UPGRADE YOUR BROWSER

We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

AR# 22424

LogiCORE CAM v5.1 - How do I use the "U' value for the enhanced ternary mode?

Description


General Description:
The v5.1 CAM LogiCore has two ternary modes. The standard mode uses the ternary values "0", "1", and "X". The enhanced mode increases functionality by providing the "U" value option. What functionality is added with the "U" value?

Solution


In general, the "U" value is supported for consistency and flexibility. However, the main function of this value is to clear the content of a specified address to an un-initialized state during run time.

Initially, unless the CAM is initialized with a COE file, the CAM contains only U's (un-initialized content). Consequently, regardless of the data entered, the CAM will never find a match because all CAM contents are un-initialized. Once the CAM contains initialized content, you can then write U's to an address. This write transaction deletes the entry at the specified address and returns it to an un-initialized state. Consequently, this address location will not match any value on a successive read.

When comparing the data on the read interface of the CAM with the CAM contents, matching occurs as follows on EACH bit:

Read Interface || CAM Contents || Result
MASK | DIN | VALUE || MASK | DIN | VALUE || Match | Notes
0 | 0 | X || 0 | 0 | X || 1 | X will match a X
0 | 1 | 1 || 0 | 0 | X || 1 | X will match a 1
1 | 0 | 0 || 0 | 0 | X || 1 | X will match a 0
1 | 1 | U || 0 | 0 | X || 0 | U never matches anything
0 | 0 | X || 0 | 1 | 1 || 1 | X will match a 1
0 | 1 | 1 || 0 | 1 | 1 || 1 | 1 will match a 1
1 | 0 | 0 || 0 | 1 | 1 || 0 | 0 will not match a 1
1 | 1 | U || 0 | 1 | 1 || 0 | U never matches anything
0 | 0 | X || 1 | 0 | 0 || X | X will match a 0
0 | 1 | 1 || 1 | 0 | 0 || X | 1 will not match a 0
1 | 0 | 0 || 1 | 0 | 0 || X | 0 will match a 0
1 | 1 | U || 1 | 0 | 0 || 0 | U never matches anything
0 | 0 | X || 1 | 1 | U || 0 | U never matches anything
0 | 1 | 1 || 1 | 1 | U || 0 | U never matches anything
1 | 0 | 0 || 1 | 1 | U || 0 | U never matches anything
1 | 1 | U || 1 | 1 | U || 0 | U never matches anything
AR# 22424
Date Created 09/04/2007
Last Updated 12/15/2012
Status Active
Type General Article