The Spartan 3 device can be made 5V tolerant by using a series resistor to limit the amount of current flowing into a forward biased clamp diode, when the input voltage exceeds 4V.
The question I have is how does this affects the AC timing in light of a diode recovery time when going from forward to reverse voltage. Let's suppose the maximum forward current is 1 ma.
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Considering the diodes are always there, and they are normally clamping whenever there is overshoot and undershoot, the recovery time must be so short that it is of no consequence.
If you are concerned, I would download the hspice models, and simulate the condition you are interested in.
Aust> The Spartan 3 device can be made 5V tolerant by using a series
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