On Thu, 01 Mar 2007 00:45:53 GMT, Joerg <notthisjoergsch@removethispacbell.net> wrote:>Specsmanship seems to be on the decline. Philips/NXP is usually top >notch but the family guide for their LVT series is, gasp, three pages >short. Information about maximum transition times on inputs: Zilch. > >In an embedded application I need to slow down the /OE of a 74LVT244 so >it turns tri-state fast but goes onto the bus slower, to avoid a brief >contention when addresses change. Is it ok for that family to slow /OE >by 200nsec or so via RC? It'll be the usual two resistor, one diode and >one cap deal. Want to avoid adding another Schmitt here.Sure is a _crude_ way to get a timing delay. Why not do it right ?:-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
74LVT transition times: How low can you go?
Started by ●February 28, 2007
Reply by ●March 1, 20072007-03-01
Reply by ●March 1, 20072007-03-01
"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message news:5npFh.5760$re4.636@newssvr12.news.prodigy.net...> Specsmanship seems to be on the decline. Philips/NXP is usually top notch > but the family guide for their LVT series is, gasp, three pages short. > Information about maximum transition times on inputs: Zilch. > > > -- > Regards, Joerg > > http://www.analogconsultants.comThis is standard practice when divisions of companies like Siemens or Philips get sold off, broken up or reincarnated. Ultimately all the useful data which you used to able to download as a single big book gets split up into lots of little pieces that are then smeared about the new abortion of a website and in the process a lot of it just disappears. It's either job creation, stupidity or job creation for stupid people. The final goal is to force you to contact an application stupid person using a stupid web form that is invariably hidden behind a registration stupid web form. If you want the information you might try it but invariably you'll become Mr Bum Bollocks Bum Bollocks & Co Ltd Bum Street Bollockshampton Just so you can say something like...... Re your p/n 123XYZ I have just placed my order for 24,999,678 units with Not Your Company Ltd Kindest ETC Mr Bollocks
Reply by ●March 1, 20072007-03-01
Jim Granville wrote:> Joerg wrote: > >> Jim Granville wrote: >> >>> Joerg wrote: >>> >>>> Specsmanship seems to be on the decline. Philips/NXP is usually top >>>> notch but the family guide for their LVT series is, gasp, three >>>> pages short. Information about maximum transition times on inputs: >>>> Zilch. >>>> >>>> In an embedded application I need to slow down the /OE of a 74LVT244 >>>> so it turns tri-state fast but goes onto the bus slower, to avoid a >>>> brief contention when addresses change. Is it ok for that family to >>>> slow /OE by 200nsec or so via RC? It'll be the usual two resistor, >>>> one diode and one cap deal. Want to avoid adding another Schmitt here. >>> >>> >>> >>> >>> Hi Joerg, >>> >>> Philips used to claim Schmitt Ips on these ? (well, the LVC series ) >>> (but not everyone does..) >>> >>> Their LVC244 data says this: >>> (as does the LVC2244A, which we have just used ) >>> >>> " Schmitt-trigger action at all inputs makes the circuit highly >>> tolerant for slower input rise and fall times." - >>> >> >> Hmm, interesting, where did you find that? > > > Try page 2 of this > http://www.standardics.nxp.com/products/lvc/pdf/74lvc2244a.pdf > > I think the LVC is the more modern family, with the LVT being phased > out ? >Are your sure the LVT is going? I am not too familiar with modern logic, as an analog guy I usually get away with 74HC and CD4000. Anyhow, this LVC driver doesn't look like Schmitt, it says 10nsec max transisitions on the inputs when using it at 3.3V. Our bus will not even be that fast. On purpose, for EMI reasons. -- Regards, Joerg http://www.analogconsultants.com
Reply by ●March 1, 20072007-03-01
Vladimir Vassilevsky wrote:> > > Joerg wrote: > > >> In an embedded application I need to slow down the /OE of a 74LVT244 >> so it turns tri-state fast but goes onto the bus slower, to avoid a >> brief contention when addresses change. Is it ok for that family to >> slow /OE by 200nsec or so via RC? It'll be the usual two resistor, one >> diode and one cap deal. Want to avoid adding another Schmitt here. > > > You can make a delay using something like 1G97.I could also do it with a 74HC14 but I wanted to avoid more chips.> But the 200ns seems like an awful long time. Why would you need that? >I might get away with 100nsec. There is going to be some intricate address decoding, more than just a 688 and a 154. -- Regards, Joerg http://www.analogconsultants.com
Reply by ●March 1, 20072007-03-01
Yuriy K. wrote:> Joerg wrote: > >> Specsmanship seems to be on the decline. Philips/NXP is usually top >> notch but the family guide for their LVT series is, gasp, three pages >> short. Information about maximum transition times on inputs: Zilch. > > > http://www.google.com/search?hl=en&q=phillips+lvt&btnG=Google+Search > > http://www.standardics.nxp.com/support/documents/logic/pdf/family.lvt.specification.pdf > > ... > RECOMMENDED OPERATING CONDITIONS > ... > Dt/Dv Input transition rise or fall rate; Outputs enabled : <10 ns/V > ... > >> In an embedded application I need to slow down the /OE of a 74LVT244 >> so it turns tri-state fast but goes onto the bus slower, to avoid a >> brief contention when addresses change. Is it ok for that family to >> slow /OE by 200nsec or so via RC? It'll be the usual two resistor, one >> diode and one cap deal. Want to avoid adding another Schmitt here. > > > Not according to specs. > > http://www.standardics.nxp.com/products/lvt/pdf/74lvt2244.pdf > ... > 9. Recommended operating conditions: > ... > Dt/DV input transition rise and fall rate; outputs enabled : <10 ns/V >I had the LVT244 in mind (sans terminator), not the LVT2244. For some reason the NXP server doesn't find its data sheet anymore this morning. Arrgh. Well, at least Digikey has a few thousand of them. -- Regards, Joerg http://www.analogconsultants.com
Reply by ●March 1, 20072007-03-01
Jim Thompson wrote:> On Thu, 01 Mar 2007 00:45:53 GMT, Joerg > <notthisjoergsch@removethispacbell.net> wrote: > > >>Specsmanship seems to be on the decline. Philips/NXP is usually top >>notch but the family guide for their LVT series is, gasp, three pages >>short. Information about maximum transition times on inputs: Zilch. >> >>In an embedded application I need to slow down the /OE of a 74LVT244 so >>it turns tri-state fast but goes onto the bus slower, to avoid a brief >>contention when addresses change. Is it ok for that family to slow /OE >>by 200nsec or so via RC? It'll be the usual two resistor, one diode and >>one cap deal. Want to avoid adding another Schmitt here. > > > Sure is a _crude_ way to get a timing delay. Why not do it right ?:-) >Yeah, I know. If it had Schmitts it would be ok. Guess it ain't... -- Regards, Joerg http://www.analogconsultants.com
Reply by ●March 1, 20072007-03-01
Genome wrote:> "Joerg" <notthisjoergsch@removethispacbell.net> wrote in message > news:5npFh.5760$re4.636@newssvr12.news.prodigy.net... > >>Specsmanship seems to be on the decline. Philips/NXP is usually top notch >>but the family guide for their LVT series is, gasp, three pages short. >>Information about maximum transition times on inputs: Zilch. >> >> >>-- >>Regards, Joerg >> >>http://www.analogconsultants.com > > > This is standard practice when divisions of companies like Siemens or > Philips get sold off, broken up or reincarnated. Ultimately all the useful > data which you used to able to download as a single big book gets split up > into lots of little pieces that are then smeared about the new abortion of a > website and in the process a lot of it just disappears. > > It's either job creation, stupidity or job creation for stupid people. The > final goal is to force you to contact an application stupid person using a > stupid web form that is invariably hidden behind a registration stupid web > form. >Yeah, it's sad. This moring the NXP server has become unable to even find the LVT244 datasheet. &*#^!!. Link broken. Sometimes it is unbelievable what people do to a formerly well-oiled machinery. What a great company this had been in the 80's. Sad.> If you want the information you might try it but invariably you'll become > > Mr Bum Bollocks > Bum Bollocks & Co Ltd > Bum Street > Bollockshampton > > Just so you can say something like...... > > Re your p/n 123XYZ > > I have just placed my order for 24,999,678 units with Not Your Company Ltd > > Kindest ETC > > Mr Bollocks >I stopped writing to top brass about serious issues a company has. Because they rarely listen. So I just quietly skadaddle over to the competition. -- Regards, Joerg http://www.analogconsultants.com
Reply by ●March 1, 20072007-03-01
Joerg wrote:> Jim Granville wrote: > >>>> " Schmitt-trigger action at all inputs makes the circuit highly >>>> tolerant for slower input rise and fall times." - >>>> >>> >>> Hmm, interesting, where did you find that? >> >> >> >> Try page 2 of this >> http://www.standardics.nxp.com/products/lvc/pdf/74lvc2244a.pdf >> >> I think the LVC is the more modern family, with the LVT being phased >> out ? >> > > Are your sure the LVT is going? I am not too familiar with modern logic, > as an analog guy I usually get away with 74HC and CD4000.Look at the dates on the data sheets, and the lack of a 'new' category on their web site. There were too many low voltage logics, and they seem to be slowly rationalising to two wide voltage familes, the LVC and the AUP> Anyhow, this LVC driver doesn't look like Schmitt, it says 10nsec max > transisitions on the inputs when using it at 3.3V. Our bus will not even > be that fast. On purpose, for EMI reasons.It explicitly says so in the data (above) - so where you have a conflict like this, grab a device and try it :) Often they still spec a Tr/Tf, in order for all the timing to be valid. eg if they allowed a slow data rise, and a slow clock rise, then the threshold match comes into play - so they give a test tr/tf, which mitigates threshold effects, and makes all the ns specs valid. Just try one: Ramp the inputs, and measure the Icc and output - there will be a Icc/Vin relationship, that is not always given, and on some devices that can be what I'd call poor. Probably does not matter in your design. -jg
Reply by ●March 1, 20072007-03-01
Jim Granville wrote:> Joerg wrote: > >> Jim Granville wrote: >> >>>>> " Schmitt-trigger action at all inputs makes the circuit highly >>>>> tolerant for slower input rise and fall times." - >>>>> >>>> >>>> Hmm, interesting, where did you find that? >>> >>> >>> >>> >>> Try page 2 of this >>> http://www.standardics.nxp.com/products/lvc/pdf/74lvc2244a.pdf >>> >>> I think the LVC is the more modern family, with the LVT being phased >>> out ? >>> >> >> Are your sure the LVT is going? I am not too familiar with modern >> logic, as an analog guy I usually get away with 74HC and CD4000. > > > Look at the dates on the data sheets, and the lack of a 'new' category > on their web site. > > There were too many low voltage logics, and they seem to be slowly > rationalising to two wide voltage familes, the LVC and the AUP >Thanks for the info, always good to know what's on the way to becoming unobtanium.> >> Anyhow, this LVC driver doesn't look like Schmitt, it says 10nsec max >> transisitions on the inputs when using it at 3.3V. Our bus will not >> even be that fast. On purpose, for EMI reasons. > > > It explicitly says so in the data (above) - so where you have a conflict > like this, grab a device and try it :) > > Often they still spec a Tr/Tf, in order for all the timing to be valid. > eg if they allowed a slow data rise, and a slow clock rise, then the > threshold match comes into play - so they give a test tr/tf, which > mitigates threshold effects, and makes all the ns specs valid. > > Just try one: Ramp the inputs, and measure the Icc and output - there > will be a Icc/Vin relationship, that is not always given, and on > some devices that can be what I'd call poor. Probably does not matter > in your design. >Or maybe I just place a couple Schmitt inverters to, as Jim put it, "do it right". -- Regards, Joerg http://www.analogconsultants.com
Reply by ●March 1, 20072007-03-01
Joerg wrote:> > Or maybe I just place a couple Schmitt inverters to, as Jim put it, "do > it right".You could, (would cover more vendors), but to this Jim, using a part with hysteresis IS 'doing it right'. We've just designed in a Philips LVC2244 for that reason. To my mind, all logic should have hysteresis by default, but I do note that the new universal gates 1G57/58/97/98 all have hystersis. ( and the better CPLDs now have it selectable by pin ) If you need to start using 'fixup gates', have a look at those universal gate series. With one of those, you should be able to save 2 if your passives. -jg
