How to generate PWM signal?

winson · Joined: 03 Dec 2008 Posts: 24

Hi,

Anybody can point me to a right direction with some sample code regarding how to generate a PWM signal? Actually i was trying to generate a 38kHz PWM signal as the carrier for my UART IR transmission, then when need to transmit data i will make the 38KHz PWM signal ON/OFF to drive an infrared LED and communicate with the IR receiver.

I was using PIC16F877A, and i haved read through the PWM section of the data sheet but just not so understand about how to set the register.

Anybody can help?

Thanks.

Ttelmah · Guest

Start by calculating the divisor needed to give your frequency. Take (for example), if you have a 8MHz clock. Then the timers run off this divided by 4, so the total divisor, becomes:

(8000000/4)/38000 = 52.63

Now a number of things apply. If the divisor is above 256, you will need to use a prescaler. Three are available, up to /16. If the value is above 4096, then the required frequency is not possible with that crystal.
Then, you can only use integers, so the nearest will be /53, giving 37735Hz.

So then you setup timer2, with:

setup_timer2(T2_DIV_BY_1,52,1);

The first value is a constant, selecting the prescaler (1, 4, 16), the second number, is the required divisor minus one. The third doesn't matter for this.

Then to enable the PWM, use:

setup_ccp1(CCP_PWM);

This for the first PWM, or ccp2 for the second.

Then to output a squarewave, use:

set_pwm1_duty(106L);

The value here should be the divisor * 2. You must include the 'L'.

To set the output low, with no pulse, use:

set_pwm1_duty(0);

Best Wishes

winson · Joined: 03 Dec 2008 Posts: 24

Thanks for reply,

This explanation really very clear. So just have a simple question, if i want to produce a PWM signal with 50% duty cycle then i just need to write like below?

Ttelmah · Guest

106L, was for the 50% duty cycle (squarewave).
The maximum count useable, is the divisor*4 (because the PWM, actually uses the master clock at it's 'heart', though the timer uses this /4). So 50%, is half this.

Best Wishes

winson · Joined: 03 Dec 2008 Posts: 24

Oh I see, thank you very much for the explanation. Very Happy

winson · Joined: 03 Dec 2008 Posts: 24

Hi everybody,

I have succesfull generate the 38KHz PWM square wave and it look nice, but after i add in the PWM code to my program, the LCD which is control by the same PIC was not working correctly and it is ok without the PWM code added. The LCD was display the same character two times like "good" it will become "ggooood".

So anybody know why this is happening? It look like affect by the PWM.

thanks

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

In this post, you explain that you use a 9v battery with a current
limiting resistor to supply power to the board. This causes problems
with the voltage every time you change the load on battery:
http://www.ccsinfo.com/forum/viewtopic.php?t=37598&start=3
Have you fixed this problem by adding a 5v voltage regulator I.C.
to your board ? If not, you should do so.

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

I second the clear explanation Ttelmah provided. But if you want to double check you calculations you can check here: http://www.micro-examples.com/public/microex-navig/doc/097-pwm-calculator

Also you can look for Mr.e's multicalc program. You can find it here: http://www.picbasic.org/forum/attachment.php?attachmentid=3283&d=1234787673

Guest · Posted: Thu Feb 19, 2009 9:20 am

winson · Joined: 03 Dec 2008 Posts: 24

winson · Joined: 03 Dec 2008 Posts: 24

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

So are you using 9V through a limiting resistor to power the LCD?
Or are you using 5V (from regulator) to power LCD?
Or 5V (from regulator) to power LCD through limiting resistor?

In any case you should drop the limiting resistor alltogether. The LCD wants to see a voltage, once you supply the voltage it will sink (i.e. use) as much current as it needs. If the LCD datasheet says it sinks 3mA, then that is what it will sink (no matter how much is available).

The regulator is there to provide a constant voltage (5V for the 7805). It will do so under load conditions, up to a maximum of 700mA (based on your numbers). So it will provide a stable 5V for anything that draws (i.e. uses) anywhere from 0mA to 700mA (there are thermal issues to consider, but I will keep this simple). I believe you are thinkin the regulator will try to source 700mA no matter what, and that is not the case.

So for your application, you should power the LCD directly from the regulator (no resistor of any kind required). Now you should have a 0.1uF (rough value) on the LCD supply line to filter any noise which may be getting to it. You should have a similar thing on the PIC's supply line.

Since the LCD does not draw much current, you could also use a zener diode, but the regulator is the best approach by far.

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

If I follow PCM programmer logic here, and this is usually a good thing, this post and the one here (http://www.ccsinfo.com/forum/viewtopic.php?t=37598) are for the same thing. Correct?

It appears you are using the PWM/CCP pins (RC1 and RC2) for both PWM and LCD functions:

winson · Joined: 03 Dec 2008 Posts: 24

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

If you still need help with this, why don't you post a small (not 1400 lines) program which includes the LCD and PWM program sections which demonstrate the problem.