Hysteresis on dsPIC ports?
in [Design]
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From: Jan Panteltje on 26 Jan 2010 18:10 On a sunny day (Tue, 26 Jan 2010 12:17:19 -0800) it happened Joerg <invalid(a)invalid.invalid> wrote in <7s90ulFookU1(a)mid.individual.net>: >Jan Panteltje wrote: >> On a sunny day (Tue, 26 Jan 2010 11:17:17 -0800) it happened Joerg >> <invalid(a)invalid.invalid> wrote in <7s8te0F2reU2(a)mid.individual.net>: >> >>> Jan Panteltje wrote: >>>> On a sunny day (Tue, 26 Jan 2010 10:28:05 -0800) it happened Joerg >>>> <invalid(a)invalid.invalid> wrote in <7s8qhqFig4U1(a)mid.individual.net>: >>>> >>>>>> it clearly shows min and max levels for the logical inputs. >>>>>> What more do you want? >>>>>> ftp://panteltje.com/pub/pic_io.jpg >>>>>> >>>>> This is what I want and what other mfgs provide: A min-max guaranteed >>>>> _spec_ for the hysteresis. >>>> >>>> No, that datasheet says: if the level is above .8 * Vdd then it will be a one, >>>> and if below .2 * Vdd it will be a zero. >>>> So you have < .2 * Vdd noise margin from ground and .8 * Vdd noise margin from Vdd. >>>> >>> Unfortunately that is not what such a spec says. It essentially says >>> that they don't guarantee anything if you are between 0.2*VDD and >>> 0.8*VDD. That has nothing to do with hysteresis. And most definitely not >>> with noise margin. >> >> No, you do not interpret this right. >> It says the input line noise can peak, for a 5V supply, >> when input is high, to Vdd .2 x 5 = 1 V below Vdd, so to >4V, >> and the noise on the input can peak to < +1 V when at zero. >> When noise stays below or above those values, the input will not flip, >> and the hysteresis will not happen. >> > >Again, this does _not_ guarantee the input won't switch to low at, say, >1.3V or high at 3.5V. It only says that they don't guarantee it will >switch there but it could (and probably does). It has nothing to do with >hysteresis and noise margin. I bet Jim could elucidate. > > >>> Well, the guy at Microchip saw it but could not help. >> >> Because there is nothing top help, you do not understand the spec. >> > >Then why did they not write back that the hysteresis is 0.2*VDD to >0.8*VDD? One of the reasons is that this would still not specify a >min-max hysteresis. Which you need in order to calculate noise margins >properly. OK, I see your point, you also want a minimum for input low going up, and a maximum for input high going down. But the minimum for input low going up is .8 *Vdd, and the maximum for input high going down is .2 * Vdd :-) Nothing else is guaranteed here. Note that that does not exclude the Schmitt trigger effect. thateffect only means it wil flip over and not come back until the otehr border value is reached. Say you start at zero with a positive ramp, it is guaranteed to flip over before you reach 4 V (for 5 V supply), if you then ramp down, it is guaranteed to flip back before you reach 1 V. The real flip over points could be 2 and 3 V... But those have nothing to do with your noise margin, UNLESS you have noise on the input while it is transitioning to cover both limits. I would say in such a case you need a better driving circuit..
From: Joerg on 26 Jan 2010 18:19 Jim Thompson wrote: > On Tue, 26 Jan 2010 14:50:56 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> Jim Thompson wrote: >>> On Tue, 26 Jan 2010 13:55:54 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> > [snip] >>>> Microchip is a little more cautious there, with 0.2 and 0.8. But that >>>> does not guarantee that it won't switch at 0.3 or even 0.4. >>> I searched for "hysteresis". Nowhere is that mentioned. I'll ask. >>> >>> But I'm pretty sure what I'll find is it's simply to clean up slow >>> inputs. >>> >> Usually that's the reason, reduces the "oh s..t, it don't work" support >> requests. But if you need it for noise immunity purposes the hysteresis >> would have to be known. That's where I must say that TI is a lot more >> diligent in their specs. Any old thunderstorm would blow a 50mV >> hysteresis aside but not a 500mV one. > > I'd expect about a VT (or around 0.35V) for this sort of animal. > That would be great. >> Right now about half my >> assignments are redesigns. Not always fun but it's got to be done. > > A lot of my work is fixing the errors/blunders of others. > Same here but it's not errors per se. Mostly just people looking for a 2nd opinion and then optimizing this, that and the other thing. Very good engineers but it's always best to have folks outside the team reviewing stuff. If all companies would do that we'd have a lot less in schedule slips and re-design rounds. Sometimes it's younger engineers. For example many have never designed a switch mode converter in their life and the prospect of having to do a fairly big one right now drives up their neck hair (it should). > Somewhat shocking, I sent a blanket E-mail to about a dozen names I > knew from my Microchip oscillator days. Four of them came back > "unknown" :-( > After only two years? Ouch, that is not so good. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 26 Jan 2010 18:35 Jan Panteltje wrote: > On a sunny day (Tue, 26 Jan 2010 12:17:19 -0800) it happened Joerg > <invalid(a)invalid.invalid> wrote in <7s90ulFookU1(a)mid.individual.net>: > >> Jan Panteltje wrote: >>> On a sunny day (Tue, 26 Jan 2010 11:17:17 -0800) it happened Joerg >>> <invalid(a)invalid.invalid> wrote in <7s8te0F2reU2(a)mid.individual.net>: >>> >>>> Jan Panteltje wrote: >>>>> On a sunny day (Tue, 26 Jan 2010 10:28:05 -0800) it happened Joerg >>>>> <invalid(a)invalid.invalid> wrote in <7s8qhqFig4U1(a)mid.individual.net>: >>>>> >>>>>>> it clearly shows min and max levels for the logical inputs. >>>>>>> What more do you want? >>>>>>> ftp://panteltje.com/pub/pic_io.jpg >>>>>>> >>>>>> This is what I want and what other mfgs provide: A min-max guaranteed >>>>>> _spec_ for the hysteresis. >>>>> No, that datasheet says: if the level is above .8 * Vdd then it will be a one, >>>>> and if below .2 * Vdd it will be a zero. >>>>> So you have < .2 * Vdd noise margin from ground and .8 * Vdd noise margin from Vdd. >>>>> >>>> Unfortunately that is not what such a spec says. It essentially says >>>> that they don't guarantee anything if you are between 0.2*VDD and >>>> 0.8*VDD. That has nothing to do with hysteresis. And most definitely not >>>> with noise margin. >>> No, you do not interpret this right. >>> It says the input line noise can peak, for a 5V supply, >>> when input is high, to Vdd .2 x 5 = 1 V below Vdd, so to >4V, >>> and the noise on the input can peak to < +1 V when at zero. >>> When noise stays below or above those values, the input will not flip, >>> and the hysteresis will not happen. >>> >> Again, this does _not_ guarantee the input won't switch to low at, say, >> 1.3V or high at 3.5V. It only says that they don't guarantee it will >> switch there but it could (and probably does). It has nothing to do with >> hysteresis and noise margin. I bet Jim could elucidate. >> >> >>>> Well, the guy at Microchip saw it but could not help. >>> Because there is nothing top help, you do not understand the spec. >>> >> Then why did they not write back that the hysteresis is 0.2*VDD to >> 0.8*VDD? One of the reasons is that this would still not specify a >> min-max hysteresis. Which you need in order to calculate noise margins >> properly. > > OK, I see your point, you also want a minimum for input low going up, > and a maximum for input high going down. > > But the minimum for input low going up is .8 *Vdd, > and the maximum for input high going down is .2 * Vdd > :-) > Nothing else is guaranteed here. > Note that that does not exclude the Schmitt trigger effect. > thateffect only means it wil flip over and not come back until the otehr border value is reached. That's what it doesn't mean, they do not state anything that says it will not flip until 0.x*VDD. > Say you start at zero with a positive ramp, > it is guaranteed to flip over before you reach 4 V (for 5 V supply), > if you then ramp down, it is guaranteed to flip back before you reach 1 V. > The real flip over points could be 2 and 3 V... > But those have nothing to do with your noise margin, UNLESS you > have noise on the input while it is transitioning to cover both limits. Aha, now you've got it :-) > I would say in such a case you need a better driving circuit.. > With a good micro controller you do not. As I showed the MSP430 specs the hysteresis properly so you can rely on it. You can probably also rely on it with the dsPIC but they failed to spec it. All one (mostly) needs in such cases is the minimum hysteresis. The maximum doesn't matter if you guarantee to get under 0.2*VDD and above 0.8*VDD. Easy to say "oh just throw in an extra Schmitt up front". But in consumer goods every penny gets turned around and around. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 26 Jan 2010 18:38 On Tue, 26 Jan 2010 15:19:31 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Jim Thompson wrote: >> On Tue, 26 Jan 2010 14:50:56 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> Jim Thompson wrote: >>>> On Tue, 26 Jan 2010 13:55:54 -0800, Joerg <invalid(a)invalid.invalid> >>>> wrote: >>>> >> [snip] >>>>> Microchip is a little more cautious there, with 0.2 and 0.8. But that >>>>> does not guarantee that it won't switch at 0.3 or even 0.4. >>>> I searched for "hysteresis". Nowhere is that mentioned. I'll ask. >>>> >>>> But I'm pretty sure what I'll find is it's simply to clean up slow >>>> inputs. >>>> >>> Usually that's the reason, reduces the "oh s..t, it don't work" support >>> requests. But if you need it for noise immunity purposes the hysteresis >>> would have to be known. That's where I must say that TI is a lot more >>> diligent in their specs. Any old thunderstorm would blow a 50mV >>> hysteresis aside but not a 500mV one. >> >> I'd expect about a VT (or around 0.35V) for this sort of animal. >> > >That would be great. > > >>> Right now about half my >>> assignments are redesigns. Not always fun but it's got to be done. >> >> A lot of my work is fixing the errors/blunders of others. >> > >Same here but it's not errors per se. Mostly just people looking for a >2nd opinion and then optimizing this, that and the other thing. Very >good engineers but it's always best to have folks outside the team >reviewing stuff. If all companies would do that we'd have a lot less in >schedule slips and re-design rounds. > >Sometimes it's younger engineers. For example many have never designed a >switch mode converter in their life and the prospect of having to do a >fairly big one right now drives up their neck hair (it should). > > >> Somewhat shocking, I sent a blanket E-mail to about a dozen names I >> knew from my Microchip oscillator days. Four of them came back >> "unknown" :-( >> > >After only two years? Ouch, that is not so good. Actually it was 8 years (2002). Time flies when you're having fun ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Tim Williams on 26 Jan 2010 19:13
"Joerg" <invalid(a)invalid.invalid> wrote in message news:7s8o8iF2o8U1(a)mid.individual.net... > Blowing off a client with a remark along the lines of "the sales guys will > dig into that if your sales volume is deemed worthy" is, ahem, not a very > polite way to deal with their most important part of biz, the sales > channel. If you merely told them your name, surely they would recognize your potential sales? ;-) I think Joel was alluding more to the fact that, regardless of sales numbers, the average user needs to be spoon-fed... imagine Microsoft help lines! They're basically paid to RTFM aloud. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |