Spartan 3 and 5V input

I think I wrote this a little too hastily.

I'm using the Digilent Spartan 3 starter board and would like to drive my design with the parallel port of my computer with as few (hopefully no) external parts as possible. All the IO banks are using 3.3V reference. If there's a particular configuration of the I/O pins that could tolerate a 5V input, I'd appreciate a short explanation. Meanwhile I'll keep reading the datasheet.

Thx,

-Nevo

Hm... I found

which lists the 5V tolerant SelectIO standards for the Spartan 2 family (and implicitly says none of the standards are 5V tolerant in the Spartan 2E family), but I'm unable to find a simlar reference for the Spartan 3 family.

Table 27, Absoulte Maximum Ratings under DC Electrical Characteristics, suggests that putting 5V into the chip is a no-no.

Hm; I think I'm going to answer my own question here by pointing myself to XAPP429 and a whole host of archived messages in comp.arch.fpga.

You need some sort of interface if you want to use 5V I/O with the Spartan 3. You have lots of options.

Leon

A 390-Ohm series resistor normally works ok. If you look at the starter kits that use Spartan3E you see that all the 5V stuff has this to connect to the FPGA.

If you don't fancy that then you can get a dedicated IC, like an LCX125... Beware that they're fairly slow, the resistor is better IMHO.

Good luck.

Ben

Nevo schrieb:

Right. Like this one:

(First google groups hit for "5v spartan-3")

So just use a series resistor.

Kolja Sulimma

Nevo, i applaud your ability to post (partial) answers to your own questions. More people need that ability. It shows the ability to recognize when you have bypassed the limits of your sure knowledge; and the perseverance to continue seeking yourself, instead of passively waiting for "an/the answer".

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

Hmm indeed. I'm glad you mentioned this, because I'd assumed the 2E and the 2 had the same 5v-tolerance characteristics... Having read ds077.pdf (Spartan-2E) as *well* as ds001.pdf (Spartan-2), I now realise my error...

Looks like I'll be using a -2 rather than a -2E for my interface-to-ancient-bus-logic :-)

Cheers, Simon