Electronic – Altera Max10 3.3V interface

3.3vintel-fpgamax10programmable-logic

Regarding Altera's MAX10 Cpld, I got some questions regarding the interface of this CPLD with 3.3V devices.

I have set pins to 3.3V LVCMOS and I got this warning message in quartus:
"Warning (169177): 100 pins must meet Intel FPGA requirements for 3.3-, 3.0-, and 2.5-V interfaces. For more information, refer to AN 447: Interfacing MAX 10 Devices with 3.3/3.0/2.5-V LVTTL/LVCMOS I/O Systems."

Looking at the AN447 document, it mentions on page 2enter image description here

To my understanding, if VCCIO is set to 3V, there is no need for anything to be done when it comes to interfacing with a 3.3V device.

But that makes little sense to me, I don't understand the reason why when VCCIO = 3V, then all issues are solved:

  1. Why bothering using 3.3V as VCCIO then, why the trouble?
  2. Could anyone confirm I'll be fine interfacing directly with a 3.3V device when VCCIO = 3V?

Thanks for your inputs. I'm sure there is a big misunderstanding there and I want to clear that out.

Best Answer

With an un-terminated line that is long enough and driven fast enough, there is a potential for overshoots with voltage level high enough to exceed the rating of a MAX10 input.

There is a "PCI clamp diode" for each MAX10 I/O pin that can clamp the voltage. The clamp diode is connected between the pin and Vccio (AN447 figure 2). Therefore the voltage is clamped to Vccio + V_forward_of_diode. At Vccio=3.3V, the clamped voltage is still high enough to exceed the rating of the pins. At Vccio=3.0V, the clamped voltage is low enough.

With Vccio=3.0V, the MAX10 I/O should interface with 3.3V without issue. The Voh could be lower than when Vccio=3.3V, but it should still be high enough for 3.3V logic that I know of. Of course, verifying an interface matches in levels is part of the design process regardless of whether using 3.0 or 3.3V as Vccio.

Some possible reasons for using 3.3V instead of 3.0V:
3.3V is already required somewhere else, so using 3.0V creates a requirement for an extra voltage source.
Lines are short enough or not driven hard enough that overshoot is not an issue.
When overshoot can be an issue, controlling it with series termination may be preferred anyway because that also reduces EMI effects.