Discussion of problem 6-20

Tristate Inverting Output Driver
Notice that the output pins of the TIBPAL16L8-25C are driven by tristate inverters. This has two consequences: 1.) You need to build the complementary function into the array in order to get correct outputs at the pins, and 2.) You need to enable the outputs.

1.) Building the complement into the array.
Suppose you want to create a three-input function like,

                                                f(A, B, C) = Σm(0,2,3,6)

and you want that to be the output. You need to program the negation of this into the array. Specifically you need to program the function shown below into the array.

                                                (A, B, C) = Σm(1,4,5,7)

Once you have identified the complementary function that needs to be programmed into the array, you should optimize it. A K-map is recommended. Then mark the appropriate fuses to program the terms needed.

2.) You need to enable the outputs
Since these are tristate outputs, you need to enable them. You can do this by connecting pin 1 to ground. The input buffer associated with pin 1 then provides a logic-1 on its inverted output. Use that logic-1 to enable outputs. Mark fuses accordingly.

An "X" mark keeps a fuse and makes a connection
On the fuse map, make an "X" mark over each intersection where you want a connection to remain. Leave other intersections unmarked. The fuses at unmarked intersections are burned out by the programming software, leaving behind only the desired connections.

Unused AND gates and Shortcut Notation
It is unlikely that you will use all the AND gates in the array. Unused AND gates must be programmed to produce a logic-0 at their outputs. This is usually done by leaving all fuses for that AND gate intact. This assures that the output of the AND gate will always be logic-0 since each input and the complement of each input is connected. To do this you could put "X" marks on every intersection along the input side of the unused AND gate. Writing all those "X" marks would be tedious however. There is a short-cut form of notation for this situation. Just put an "X" mark on an AND gate itself. That signifies that all the fuses on all the inputs for that AND gate should remain intact, just as if you had made "X" marks all along the row.

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