developing 8 to 256 bit decoder

[karthyekrajhaa]

decode 256 bit

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

 

[nsit1234]

8 zu 256 decoder

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?????

 

[newelltech]

256 bit decoder

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.