Since there are 8 inputs it is called as octal input and since there are three outputs it's also called binary output.Required number of 2:4 Decoder for 3:8 Decoder = 8/4 = Required number of Lower Order Decoder = m2 / m1 Where, m2 -> number of outputs for lower order Decoder m1 -> number of outputs for higher order Decoder In our case, the value of m1 will be 4 and the value of m2 will be 8, so applying these values in the above formulae we get.Thus, this enable can be used as a switch by many devices in order to keep this device on only in the time of need From the above Boolean expressions, the implementation of 3 to 8 decoder circuit can be done with the help of three NOT gates & 8-three input AND gates Entity decoder is port ( x : in std_logic_vector (2 down too) Y : out std_logic_vector ( 0 down to 7 ) en : in std_logic) End decoders Architecture behavioral of decoder is signal Y 1 : std_logic_vector (7 down to 0) Begin Y1 the output becomes zero no matter the input. Mean to say, If E equals to 0 then the decoder would be considered as disabled regardless of what inputs are, If E equals to 1 then the decoder would work as per inputs From the above truth table of 3 lines to 8 line decoder, the logic expression can be defined as. E input can be considered as a control input. The three inputs A, B and C are decoded into eight outputs, each output representing one of the midterms of the 3-input variables Now, it turns to construct the truth table for 3 to 8 decoder. Using the above expressions, the circuit of a 3 to 8 decoder can be implemented using three NOT gates and eight 3-input AND gates as shown in fig (1). Home Truth table for an active low 3 to 8 decoder
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