I am studying a lesson on decoders, when I came across this exercise. I do have a brief understanding on decoders and what they do, but I don't understand this example in particular:
We have available 74138 decoders and wish to select one of 32 devices.
(What is meant by this?)
We would need four such decoders.
(On what basis did they assume we needed four decoders?)
Typically, one of these would select one of the first eight addressed devices; another would select one of the next eight, and so forth.
(what are the first eight addressed devices?)
Thus, if the address were given by bits a, b, c, d, e, then c, d, e would be the inputs (to C, B, A in order) for each of the four decoders, and a, b would be used to enable the appropriate one.
(On what basis did we decide we needed this many bits?)
Thus, the first decoder would be enabled when a b 0, the second when a = 0 and b = 1, the third when a = 1 and b = 0, and the fourth when a = b = 1.
Since we have two active low enable inputs and one active high enable, only the fourth decoder would require a NOT gate for the enable input assuming a and b are not available. (The answer is attached to this post)
Any help is greatly appreciated!
We have available 74138 decoders and wish to select one of 32 devices.
(What is meant by this?)
We would need four such decoders.
(On what basis did they assume we needed four decoders?)
Typically, one of these would select one of the first eight addressed devices; another would select one of the next eight, and so forth.
(what are the first eight addressed devices?)
Thus, if the address were given by bits a, b, c, d, e, then c, d, e would be the inputs (to C, B, A in order) for each of the four decoders, and a, b would be used to enable the appropriate one.
(On what basis did we decide we needed this many bits?)
Thus, the first decoder would be enabled when a b 0, the second when a = 0 and b = 1, the third when a = 1 and b = 0, and the fourth when a = b = 1.
Since we have two active low enable inputs and one active high enable, only the fourth decoder would require a NOT gate for the enable input assuming a and b are not available. (The answer is attached to this post)
Any help is greatly appreciated!
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