On Wed, 25 Nov 2009, Joachim Thiemann wrote:
I'm planning to use the Xilinx chips with my old
Commodore stuff too,
and one of the problems is the use of 3.3V rather than 5V logic. The
official recommendation from Xilinx for their Spartan devices is to
use simple resistor for connecting 5V out to 3.3V in, and just connect
straight-through for 3.3V out to 5V in. I have no idea how I'd handle
a bidirectional port though; is a resistor good enough or is a level
shifter needed?
Resistors are fine for low speed/low drive situations.
A good choice for higher speed/high drive applications is one of the passive
bus switches (made by Pericom etc). They are nice because theres almost no
propagation delay and no direction signals involved (they are bi-directional).
They are basically arrays of NMOS series transistors that just turn off when
the input voltage gets too high (and they lose gate drive)
Peter Wallace