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Subject: Re: Shifting TTL to 5V or 3.3V CMOS levels
Date: Fri, 27 Sep 2002 17:10:36 +0100
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"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?
> 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..
> 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..
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