I am in a bit of a dilemma. We use ISA architecture here where I work that I would like to use an FPGA solution with. The only issue I am having is that all the FPGAs that have the resources to do what I need it to do run at 3.3V. I was wondering what the threshold for a logical HIGH and LOW signal voltages were. If I drive a signal (IO16 or Data bus) with the FPGA's 3.3V high, is that high enough to merit a HIGH voltage on the output?
Thanks, Matt
If it's this:
Then asfaik, ISA uses TTL, which should be voltage level compatible with LVTTL.
BUT, I only verified this work for Xilinx Spartan FPGAs. Where you can connect a 300 ohm resistor in series to get "5V tolerance". Just make sure the bus capacitance isn't bogging down your risetimes too much. If risetimes become an issue. You will have to use level translator chips. (For which you can search for using altavista/google etc..)
What hinders you from dropping the ISA bus altogether?
The ISA bus is a part of an expensive legacy system. A bus change will take place sometime down the road, but I am afraid I have no where the political power or money to suggest a bus change at the moment. The bus I am running is a 16-bit ISA. I started looking a bi-directional bus voltage translators but all the ones I found were 8-bit so I would have a large bank of these translators just to use and FPGA. I was hoping there may have been an easier way, but I may be stuck in doing just that. The FPGA I have found actually has about 30 pins that can be 5 Volt tolerant using a clamp diode. I was planning on using this for the address bus and the data lines and then use the bus translator for all the control lines if 3.3V is not considered a high on an ISA system.
Thanks for the input
Driving the bus, 3.3 should be a good high. But the fpga would have to be 3.3 volt tolerant in the input direction, as the motherboard or other boards may pull the bus lines higher, maybe all the way to +5. FPGA 5 volt tolerance is *not* the wave of the future. To be safe, insert some of those bus level translators, one of those SN74ALVC164245 sorts of gadgets. They also tend to make the FPGAs more reliable, protecting them from various insults.
John
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