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Subject: Re: Shifting TTL to 5V or 3.3V CMOS levels
Date: Fri, 27 Sep 2002 22:53:13 +0100
References: <3D9486FE.2529F574@webaccess.net> <3D94C243.4F866D5B@webaccess.net>
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"Chuck Simmons" wrote in message
> carltons wrote:
> > In article <3D9486FE.2529F574@webaccess.net>, Chuck Simmons
> > wrote:
> > > markp wrote:
> > > >
> > > > "Sir Charles W. Shults III" wrote in message
> > > > news:Sq_k9.6977$O8.email@example.com...
> > > > > I can think of a hundred ways to do this- start with a simple
> > > > > divider. You can use a pair of resistors to cut the logic levels
> > > > 3.3
> > > > > volts easily. Now think "switchable". MOSFET on the upper leg
> > > > Or a
> > > > > simple driver?
> > > > > There are even chips that will to it easily enough and some
> > > > 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 -
> > > >
> > > > Thanks, but there are 16 (actually 17) of them, and I need to
> > > > TTL to 5V CMOS or 3.3V CMOS outputs on all, so adding discrete
> > > > each signal on the output is not really practical (I might as well
> > > > level shifting buffer which I don't want to do). The resistor
> > > > also work for really slow things but the resistance needed to keep
> > > > down would slug the line too much.
> > > >
> > > > The logic devices such as HCT translate to 5V CMOS directly from
> > > > are specified 4.5V - 5.5V for Vcc, so can't use them for 3.3V
> > > > may be possible with some families to run at 3.3V but the input
> > > > are then not specified in the datasheets). Switching times needed
> > > > 60ns max so not too restrictive. Most CMOS type devices are
> > > > 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
> > > > PLD with selectable I/O voltage..
> > > >
> > > > Mark.
> > >
> > > A lot of 3.3 volt CMOS devices are 5 volt tolerant. In one iteration
> > > 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
> > > 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.
> > >
> > > Chuck
> > 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
> > look out and email me, please. This can be a pain.
> > Steve
> We are running pretty slow. We do have another part (3.3 volt) that does
> ATA100 but we are content on my project with ATAPI at 33MHz. The clocks
> are synthesized and are rather arbitrary. The part I mostly work with
> uses 40MHz for a base clock. I have a lot of mixed signal which limits
> clocks. For example, I run my A/D SAR at 20MHz for a leisurely 0.6usec
> 10 bit conversion. The board is large. I have more trouble with distance
> than anything. I can drive a 40MHz clock about 6 inches. Beyond that
> (and I don't need it) I will have to do transmission lines. The 3.3 volt
> sags a lot with distance (cannot drive other 3.3 volt parts). Series
> terminators at the driving end can do wonders (see the ATA100
> specification for an example - the whole damned bus).
I think I've solved my problems! I've decided to use a Philips 74LVC4245A,
which is a level shifting bi-directional buffer. The great thing about it is
1.5V-3.6V on one port, and 1.5V-5.5V on the other (assuming Vcca >= Vccb).
Tpd is around 6.5ns. Check out the specs:
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