level translators

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Can I make level translaters to go between 5v and 3v logic using diodes like
this ?


5V --+--->|---->|-----+-- 3.3v
         |                          |
        +-----|<---------+


Rob





Re: level translators
I'm not sure I understand your ASCII schematic.  It looks like the 5V
driver on the left drives a 3.3V input on the right through 2 diodes
(-1.4V) that will possibly create a 3.6V output if the input is at 5V.
 If the 5V driver is a 0V the 3.3V input will float.  What is the
single diode below the top wire?  I would think that you need a pull
down resistor on the 3.3V input to finish the job but the signal will
not be allowed to toggle much faster than a MegaHertz.

Also,  If the driver is TTL or highly loaded (or fast switching) CMOS
the output may not get up to 5V so the two diode drops will
potentially not allow the 3.3V input to see the signal as high.  For a
cheap solution try:

                          3.3V supply
                           |
                         __|__
                           ^
                          / \   <--- 1N914 Diode
                           |
5V driver >----\/\/\/\/\/--+----> 3.3V input

                 270 Ohm resitor

Depending on the input capacitance of the 3.3V receiver this can run
at about 20 to 50 MHz.  If the receiver has input clamping diodes (you
can do some testing to find out) then you can omit the diode and just
put a resistor in series with your signal.

The more expensive but better performing solution is to buy a 74HCT245
type buffer and power it with the 3.3V supply.  Check the TI data
sheets as I'm not completly sure that the HCT is the family that I
remember can tolerate 5V inputs.

I hope this helps.

Ted.

P.S. You can see my crap at:

http://www.geocities.com/ted_rossin/Electronics/Electronics.html


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Re: level translators
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It probably won't work.  If the 3v side outputs LOW, the 5v side might
see something like 1.6v (not a low for most TTL/CMOS chips).  When the
5v side outputs a HIGH, the 3v side might be forced some 3.9v (depending
on the diodes).  Etc.

If you want a simple circuit that works for a prototype (not production),
use a simple series resistor, and make sure that the 5v side is "TTL
compatible" (eg 74HCT).

Else, build a level translator yourself.  74VHC/VHCT are good compontents
for this purpose.

Marc

Re: level translators
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Please forget such "circuits" and use level translators or 5V tolerant
3.3V gates instead, e.g. 74LVCxxx series (they are very cheap).


Regards,

--
Bernhard Roessmann
Don't Fear The Penguins!


Re: level translators
TI makes a whole series of chips for this - the LVC4245A should do this
for you nicely with no thought or simulation - the diodes will cost more
to mount than buying the chip, and board space will go down big time.

Andrew

Robert Finch wrote:

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Re: level translators
If your signal is uni-directional (one way only), a transceiver
is not required. Consider the 74LCX07, a non-inverting buffer
with 5-Volt-tolerant I/O.  With this 20-cent chip, I have interfaced:

  - a 5-Volt device to a 3.3-Volt Microcontoller, with uni-directional
    signals in and out.

  - a 3.3-Volt microcontroller to a 3.0-Volt device in a similar manner.

...here is how:

1. Power the 74LCX07 with the lower of the two voltages, i.e. 3.3 volts
    for a 3.3-to-5.0 Volt interface.

2. Run the signals from either (voltage) device into the 74LCX07.

3. Pull up the outputs (with a 10K or 33K resistor) from the 74LCX07,
    to 5 or 3.3 volts, depending on their destination.

That simple; cheap, and works good!

These parts are sourced by at least three different vendors, including
Fairchild, On Semiconductor, and Toshiba, as part of the "Low-Voltage
Logic Alliance," so you'll always have a source.

--
Douglas Beattie Jr.      http://www.hytherion.com/beattidp /


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Beware alliances
....

Check EXACTLY what the alliance is. Sometimes this can mean that each
company only SELLS in one part of the globe (Americas, Europe, Asia), the
parts may well be made by one company and STAMPED with whatever company.
In product lines like this you may well find they are each making a third
of the devices in wafer form at least even if they all could make them.
Each company is then selling into one region.

An example of this is Fujitsu and AMD with Flash chips. You can end up with
as soon as one company stops making a particular device they ALL stop making
at the same time!

The alliance is for the manufacturers' benefit NOT the customer.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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