5 V inputs to 3.3 V CPLD

I'm working on a design with a Xilinx XC95XL series CPLD.  The inputs
would be coming from a system with a 5 V (possibly higher, I don't
have the exact number yet) output.  I need 3.3 V outputs, so using the
separate I/O power supply is not a good solution.  As a novice
engineer, I have a few ideas, but don't know which is best.  Is there
a problem with using a simple resistor divider to create lower
voltages?  Should I have to use a separate level shifter IC instead to
change the 5 V signals to 3.3 V?  Thanks,
Matt Cohen

a simple series resistor to limit the input current is sufficient for 5v
signals to 3.3v inputs assuming the 3.3v inputs are NOT already 5v
tolerant (if they are, then you don't even need the resistor).

Matt Cohen wrote:

> I'm working on a design with a Xilinx XC95XL series CPLD.  The inputs
> would be coming from a system with a 5 V (possibly higher, I don't
> have the exact number yet) output.  I need 3.3 V outputs, so using the
> separate I/O power supply is not a good solution.  As a novice
> engineer, I have a few ideas, but don't know which is best.  Is there
> a problem with using a simple resistor divider to create lower
> voltages?  Should I have to use a separate level shifter IC instead to
> change the 5 V signals to 3.3 V?  Thanks,
> Matt Cohen

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401/884-7930     Fax 401/884-7950
email ray@andraka.com
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  temporary safety deserve neither liberty nor safety."
                                          -Benjamin Franklin, 1759

Matt Cohen wrote:
> 
> I'm working on a design with a Xilinx XC95XL series CPLD.  The inputs
> would be coming from a system with a 5 V (possibly higher, I don't
> have the exact number yet) output.  I need 3.3 V outputs, so using the
> separate I/O power supply is not a good solution.  As a novice
> engineer, I have a few ideas, but don't know which is best.  Is there
> a problem with using a simple resistor divider to create lower
> voltages?  Should I have to use a separate level shifter IC instead to
> change the 5 V signals to 3.3 V?  Thanks,
> Matt Cohen

It depends on the details of the chips you are driving and the CPLD. 
Most 3.3 volt chips can tolerate 5 volt signals on their inputs.  If
your 5 volt signals are actually TTL level driven, the thresholds will
match well.  If they are CMOS level driven then they will likely work as
well, but heavy loading can cause the low state to rise above the
acceptable 0.4 volt on the input.  I doubt that you will have heavy
loads on your inputs.  

The output of 3.3 volt logic is what you say you need.  So if you power
the IOs from 3.3 volts and use a 5 volt tolerant CPLD, you should be in
good shape.  

If you really want to use a 5 volt supply on the CPLD, then the way you
interface to the output loads depends on the loads.  If the load devices
are not 5 volt tolerant, then you will need to add a series resistor or
even a divider as you say.  But if they are 5 volt tolerant, then you
need do nothing.  Check the data sheet on those devices.  

-- 

Rick "rickman" Collins

rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.

Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design      URL http://www.arius.com
4 King Ave                               301-682-7772 Voice
Frederick, MD 21701-3110                 301-682-7666 FAX

"Matt Cohen" <matthewlawrencecohen@yahoo.com> wrote in message
news:81fdc5f7.0406021212.49a57321@posting.google.com...
> I'm working on a design with a Xilinx XC95XL series CPLD.  The inputs
> would be coming from a system with a 5 V (possibly higher, I don't
> have the exact number yet) output.  I need 3.3 V outputs, so using the
> separate I/O power supply is not a good solution.  As a novice
> engineer, I have a few ideas, but don't know which is best.  Is there
> a problem with using a simple resistor divider to create lower
> voltages?  Should I have to use a separate level shifter IC instead to
> change the 5 V signals to 3.3 V?  Thanks,
> Matt Cohen

I think you are in luck. The datasheet for the XC9500XL series specifically
states that the inputs are 5V tolerant. In fact I am using one at the moment
that is working just like that!

James

"I think you are in luck. The datasheet for the XC9500XL series
specifically states that the inputs are 5V tolerant. In fact I am
using one at the moment that is working just like that!"

Yes.  Yes it does.  Somehow I missed that.  I just opened up the data
sheet to check, and the very first thing I saw was "5V tolerant I/O
pins accept 5V, 3.3V, and 2.5V signals".  Thanks a lot everybody.
Matt