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AR# 1987

8.1i CPLD - How do I preserve, keep, save, or prohibit software from using certain pins?

Description

Keywords: 7.1i,6.3i, 6.2i, 6.1i, 5.2i, 5.1i, 4.2i, 4.1i, 3.1i, reserve, XC9500, XC9500XL, XC9500XV, CoolRunner XPLA3, CoolRunner-II

I want to reserve particular I/O pins on the device for future use. Logic is not yet connected to these pins in my design, but I still want the software to use these pins for any other signals.

解决方案

1

The Prohibit attribute performs this functionality. (This constraint is available for all families.)
This attribute prevents the tools from mapping any I/O to the specified pin.

Beginning in 6.1i, the Assign Package Pins process provides a graphical method for using the Prohibit attribute. You can prohibit pins by selecting Tools -> Prohibit Mode, and then clicking any pin that you want to preserve. To undo a prohibited pin, select Allow Mode.

For previous versions of ISE, you must add this attribute in the User Constraint File (UCF). In the UCF, add a line for each pin that you want to preserve.

For example:

CONFIG PROHIBIT = P26;
CONFIG PROHIBIT = P17;

2

If you want to avoid using the UCF, you can try this alternate method:

To preserve a given I/O pin, use the following circuit:

Schematic
Schematic


The software does not trim this logic out because it does not interpret the logic associated with the pin as sourceless or loadless. Also, since it is "used," it does not route any other signals to this pin. As a result, the pin is permanently placed in a 3-state condition. Apply the appropriate LOC attribute to preserve the desired pin.

For other common CPLD questions, please see the CPLD Tech Tips FAQ:
http://www.xilinx.com/xlnx/xil_tt_faq.jsp?iLanguageID=1&sProduct=Xilinx+CPLDs
AR# 1987
创建日期 03/05/1997
Last Updated 04/26/2006
状态 Active
Type 综合文章