Cascading Decoder Circuits
There can possibly be a situation where the desired number of input and output lines is not available in IC decoders. More than one of these devices of a given size may be used to construct a decoder that can handle a larger number of input and output lines. For instance, 3-to-8 line decoders can be used to construct 4-to-16 or 5-to-32 or even larger decoder circuits. The basic steps to be followed to carry out the design are as follows:
1. If n is the number of input lines in the available decoder and N is the number of input lines in the desired decoder, then the number of individual decoders required to construct the desired decoder circuit would be 2N−n.
2. Connect the less significant bits of the input lines of the desired decoder to the input lines of the available decoder.
3. The left-over bits of the input lines of the desired decoder circuit are used to enable or disable the individual decoders.
4. The output lines of the individual decoders together constitute the output lines, with the outputs of the less significant decoder constituting the less significant output lines and those of the higher– order decoders constituting the more significant output lines. , which gives the design of a 4-to-16 decoder using 3-to-8 decoders.