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74LVT transition times: How low can you go?

Started by Joerg February 28, 2007
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.

-- 
Regards, Joerg

http://www.analogconsultants.com
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.
Sounds reasonable to me. Not a lot can go wrong here. John
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." - -jg
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.
100mV/ns and 1/2 &#2013266101;s are specified. I'd go with that solution. -- Johannes You can have it: Quick, Accurate, Inexpensive. Pick two.
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? -- Regards, Joerg http://www.analogconsultants.com
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?
Attention: He is talking about lvC and you about lvT! -- Johannes You can have it: Quick, Accurate, Inexpensive. Pick two.

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. But the 200ns seems like an awful long time. Why would you need that? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
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 ? -jg
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 -- WBR, Yuriy. "Liberalism is a mental disorder"
John Larkin wrote:
> <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. > > Sounds reasonable to me. Not a lot can go wrong here.
Is that a CMOS package? If so, slow transition times will seriously increase the power consumption, and (if excessive) can actually destroy the chip. The reason is that at intermediate levels both the pull-up and pull-down components are on, and are fighting each other. -- Chuck F (cbfalconer at maineline dot net) Available for consulting/temporary embedded and systems. <http://cbfalconer.home.att.net>