7 replies to this topic

[Ryan Mick]

Still new at all this stuff, but I have encountered a situation where I need to run PWM at 32Khz. Problem is I have no idea how to calculate the correct period and duration to use with SetPulse. If someone can point me in the right direction I would greatly appreciate it. Thanks, Ryan

[Justinius]

Someone else will have to help with implementation. PWM runs at a set frequency so you would run at 32kHz. It would require you to change the duty cycle. 32kHz means you would have to create a new pulse every 3.125e-5 seconds. So a 50% duty cycle would have a hi value for half that. Whatever you are connecting with will have some information about what the different duty cycles mean on the that side.

[Ryan Mick]

From what I have read elsewhere on the forums PWM runs at a default 10 Khz and that changing duty cycle only affects the default value. But from what I read they said you can change the frequency by using the SetPulse method by giving it the appropriate period and duration.

Maybe some info on what I am trying to do will help. I have picked up a barometric sensor HP03 from Futurlec and it requires a 32 Khz clock input. So I really have 2 choices, figure out how to set a PWN line to 32 Khz or use a 32 Khz crystal. Since I am still new to electronic components and not sure how I would wire in a crystal I thought throwing code at it would be easiest at the moment. But to throw code at it I need to know how I can calculate period and duration. If the easy solution is to wire it to a crystal and someone can share the proper way to do so I would be very glad to hear it.

Thanks,
Ryan

Reference:
PWM Demystified

[Justinius]

So I did some reading. The SetPulse does take a period and duration. So period = 1/frequency, so you can set that. Although 32kHz is 31.25uS, and the SetPulse seems to only take integers. Then the duration would be a fraction of your set period. So if I set the period at 32uS and wanted a duty cycle of 50% I'd set the duration at 16uS. see: http://10rem.net/blo...-modulation-pwm Anyone else want to comment on this? Hope this helps.

[BitFlipper]

/// <summary>
/// Set the dutycycle of the 32kHz signal
/// </summary>
/// <param name="dutyCycle">A value between 0.0f and 100.0f</param>
private void SetPwm(float dutyCycle)
{
    var period = (uint)(1000000.0f / 32000.0f + 0.5f);
    var duration = (uint)(dutyCycle * 0.01f * period + 0.5f); 

    m_pwm.SetPulse(period, duration);
}

Unfortunately this will result in a period of 31.25, which when rounded will be 31, or an actual frequency of 32,258.06 Hz.


EDIT: OK I re-read your post, and seems you only need 50% duty cycle. In that case all you need to do is something like:

m_pwm.SetPulse(31, 15);

Once again unfortunately you can't get exactly 32kHz. I wonder how critical it is to be exactly 32kHz.

[Ryan Mick]

Thanks guys, that is the info I am looking for. I hear an future update to PWM is going to have the ability to set a frequency directly which will make this kind of stuff even easier. Thanks again, Ryan

[Justinius]

I hope these comments helped. If you get the sensor up and running document it in the Wiki, if you can. It would be a good guide.

[Ryan Mick]

Funny thing is I never noticed the wiki link at the top before. Will do, I am also working with the HH10D humidity which takes some extra steps to get working as well. When it's all working well I will put them both in the wiki.