Thread: developing 8 to 256 bit decoder

will u help me in developing 8 to 256 bit decoder using 3 to 8 bit decoder circuits?

u can make it in many ways
for that u have to know what is internal structure of the decoder
means in 3 to 8 bit decoder consists of 8 no.s of 3-input and gates or nand gates and 3 inverter gates
and what u want to do ,it needs 256+8 inputs
and u have to specify which types of other gates(two i/p or 3 i/ps and gates or nand gates)
etc. u need then u can make it
first tell me with what gates,decoders etc. u need to make that ckt. or u need to make the ckt. using only 3to8 decoders and how much no.s of that?????

There is really only one way to do this assuming your decoder is standard and has 3 select pins and one enable pin.
Work backwards and start with the output. You will need 32 decoders to produce 256 outputs. Each of these decoders will use 3 bits of your 8 bit input, say I0,I1 and I2.

Next you will need to control the enable pins of your 32 decoders. For this you will need 4 more decoders. Each of these decoders will use 3 more bits of your 8 bit input, I3, I4 and I5. The outputs of these 4 decoders will give you 32 enable controls.

Finally you will need to control the enable pins of your group of 4 decoders. For this you will need 1 more decoder. This decoder will use the last 2 bits of your 8 bit input, I6 and I7 and the third input selector for this decoder will just be grounded. The outputs of this decoder will give you 4 outputs which will control the enable pins in the group of 4 decoders.

You will need to check how your enable pin works. It may set all the outputs to 0 which is what you want. If it tri-states the outputs, then you will have to add pull-down resistors if you're using CMOS logic. If so you can use the resistor packs that have 8 resistors in a dil package.