Re: parallel port and cyclone?

Michele, 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:

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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:

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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!

Sincerely, Greg Steinke snipped-for-privacy@altera.com Altera Corporation

I want to interface altera cyclone(3.3V) to the pc parallel port(5V). Is it > possible without using further external devices? > > Thanks
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Greg Steinke
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