Design a 3 to 8 line decoder using two 2 to 4 line decoder and explain it
The 3 X 8 decoder constructed with two 2 X 4 decoders figure shows how decoders with enable inputs can be connected to form a larger decoder. Two 2-to-4-line decoders are combined to achieve a 3-to-8-line decoder. The two least significant bits of the input are connected to both decoders. The most significant bit is connected to the enable input of one decoder and through an inverter to the enable input of the other decoder.
It is assumed that each decoder is enabled when its E input is equal to 1. When E is equal to 0, the decoder is disabled and all its outputs are in the O level. When A, = 0, the upper decoder is enabled and the lower is disabled. The lower decoder outputs become inactive with all outputs at 0. The outputs of the upper decoder generate outputs Do through D3, depending on the values of A1 and A0 (while A, = 0). When A, = 1, the lower decoder is enabled and the upper is disabled. The lower decoder output generates the binary equivalent D4 through D, since these binary numbers have a 1 in the A, position.