Connecting XEM Board with PC parallel port


Hi everybody,
Does anyone know if it is possible and safe to connect // port pins to some IOs, from say JP2 or JP3 ?
What i have in mind is signal level : // port is TTL : is TTL compatible with LVTTL ? does the xem board accept +5V ?

Thanks in advance.


The Spartan-3 on the XEM3001 is not 5v tolerant. You will need some sort of level translation to provide the conversion to and from the 3.3v levels. Maxim, TI, IDT, and others make a number of level translation ICs that will work for 3.3v to 5v.


Coming back on this subject : i read on some internet forums that it is possible to drive Spartan3 with TTL (i.e logic ‘1’ ~5 v ) just by inserting a 100 Ohms resistor in serie which limits the clamping diode current at 10 ma ?

What do you think (i know your first & official answer is : DONT DO THAT)

I really need to quickly try something.


The FPGA inputs sink a certain amount of current (I’m not sure what it is). You can put a series resistor there so as to drop the voltage enough to avoid the clamping condition.

Unfortunately, I have not seen anything from Xilinx that says what values to use to do this correctly. They -do- suggest a method in their datasheet for dealing with the 2.5v AUX-powered pins from a 3.3v supply, but as far as I know, that’s the only info they give.

Your best bet is to use a proper translator like the MAX3372-MAX3393 or something.


Yes i already think to drop voltage using a resistor but i assessed that the current is only … 60 nA ! ! so to drop voltage of 1.5 V i need a 25 Mega Ohms resistor ???

I am surprised that an input pins only consumes far less than 1µA ???

By the way, i measured my // port outputs voltage and tehlogic ‘1’ is only …3.35 V !!!

(but in fact i need the voltage transaltion for another reason : to link the XEM with a TTL circuit )


I made it with // port ( PC // port output --> XEM 3001 input ) : OK, it works !
I however advise anyone to first check // port output Voltage before proceeding, i guess voltage level could depend on actual circuitry.


Well i did an accurate current sink measurement : about 3 µA under 3.5 V so Spartan3 IO offers roughly an input impedance of 1 Mega Ohm.
So when i put a 500 KOhms in serial , the input voltage is multiplied by 0.66.
==> this allow to go from 5V to 3.5V.:slight_smile:

PS : the previous 60na measurement was made with a measurement device which was out of battery…:smiley: