolution, you can have a look at the other sample, 16f88_pwm_led_highres.jal. As you'll see, there are very few differences.

Now let's dive into the loop...

forever loop
   var byte i
   i = 0
   -- loop up and down, to produce different duty cycle
   while i < 250 loop
      pwm1_set_dutycycle(i)
      _usec_delay(10000)
      i = i + 1
   end loop
   while i > 0 loop
      pwm1_set_dutycycle(i)
      _usec_delay(10000)
      i = i - 1
   end loop
   -- turning off, the LED lights at max.
   _usec_delay(500000)
   pwm1_off()
   _usec_delay(500000)
   pwm1_on()

end loop

Quite easy right ? There are two main waves: one will light up the LED progressively (0 to 250), another will turn it off progressively (250 to 0). On each value, we set the duty cycle with pwm1_set_dutycycle(i) and wait a little so we, humans, can see the result.

About the result, how does this look like ?





"I wanna try, where are the files ?"

To run this sample, you'll need the last jallib pack (at least 0.2 version). You'll also find the exact code we used here.



Sébastien Lelong