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An example of converting an equation to minterm format to derive the select
code.
_
D = AB+AC (Starting equation)
_ _ _
D = AB(C+C)+AC(B+B) (Multiplying by 1)
_ _ __
D = ABC + ABC + ABC + ABC (Final minterms)
The final form contains only terms that contain all of the input sources.
These are the minterms you use. These minterms are selected with the minterm
enable bits LF7-LF0 as shown below:
_ _ __ _ _ _ __ ___
ABC ABC ABC ABC ABC ABC ABC ABC Available minterms
1 1 0 0 1 0 1 0 BLTCOM0 bits LF 7-0 Binary
C A LF 7-0 Hex
Below is a Venn diagram to aid in selecting minterms. The diagram shows a set
of three squares A, B and C. In the diagram, the numers 0 through 7 in various
areas correspond to the minterm number LF 7-0 above.
0
BBBBBBBBBBBBBBBBBBBBB
AAAAAAAAAAAAAAAAAAAAA B
A B A B
A 4 B 6 A 2 B
A B A B
A CCCCCCCBCCCCCACCCCCCC B
A C B A C B
A C 5 B 7 A 3 C B
A C B A C B
A C BBBBBBABBBBBBBBBBBBBB
AAAAAAAAAAAAAAAAAAAAA C
C C
C 1 C
C C
C C
CCCCCCCCCCCCCCCCCCCCC
To select a function D = A (that is, destination = A source only), you can
select only the minterms that are totally enclosed by the A-square. In this
case minterms 7,6,5,4. When written as a set of 1's for the selected minterms
and 0's for those not selected. the value becomes:
7 6 5 4 3 2 1 0 Minterm Numbers
1 1 1 1 0 0 0 0 Selected Minterms
F 0 Hex
If you wish to select a combination of two sources, you look for the minterms
enclosed by both squares in their common area. Example, the combination AB
( A "and" B) is represented by the area common to both the A and B squares.
This area encloses both minterms 7 and 6.
7 6 5 4 3 2 1 0 Minterm Numbers
1 1 0 0 0 0 0 0 Selected Minterms
C 0 Hex
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If you wish to use a function that is "not" one of the sources ( A ), you
take all of the minterms not enclosed by the A-square.
To combine minterms, you need only "or" them together. Example AB+BC results
in:
AB = 1 1 0 0 1 0 0 0
BC = 1 0 0 0 1 0 0 0
---------------
1 1 0 0 1 0 0 0 = $C8
Hope this helps
Bob
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