Re: parallel port and cyclone?

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While the Cyclone device is not directly compliant to 5V interfaces,
there are some possible techniques for solving this problem. I don't
know the exact requirements for a 5V parallel interface, but the
following applies to 5V interfaces in general. Let's consider the
input and the output sides:

Input (driving to the Cyclone device):
The key here is that the Vin must never exceed 4.1 V (the maximum
Cyclone device recommended operating conditions show in the
datasheet). With a 5V driver this can be a problem. However, you can
use a simple external circuit to ensure that Vin is below 4.1V. The
basic idea is that you use a series resistor and a diode connected to
the 3.3V supply. The diode limits the maximum excursion of the Vin,
and the resistor limits the current from the 5V driver through the
diode to the 3.3V supply. We have published a design based on this
technique for APEX 20KE devices, shown here:
We have not worked out the details for Cyclone devices, but the same
basic idea holds.

Another option is to use a QuickSwitch device from IDT, as shown in
this document:

Output (driving from Cyclone):
This may be much simpler. 3.3V LVTTL and 5.0V TTL share the same VOH
and VOL specs. Cyclone devices can drive 3.3V LVTTL. So if the
parallel port uses 5V TTL specs then there's no problem. However, if
it uses 5V CMOS, then there is an issue since the VIH for 5V CMOS is
higher than the VOH for 3.3V LVTTL. You can't simply use a pullup
resistor to 5V since you then are pulling the Cyclone pin too high as
well. So if the interface is 5V CMOS then you would probably need an
external buffer.

You also may want to consider using an ACEX 1K or FLEX 10KE device.
The advantage here is that it is natively 5V-tolerant, so there's
nothing special to do. The output is still LVTTL, so driving 5V TTL is
no problem but driving 5V CMOS would require a buffer. The driving
factor behind this choice would be performance, density, perhaps
price. The biggest Cyclone device is bigger than the biggest ACEX 1K
or FLEX 10KE device, which may be relevant to your design.

One of the options discussed here ought to implement what you need.
Have fun!

Greg Steinke
Altera Corporation

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