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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Shifting TTL to 5V or 3.3V CMOS levels
Date: Fri, 27 Sep 2002 16:27:18 GMT
NNTP-Posting-Date: Fri, 27 Sep 2002 09:27:18 PDT
> "Sir Charles W. Shults III" wrote in message
> > 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
> > 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
> > 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..
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.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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