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

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Matt Cohen
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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.

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rickman

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Ray Andraka

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

Reply to
James Alston

"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

Reply to
Matt Cohen

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