Virtex-4 driving a 5V CMOS

Hi all,

I need to drive a 5V CMOS input from a 2.5V Virtex-4 bank. Is there anything wrong with simply using a pullup to 5V? The speed doesn't matter.

Thanks.

Reply to
fpgauser
Loading thread data ...

The output transistors are not rated for 5 Volt.

Look for thre zillions of articles about level translation...

--
Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
 Click to see the full signature
Reply to
Uwe Bonnes

Depends - is that 5V device rated with TTL thresholds, or CMOS ones ? A virtex OP will not go to 5V, but an open drain one will pull one diode-clamp above 2.5V, and a TTL pin usually has a span of < 0.8V and > 2.0V, with a typical actual threshold of ~1.3V

-jg

Reply to
-jg

Most 5-volt cmos parts will draw input-stage shoot-through current if the input isn't close to the 5-volt rails. Some 5-volt parts will get quite hot if Vhigh is, say, 3.3.

John

Reply to
John Larkin

Generally ir won't work because the clamp diode on the I/O will limit the pullup to a diode drop above the 2.5V i.e. circa 3.2V. You could put a series resistor to the I/O and then pullup but you will need to be careful of the values as they will form a potential divider network. but provided the current through the clamp diode is limited it is usually ok. I think Xilinx have one or more applications note on this. Alternatively put a FET, or single gate logic chip, or bus switch in the way as they are nicer ways to achieve the result. You can see lots examples of bus switches used for this function in a number of our products.

John Adair Enterpoint Ltd. - Home of Darnaw1. The PGA FPGA Solution.

Reply to
John Adair

This is not recommended practice... the FPGA's protection diodes aren't going to be happy. If you only have 1 signal, and you don't care if it's slow, you can just use a SMD transistor to make an open collector/drain from your FPGA output, or an HCT IC, or an HCT picogate... or even simpler replace your

5V CMOS IC by an HCT if possible. If you need something more elaborate, there are zillions of voltage translator chips...
Reply to
PFC

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.