Binary decoder help!

[Gilsdank]

I didn't know where to post this but I have a personal project I'm working on. How can I make an 8x256 decoders out of 4x16 decoders? To have enough pins I'd imagine I need probably 16 or more 4x16 decoders.. Any help or advice would be greatly appreciated. Or an easier way to make an 8x256 decoder?

[Ciccioo]

the way i'm thinking requires 17 4x16 decoders, and it works like this:

 

let's call the 8 bits of your 8x256 decoder INPUT (because i don't remember the formal naming)

let's call 16 of the 17 4x16 decoders A, B, C, ..., O, P

let's call a 4x16 decoder SELECTOR

 

the higher 4 bits of INPUT will go into SELECTOR

every one of the 16 outputs of SELECTOR will be an ENABLE input of A, B, C, ..., O, P

A, B, C, ..., O, P will all have the lower 4 bits of INPUT as input

SELECTOR will have enable set to 1

edit: the outputs of A, B, C, ..., O, P taken all together in the right order will be the output of your 8x256 decoder

 

maybe i made this explaination too long, but hopefully i explained what i think of

i dont think there is an easier way to accomplish this

[Gilsdank]

@ciccio wow that looks like it work perfectly.. I'm going to be using this for a memory address so 2 8x256 decoders should give me 65536 bytes of ram I've been struggling over this so I really appreciate your quick response. I think this solution is going to work great and is very inexpensive

[Ciccioo]

@ciccio wow that looks like it work perfectly.. I'm going to be using this for a memory address so 2 8x256 decoders should give me 65536 bytes of ram I've been struggling over this so I really appreciate your quick response. I think this solution is going to work great and is very inexpensive

ohh i see, nice!

do you only own 4x16 decoders? can't you find 8x256 decoders somewhere?

and most importantly, if i can ask... what and how are you going to do with these addresses? cabling 65536 flip-flops  ain't no fun

[juretrn]

Can you give more information?nvm, missed previous comments

[Gilsdank]

ohh i see, nice!

do you only own 4x16 decoders? can't you find 8x256 decoders somewhere?

and most importantly, if i can ask... what and how are you going to do with these addresses? cabling 65536 flip-flops  ain't no fun

What I'll be doing is have 2 registers I'll call r0 and r1 connected to the bus. The Cpu will put one bye on the bus and set r0 and then put another byte on the bus then set that byte into r1 and when I enable those registers I'll get the cross point of the 2 decoders... How is this method?

[Gilsdank]

And @Ciccioo all I could find online without having to code my own logic chips is 4x16

[Ciccioo]

What I'll be doing is have 2 registers I'll call r0 and r1 connected to the bus. The Cpu will put one bye on the bus and set r0 and then put another byte on the bus then set that byte into r1 and when I enable those registers I'll get the cross point of the 2 decoders... How is this method?

And @Ciccioo all I could find online without having to code my own logic chips is 4x16

i didn't understand what you're trying to do, and using what (i thought it was about a breadboard and a bunch of cables?)

also, what do you mean when you say 'having to code my own logic chips'? i found these on amazon, give it a look

[Gilsdank]

i didn't understand what you're trying to do, and using what (i thought it was about a breadboard and a bunch of cables?)

also, what do you mean when you say 'having to code my own logic chips'? i found these on amazon, give it a look

I will be using a breadboard. I can upload some schematics when I get home of what I'm doing. So a 3-line to 8 is better? Sorry I'm fairly new to all of the ICs

Imagine 2 8x256 decoders with the outputs crossing perpendicularly making a grid. When I send a byte to the register that controls the top decoder it will choose on of the 256 outputs and the second bit will then be sent to the other decoder that will then choose one of the 256 outputs and the point where they intersect will have a register that has some byte, making my memory array

[Gilsdank]

At each intersection is an and gate so both wires of the and gate have to be on to select that 'cell' and then the register at that intersection is connected to the bus and when I enable or set ram I can write or read tgat ram address

[Ciccioo]

So a 3-line to 8 is better? Sorry I'm fairly new to all of the ICs

oh sorry, i didn't see that, just forget what i wrote, i don't even know what a 3-line to 8 is

 

 

the biggest problem i still see in your method is the magnitude of things: working with THAT many circuits is really, really a big mess

 

would it work? yes, i believe, but i'm really not good in electrotecnichal things

my suggestion is to try with a couple of 2x4 decoders first (just use the 4x16, ignoring the exceeding pins)

[Gilsdank]

oh sorry, i didn't see that, just forget what i wrote, i don't even know what a 3-line to 8 is

 

 

the biggest problem i still see in your method is the magnitude of things: working with THAT many circuits is really, really a big mess

 

would it work? yes, i believe, but i'm really not good in electrotecnichal things

my suggestion is to try with a couple of 2x4 decoders first (just use the 4x16, ignoring the exceeding pins)

I agree with you because that means I'll have 65553 intersections all with an and gate and register... What I am trying to do is make a big Frankenstein computer out of cmos chips haha... This 65kb array would be the ram for my system

[Ciccioo]

I agree with you because that means I'll have 65553 intersections all with an and gate and register... What I am trying to do is make a big Frankenstein computer out of cmos chips haha... This 65kb array would be the ram for my system

that's a cool thing, but if you keep things in this scale, you risk to just die, killed by an invasion of cmos and little cables and breaboards

i mean... 65536 chips... every chip is, say, 1cm long and 0.5cm wide, that's 0.5cm^2

all the chips would be 0.5 * 65536 = 32768 cm^2 = 3 m^2 = three goddamn square meters

lol, and that would be with a perfect packaging, small circuits, and perfect cable management

 

i say: aim lower, you'll do better things

[Gilsdank]

that's a cool thing, but if you keep things in this scale, you risk to just die, killed by an invasion of cmos and little cables and breaboards

i mean... 65536 chips... every chip is, say, 1cm long and 0.5cm wide, that's 0.5cm^2

all the chips would be 0.5 * 65536 = 32768 cm^2 = 3 m^2 = three goddamn square meters

lol, and that would be with a perfect packaging, small circuits, and perfect cable management

 

i say: aim lower, you'll do better things

Haha yeah you're right... What I'll probably do to start is just have 2 4x16 decoders for 256 bytes of ram.. Not bad and I'll be able to do some small programs on that.. I'm planning on building all of it, the processor ram and everything and I'm gonna post it as a build log here on the forum

[Ciccioo]

Haha yeah you're right... What I'll probably do to start is just have 2 4x16 decoders for 256 bytes of ram.. Not bad and I'll be able to do some small programs on that.. I'm planning on building all of it, the processor ram and everything and I'm gonna post it as a build log here on the forum

looking forward to that   good luck

[Gilsdank]

looking forward to that   good luck

Thanks man! I'll keep you posted