Welcome to digital electronics lecture series. I professor Hitesh Dolakia is going to explain you 3 cross 8 decoder in this session. So in my previous video I have already explained you what is decoder.
So decoder is a circuit which decodes an input to 2 to the power n number of outputs. So here you can see we have three inputs. So total outputs will be 2 to the power 3 means 8 right so here let me draw this basic Block diagram which is there with 3 cross 8 decoder.
So that will give us proper understanding like see 3 cross 8 decoder means here we have 3 input terminals. Let us say this 3 input terminals are A, B and C. Then at output side we have 8 outputs.
So let us say this is Y0, Y1, Y2, Y3. Y4, Y5, Y6 and Y7 that is how total 8 outputs are there and to enable this decoder here we have enable terminal and this is very important terminal where if this enable terminal is zero in that case this decoder will be disabled. Right. So here E is enabled terminal. Where if you see E is equals to 0 then we can say decoder is disabled and if E is equals to 1 in that case we can say this decoder is enabled.
So disabled means this decoder output will be 0 no matter what is this lines right. And if we say this lines. which is input lines so that is selection line or one can say selection inputs right so if e is 0 then output is not depending on a b c output will be always 0 and if e is 1 then based on this selection line a b c output will get selected now see how output will get selected so output selection that is purely based on values of a, b and c.
If it is 0, 0, 0, then y0 will get selected. If it is 0, 0, 1, then y1 will get selected. Likewise, as if a, b, c is 1, 1, 1, in that case, y7 will get selected. Right.
So let us try to understand all those things by truth table. So it will be more clear, like see how we can design circuit. Right. So in truth table, Let us say here we have enable line E then A, B and C and then we have outputs Y0, Y1, Y2, Y3, Y4, Y5, Y6 and Y7.
Right. And now in this enable line as if this enable line is 0 then output is not depending on ABC means. Don't care that I need to say an output will be 0 in that situation. Right. So all seven lines that will be 0. But when enable is 1 at that time ABC is having total eight combinations.
So let me mention all eight combinations first. So here you can observe see ABC. So these are our selection. inputs and y0 y1 y2 y3 y4 y5 y6 and y7 those are my outputs Now let us try to understand how decoding operation is happening. So see as if ABC that is 0 0 0 then Y0 will be 1 and other outputs will stay 0. So here we can say Y0 decoding is happening when ABC is 0 0 1 at the time Y1 will get Decoded means it is one and other outputs will stay zero.
In case if 0 1 0, y2 will get decoded and other output stays 0. In case of 0 1 1 of selection a, b, c, y3 will get decoded and other output will stays 0. In case of 1 0 0, y4 will get decoded and other output will stay 0. In case of 101, Y5 will get decoded and other output will stay 0. In case of 110, Y6 will get decoded and other output will stay 0. And in case of 111 means 7. So Y7 will get decoded and you'll be finding. other output will stay 0. So see this is how decoding is happening at output side. Now I'll explain you how we can make circuit from this truth table right.
So to make a circuit first see we need to make it with respect to selection line ABC. So here I'm making selection line let B and C and these are active high line which is there with selection lines and here in parallel with that I am also making inverting lines right. So here A bar B bar and C bar that is what I am connecting right now If you want to make a circuit for Y0 decoding then you see when ABC is 0 0 0 at that time Y0 will get decoded right.
So here if I say we have Y0 then ABC is 0 0 0 means A bar B bar C bar that we need to connect. So here you see C bar B bar and A bar that I am connecting. Right.
To have decoding of Y1, ABC should be 0 0 1 means A bar B bar C that we need to connect. So here we have Y1 and A bar B bar and C that we need to connect. To have decoding of Y2, C. ABC is 010 means A bar B.
C bar. So A bar, B and C bar that we need to connect. To have decoding of Y3, you see it is 1 over here means ABC is 011 means A bar.
B and C that we need to connect to have decoding of Y4 we need to have ABC that is 100 means A, B bar, C bar. Right. So here we have A then B bar then C bar that we need to connect for Wi-Fi decoding.
You see this is Wi-Fi which is 1 where ABC is 101 so A, B bar, C. So here this is A, this is B bar and this is C. That is how we decode Wi-Fi and for Y6 means this 1 so at that time ABC is 110 means A B and C bar that we need to connect and for y7 decoding ABC that should be one over here right ABC that should be one over here for y7 decoding. So ABC that should be active high. So you see I'm connecting it and Very important terminal that we should not forget that is enable terminal.
So this enable terminal that should stay active high. So it should be connected as active high to all these AND gates. Right. So this is what fourth terminal that I'm connecting to input of all AND gates.
And that is what resulting into decoding of output Y0 to Y7 with respect to selection line ABC. Right. So here I'll give you one practical example like see if E is equals to 1 if E is equals to 1 and ABC is 1 1 0. So if E is equals to 1 means it is enabled and if ABC that is 1 1 0. So 1 1 0 means what C A is 1 B is 1 and C is 0 means 6. So this Y6 that will get on.
Right. So as per that if you want to make Boolean function then I'll give you one example of y7 see y7 that has to be a b c e right as per this a b c and e if you want to write what is y6 then y6 is a b c bar e see that is how this connection is there right so this is how 3 cross 8 decoder functions right and here this decoder that we use it to select ICs. So this decoder that we can use it to select eight different ICs and usually I have seen 3 cos 8 decoder that is very popular in memory interfacing where if you have multiple ICs of memory in that situation to have selection of memory IC we use this decoder. So you see that memory interfacing In 8085 memory interfacing as well as 8086 memory interfacing I have explained how we can select by using decoder where sometimes we use this type of decoder. I hope that you have understood this.
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