Post by thread:[PIC]: PWM stabilty

16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap. With the registers holding a static value, would you expect the output voltage to be stable? Its acting as if the lsb's are toggling. _________________________________________________________________ MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*. http://join.msn.com/?page=features/virus -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu>

On Friday 11 April 2003 06:58 am, Micro Eng wrote: > 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap. > > With the registers holding a static value, would you expect the > output voltage to be stable? Its acting as if the lsb's are > toggling. What frequency PWM? You're going to see some ripple on the output of the filter, anyway. What does it look like on a scope? -- Ned Konz http://bike-nomad.com GPG key ID: BEEA7EFE -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu>

>> 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap. >> >> With the registers holding a static value, would you expect the >> output voltage to be stable? Its acting as if the lsb's are >> toggling. > >What frequency PWM? You're going to see some ripple on the output of >the filter, anyway. What does it look like on a scope? That is my reaction too. That RC does not sound like it will filter too much even if the PIC is running at 20MHz, with a 10 bit PWM. -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu>

> 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap. > > With the registers holding a static value, would you expect the output > voltage to be stable? Its acting as if the lsb's are toggling. The filter rolloff is about 1.6KHz, so a little ripple should be expected depending on what your PWM frequency is. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu>

Hi Olin. Olin Lathrop wrote: > > > 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap. > > > > With the registers holding a static value, would you expect the output > > voltage to be stable? Its acting as if the lsb's are toggling. > > The filter rolloff is about 1.6KHz, so a little ripple should be expected > depending on what your PWM frequency is. How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula ) WBR Dmitry. -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu>

At 04:42 PM 4/11/03 +0400, Dmitriy A. Kiryashov wrote: >Hi Olin. > >Olin Lathrop wrote: > > > > The filter rolloff is about 1.6KHz, so a little ripple should be expected > > depending on what your PWM frequency is. > >How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula ) 1 / (2 * Pi * R * C) dwayne -- Dwayne Reid <dwaynerspam_OUTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 19 years of Engineering Innovation (1984 - 2003) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-requestKILLspammitvma.mit.edu>

Dmitriy A. Kiryashov said: > How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula ) Hi Dimitri, f = 1 / (2*pi*R*C) f = hertz pi = 3.14159265.... R = ohms C = farads That gives you the half-power (-3db) point. Cheers, Bob -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu>

> How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula ) The rolloff frequency (-3dB point) of a single pole R/C filter is F = 1 / (2Pi * R * C) where F is the frequency in hertz, R the resistance in ohms, and C the capacitance in farads. This also means that F * R * C = 1 / 2Pi This is such a handy formula that I keep 1,000,000 / 2Pi = 159,155 permanently in register 7 of my calculator. I can start with the register 7 value and divide it by any two of the frequency in herz, the resistance in ohms, or the capacitance in microfarads to get the third value. Very handy. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu>

Got it. I was confused with cut off frequency ( not -3 db ) Thanks guys. ;) WBR Dmitry. Bob Blick wrote: {Quote hidden}> > Dmitriy A. Kiryashov said: > > > How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula ) > > Hi Dimitri, > > f = 1 / (2*pi*R*C) > > f = hertz > pi = 3.14159265.... > R = ohms > C = farads > > That gives you the half-power (-3db) point. > > Cheers, > > Bob > > -- > http://www.piclist.com hint: To leave the PICList > spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Thank you Olin. ;) WBR Dmitry. Olin Lathrop wrote: > F * R * C = 1 / 2Pi > > This is such a handy formula that I keep 1,000,000 / 2Pi = 159,155 > permanently in register 7 of my calculator. I can start with the register > 7 value and divide it by any two of the frequency in herz, the resistance > in ohms, or the capacitance in microfarads to get the third value. Very > handy. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Dmitriy A. Kiryashov wrote: {Quote hidden}> > Thank you Olin. ;) > > WBR Dmitry. > > > Olin Lathrop wrote: > > > F * R * C = 1 / 2Pi > > > > This is such a handy formula that I keep 1,000,000 / 2Pi = 159,155 > > permanently in register 7 of my calculator. I can start with the > register > > 7 value and divide it by any two of the frequency in herz, the > resistance > > in ohms, or the capacitance in microfarads to get the third value. > Very > > handy. > Hey Dima, keep on asking about formula. I suspect that RC circuit discharges according to the exponential low. (When input voltage is constant, of cause.) It was so long ago at a school I was told about this. The original posting was about ripple on capacitor that is fed from PWM output over resistor. When the ripple is small compared to amplitude of voltage over resistor then the ripple curve should look exponentially, I think. By definition R and C should discharge to 1/e value over 1 second. Where did guys get 2*Pi, I don't know. To estimate the ripple very roughly, we may keep in mind the next: over 0.1*RC PWM cycle voltage change would be very aprox V/2.7/10 (0.2v when V=5v) Don't blame me hard if I'm wrong: Have been developing some software under MS tools over last years. Mike Singer. -------------------- P.S. I use not to use calculators at all when estimating such timings with non-sinusoidal signals. Very, very handy(or non-handy, hands-free) :-) -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> Got it. I was confused with cut off frequency ( not -3 db ) The -3dB point is often called the "cutoff frequency", although I prefer the term rolloff frequency. The former implies a sudden transition, which this certainly is not. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> By definition R and C should discharge to 1/e value > over 1 second. Where did guys get 2*Pi, I don't know. That is correct. Note that when the R*C time constants is significantly longer than the period of the incoming square wave, the result approaches a triagle wave (the exponential becomes a line because it doesn't have time to change slope). > To estimate the ripple very roughly, we may keep in > mind the next: over 0.1*RC PWM cycle voltage change > would be very aprox V/2.7/10 (0.2v when V=5v) Right idea, but the details are a bit off. Assuming square wave input, the output will always be "near" 2.5V. Also, you have 2 exponentials per cycle, one for each half of the square wave. I haven't checked your calculations, but these two factors should make the ripple 1/4 of what you computed. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

I wonder, if the best of the best student and fast-learner could calculate the ripple when PWM-ing RC? By the way, headhunters love to ask this type very basic questions. Mike. Marc Nicholas wrote: {Quote hidden}> Subject: Re: [AD]: Co-op student for hire (me)! > > Jai... > > Don't forget that you happen to attend one of the best EE/comp sci > schools > in North America ;-) > > > -marc > > On 14/4/03 13:16, "Jai Dhar" <TakeThisOuTjdharEraseMEspam_OUTENGMAIL.UWATERLOO.CA> wrote: > > > [snip] > > > >> Comps > >> I can understand, most of them will never touch real hardware, but > EE's? > >> TTYL > > > > I beg to differ. In 2nd year Comp (not EE), we have already done > wirewrap (and > > breadboard), and have PLENTY of real hardware to go. Maybe the > programs have > > changed over the years (I am sure they have somewhat), but our > program is > > focused around mixing software and hardware, not just computers... > so we still > > have plenty of "real" hardware exposure ahead of us. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Saturday 12 April 2003 12:42 am, Mike Singer wrote: > By definition R and C should discharge to 1/e value > over 1 second. Where did guys get 2*Pi, I don't know. If you look at the frequency response of the RC filter, you get an expression with the frequency in radians per second in the denominator. To get from cycles/second (Hz) to radians/second you need 2 * pi... So the 3dB down point (in Hz) of the RC filter is at 1/(2*pi*RC) or at 1/RC, if you want to talk in terms of radians/second. -- Ned Konz http://bike-nomad.com GPG key ID: BEEA7EFE -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Ned Konz wrote: > On Saturday 12 April 2003 12:42 am, Mike Singer wrote: > > By definition R and C should discharge to 1/e value > > over 1 second. Where did guys get 2*Pi, I don't know. > > If you look at the frequency response of the RC filter, you get an > expression with the frequency in radians per second in the > denominator. > > To get from cycles/second (Hz) to radians/second you need 2 * pi... > > So the 3dB down point (in Hz) of the RC filter is at 1/(2*pi*RC) > > or at 1/RC, if you want to talk in terms of radians/second. This isn't sinusoidal signal, at first. At second, what should be done with all these things to figure out the value of a ripple? Mike. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservEraseME.....mitvma.mit.edu with SET PICList DIGEST in the body