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Question & Answer
Classification:
Date: Mar 19, 2001
A simple rule-of-thumb to prevent permanent disabling of the XSV interface CPLD
Q:
I'm wondering if anyone has an SVF or source code for the XC95108 that will maintain the standard Parallel port functionality and also pass data to the Virtex from the Serial port over the flash ram bus. I've written RS232 HDL before, but given that if I make a mistake it'll render the entire board useless I'm rather wary about attempting to do this integration myself. All i really need is a pass through, I can do the serial processing myself on the Virtex.
A:
Some people never read the manual and they end up programming the CPLD so that their board becomes inoperable, while others read the instructions and become overly cautious. I can't make the first group of people smarter, but I can give the second group of people a rule-of-thumb that they can follow that will always keep them out of CPLD trouble. Here it is:
RULE: Never configure pins 63, 71, 73, or 78 as outputs.
If you follow this rule, then you can never block the JTAG programming signals that go to the CPLD. So you can always reprogram the CPLD and you will never lose control of the XSV Board.
There are ways to violate the rule and still regain control of the CPLD. You can figure it out if you look at the schematic and know how the CPLD is configured through the parallel port. But the simple rule given above is the easiest to follow.