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From: email@example.com (carltons)
Subject: Re: Shifting TTL to 5V or 3.3V CMOS levels
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Fri, 27 Sep 2002 19:44:50 GMT
Date: Fri, 27 Sep 2002 19:44:50 GMT
In article <3D9486FE.2529F574@webaccess.net>, Chuck Simmons
> markp wrote:
> > "Sir Charles W. Shults III" wrote in message
> > news:Sq_k9.6977$O8.firstname.lastname@example.org...
> > > I can think of a hundred ways to do this- start with a simple voltage
> > > divider. You can use a pair of resistors to cut the logic levels down to
> > 3.3
> > > volts easily. Now think "switchable". MOSFET on the upper leg perhaps?
> > Or a
> > > simple driver?
> > > There are even chips that will to it easily enough and some are old
> > style
> > > CMOS, but I don't know what your switching times are going to be..
> > >
> > > Cheers!
> > >
> > > Chip Shults
> > > My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
> > Thanks, but there are 16 (actually 17) of them, and I need to translate from
> > TTL to 5V CMOS or 3.3V CMOS outputs on all, so adding discrete devices for
> > each signal on the output is not really practical (I might as well use a
> > level shifting buffer which I don't want to do). The resistor divider may
> > also work for really slow things but the resistance needed to keep the power
> > down would slug the line too much.
> > The logic devices such as HCT translate to 5V CMOS directly from TTL, but
> > are specified 4.5V - 5.5V for Vcc, so can't use them for 3.3V (Actually, it
> > may be possible with some families to run at 3.3V but the input thresholds
> > are then not specified in the datasheets). Switching times needed are around
> > 60ns max so not too restrictive. Most CMOS type devices are specified
> > 0.3*Vdd for Vil and 0.7*Vdd for Vih, which puts it at 3.5V for 5V Vcc, and I
> > only have TTL levels coming in!
> > There might be a logic family out there specified for this, or even a small
> > PLD with selectable I/O voltage..
> > Mark.
> A lot of 3.3 volt CMOS devices are 5 volt tolerant. In one iteration of
> a board design (a large board with mostly 3.3 volt parts) I have 3.3
> volt devices, one 3.6 volt device and a good many TTL and 74F devices.
> The 3.3 volt stuff all was designed with 5 volt tolerant inputs so I had
> no problem. 3.3 volt CMOS drives TTL inputs just fine and 5 volt
> tolerant inputs really are. The mix is not ideal but the interface is
> IDE (5 volt) and the supplies are +5 and +12. The IDE is driven with a
> 3.3 volt part and meets specification.
What are your clock speeds? If you are going down in voltage, use a 5v
tolerant part like recommended before in the thread. If you are going up,
look out and email me, please. This can be a pain.
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