5V inputs with series resistor on Spartan-3

Eric,

Spartan 3 has clamp diodes that always prevent the voltage on the pin from being more than a diode drop above Vcco (or below ground).

So, if Vcco is 3.3V, and the diode drop is ~ 0.5V at the current thru the 270 ohms, then the max V is 3.8V, well below the abs max limit of

4.05V in the data sheet.

But, take care, as if you have Vcco = 3.45V, then you are getting uncomfortably close to the 4.05V abs max limit.

As it turns out, the characterization work we have done has shown that the nmos gate is very robust, and the pmos gate is the one limited to the no greater than 4.05V restriction (in .25u IO used by V2P, S3, and V4). In order to stress the pmos gate, you would have to have a voltage below ground, AND a high Vcco! So for the 5V and a resistor case, even if you had 3.45V Vcco, it still does not represent a reliability issue with the nmos gate oxide.

I have stated here before, that the pmos stress >4.05V results in a leakage of ~ 10uA over time, and a weakening of the pmos output transistor by as much as 10%. This is not affected or accelerated by temperature. If the IO is always used as an input, then you don't care about the driver becoming weaker over time. If the input is thru a 270 ohm resistor from a 5V CMOS output, you probably also don't care about

10uA of input leakage. But, the leakage is an indication of gate oxide damage of the pmos pullup, so we can not recommend operating it under these conditions.

Aust> I was looking for information on driving Spartan-3 inputs from 5V CMOS

And CoolRunner-II do not?

Thats correct. CoolRunner-II devices do not have clamp diodes...

Thanks, Mark

Eric Smith wrote:

If I want to drive a Spartan 3 input from 5V TTL (not CMOS), such as an SN74LS14, do I need a series resistor, and if so, what value? The data sheet gives a typical output voltage of 3.4V at -0.4 mA (which is the recommended maximum Ioh). They don't give a maximum Voh. Since it's bipolar, it's obviously less than the supply rail. I suppose I could size the series resistor as if it were a 5.5V CMOS output, but I'd prefer to use a lower value if it can be guaranteed not to exceed the maximum current of the S3 clamp diode.

Thanks, Eric

Eric,

I would run a spice or IBIS simualtion of the driver into an open.

Then you would know.

Aust> Aust>

Is 74LS still available? Why would you use it and who still makes it?

In theory, you are probably OK electrically. In practice you need to watch the production to make sure no one does a 'Heck, 74LVC14 (etc) are the same as 74LS14', replacement :), plus LS14 have to be getting near EOL....

-jg