Difference between VDD and VDDIO?

[illucius]

On a number of component data sheets I've been reading, I see two different voltage supplies, VDD and VDDIO. They are usually called the analog and digital supplies or the power and I/O power.

My question is what are these two different supplies for? I understand that these components may be made of analog and digital circuits, but why do they need to be supplied by different voltages and why are they usually rated for different voltages as well? Ie VDD is usually higher than VDDIO.

 

[keith1200rs]

In a CPLD, VDD will usually be the internal VDD which can be lower the VDD of the I/O. For example, you might have a CPLD core with 2.5V Vdd but want to interface to 3.3V logic. Faster logic on smaller geometries tends to be lower voltage - a lot are well under 2V now but you are still likely to need to interface to devices with a higher I/O voltage.

Keith.