Digilent Nexys2 question

[ryu2112]

Hi,

I was wondering if the Nexys2 I/O can accept 5.0v logic as opposed to 3.3v (specifically on the FX2 expansion port). I looked on their website but cannot find the answer. I can't even find a data sheet for board. Any help would be appreciated. thanks in advance.

Here is the board:
http://www.digilentinc.com/Products/Detail.cfm?Prod=NEXYS2

 

[Jack// ani]

Spartan 3E FPGA can not handle 5V (TTL logic) directly, Nexys2 board has nothing to do with this issue.

At best it will support 3.3V (LVTTL logic), so you have to figure out way to convert 5V logic level first.

Good luck

 

[Flurkje]

I was looking for the same info.
I'm not an expert on this but according to this link:
**broken link removed**

It should be possible to use the ports with 5V input if you place a 300 ohm resistor in series.

According to the Nexsys2 schema:
http://www.digilentinc.com/Data/Products/NEXYS2/Nexys2_sch.pdf

There is already a 200 ohm resistor present for shortcut protection. So I assume it's save to use the inputs for 5V as long as you add an extra 100 ohm resistor.

I'll let you know if I blow up my board

David

 

[Jack// ani]

I was looking for the same info.
I'm not an expert on this but according to this link:
**broken link removed**

It should be possible to use the ports with 5V input if you place a 300 ohm resistor in series.

According to the Nexsys2 schema:
http://www.digilentinc.com/Data/Products/NEXYS2/Nexys2_sch.pdf

There is already a 200 ohm resistor present for shortcut protection. So I assume it's save to use the inputs for 5V as long as you add an extra 100 ohm resistor.

I'll let you know if I blow up my board

David

Those 200 ohm series resistor are current limiting resistors at 3.3V.

That does not mean you can put 300 ohms series resistor and feed in 5V, unless until you want to fry it!

 

[mrflibble]

The trick is to properly use and abuse the clamp diodes on the iob's. The spartan-3e can easily take a 5 volt input as long as you make sure the current stays within acceptable limits. Hell, I've got a goodie right here that abuses it at 12 volt, BUT with a sensible resistor in front of it. The resistor burns most of it and the clamp diode clamps it down to Vcc + "a tiny bit".

If you don't want to believe me, read **broken link removed** closely. Also check the IBIS model of the spartan-3e, it has some useful clues on how to abuse the IOs.

Or let me put it another way by use of an example. The circuit I use has the spartan-3e configured as 2.5 Volt iob's. Which means the absolute maximum that is tolerated at the IO pad is 3.0 Volt. Suppose I try to put a higher voltage on the pin? Well, the internal clamp down will try to clamp it down to the 2.5 Vcc to the best of it's ability. The clamp diode will try to take care of the excess voltage. And will succeed as long as the current into the pad is not too high.

Just take the voltage you want to apply (5 volt), take your rated maximum (3.3 volt + 0.5 volt = 3.8 volt) and calculate how much a resistor would have to dissipate. This gives you a current, which you can use in the IBIS model to check how the clamp diode would do...

So in my case it takes 12 volt at fairly low switching speeds (25 kHz). A mere 5 volts should be too easy with I dunno, a 1 k resistor. Probably less, but too lazy right now.

 

[Flurkje]

Thanks, that confirms what I understood from the Xilinx support article.

You mention the speed that is required by your application. Is that because this 'voltage abuse' slows down the maximum speed of the port? In my application I need to process video signals, so that might be an issue.

David

 

[mrflibble]

You mention the speed that is required by your application. Is that because this 'voltage abuse' slows down the maximum speed of the port? In my application I need to process video signals, so that might be an issue.

Nope, it is just because 25 kHz switching is all the speed I need in that particular part of the circuit. It runs synchronous with a couple of mosfet drivers that PWM the coil currents in stepper motors. The 25 kHz is high enough to not be audible and have low current ripple in the coils. And it is low enough so I don't get excessive switching losses in the FETs, which keeps things nice and cool. I recall I tested it to 3.something MHz (50 MHz / 16) without problems.

I suspect you will get some speed degradation with the limiting resistors in place, so it would all depend on high speed your high speed is going to be.