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00 Ebooks

posted Nov 15, 2012, 7:16 AM by Neil Mathew   [ updated Mar 28, 2013, 8:37 PM ]


Pretty good PDF. 

P-5 Decoders

posted Nov 4, 2012, 2:58 AM by Neil Mathew   [ updated Nov 4, 2012, 3:36 AM ]



Making the 2:4 decoder should be fairly simple.

The only thing that continues to confuse me is the truth table. 
I've explained it here.

When using AND gates to make a decoder, the truth table is as follows:




However, the interior of the 74155 IC is designed as follows:


Notice the NOT gates before A, B (select lines) 

and C (data).  <<?? circle before AND?

MAINLY, the use of NAND gates instead of AND gates.

This causes the truth table to be as given below.
(Basically, inverting all the values. 0 when chosen, 1 elsewhere)





STROBE?

Here, it's G or G Dash. I understand that it acts as an enable.

C is the data.

How do I tell what value the enable wants?

I understand it from the IC.
If G dash is mentioned in the circuit, give it a ZERO. 



3:8 DECODER


One way is to use two ICs as two separate 2:4 decoders to make a single 3:8 one.



Based on the above interior of the IC 74155
we can use the same IC as a 3:8 decoder if we ensure certain things.

(I haven't performed this on my own yet, but assume my theory here is right.)

A, B is same.
1G' and 2G' has the same line.
1C and 2C'2 has the same line too.
 


P-4 ALU circuit using MUX

posted Nov 4, 2012, 2:26 AM by Neil Mathew   [ updated Nov 4, 2012, 2:32 AM ]


Once you've got the truth table and the IC Number of the 4:1 Mux
this should be pretty simple.

IC of 4:1 MUX: 74153

INPUTS: A, B, S0, S1 

S0

S1

OUTPUT(y)

0

0

AND

0

1

OR

1

0

XOR

1

1

NOT



            > A O X N >




P-3 Tri State Buffer and Bus

posted Nov 4, 2012, 2:18 AM by Neil Mathew   [ updated Nov 4, 2012, 2:20 AM ]


Tri-State buffer


Tri State Buffer Bus






P-2 Shifter

posted Nov 4, 2012, 2:09 AM by Neil Mathew   [ updated Nov 4, 2012, 2:15 AM ]


Let the picture do the talking.


Click on it for a larger view.

03 Setting the Digital Clocks

posted Nov 4, 2012, 1:37 AM by Neil Mathew


The DigiClocks can be found in the SOURCE Library.

I've classified them in the ways I've used them. 
This may not be the conventional method, but it works for me.


My Type #1: Normal Varying Clock Signal (INPUT)
OFFTIME- duration of low (0)
ONTIME- duration of high (1)

These control the duration of the high and low cycles of the clock.
Both are set equal at 0.2uS so that they are high and low for equal durations.

When there are many clock inputs required, inorder to see my output clearly, 
I keep each clock with a different duration, usually in multiples.
eg: 0.1uS , 0.2uS , 0.4uS , 0.8S

STARTVAL - the starting value of the clock
Here, set as 0.


My Type #2: Fixed Value Clock Signal (ENABLE)

Here, I alter the OFFTIME and ONTIME by assigning one of them a value of 0uS

That way, it will remain in only one state: low or high.

Always 0:

Always 1:

 

Notice how the STARTVAL does not matter here.



My Type #3Changing the Delay

In order to distinguish between the various input clock signals, I initially used delay instead of changing the ontime and offtime. They used to create excessive propagation delays which I found unfavorable.

Hence, I don't use this type anymore.



02 Closer Look at ICs

posted Nov 4, 2012, 1:37 AM by Neil Mathew   [ updated Nov 4, 2012, 1:15 AM ]


All The ICs used

> DUAL, QUAD here means how many such components are available in that one IC.

** 74157 IC TTL quad 2-to-1 data selector multiplexer with non-inverted data outputs
(74157 was a unique IC. All four 2:1 MUX had the same select inputs)


IC Number: Name  Where it's used
   
7400
 2-input NAND
 
7402 
 2-input NOR
 
7404
 1-input NOT
 



7408
 2-input AND
 
7432
 2-input OR
 
7486
2-input EXOR 
 
 
 
7483A
4-Bit Binary Full Adder
Adder-Subtractor
Decrementer (add 1111 ~ -1) 
   
74153
Dual 4:1 MUX
 ALU circuit
74157
Quad 2:1 MUX **
 4-bit Shifter
 
 
 
74155
Dual 2:4 DECODER
3:8 Decoder 
Tri State Buffer Bus
 
 
 
74125
Tri-State Buffer 
Tri State Buffer
Tri State Buffer Bus

(CLICK HERE FOR MORE ICs)



ScreenShots of a few good ones


4-bit Full Adder Circuit

Note the last, C0 is the select input (m) or Input Carry. 

Left: A4 A3 A2 A1 B4 B3 B2 B1 C0
Right: C4 SUM4 SUM3 SUM2 SUM1





 DUAL 4:1 MUX
Usually just one 4:1 MUX is used. 
EA' should be supplied 0. 



DUAL 2:4 DECODER
Again, only one of the two decoders is used.
A, B are the Inputs. (Treat as B A as in 1 0 = 2)
G' is the enable (Supply it 0)
C is the store (Supply it 1)




 QUAD 2:1 DATA SELECTOR MUX

STROBE G' (Supply it as 0)
SELECT A'E ( Varying 0 and 1 since its for selection)




Tri State Buffer
Pretty easy this one. 
When C = 1 Output's the same.
When C = 0, Output is Z, due to high impedance (Z).


P-1 Adders & Subtractors & Adder-Subtractor & Incrementer & Decrementer

posted Nov 4, 2012, 12:53 AM by Neil Mathew   [ updated Nov 4, 2012, 1:23 AM ]


We've done this countless times in so many different ways.

That said, I say it's easier if I just mention the functions used:

NOT    7404
AND    7408
OR      7432
XOR   7486



HALF 
ADDER

Sum=A XOR B

Carry=A AND B



FULL
ADDER


Sum=A XOR B XOR Cin


Carry=(A AND B) OR (B AND Cin) OR (Cin AND A)
 
HALF
SUBTRACTOR

D=X XOR Y

B=(NOT X) AND Y



FULL 
SUBTRACTOR


D=X XOR Y XOR Z

B=(NOT X AND Y) OR (NOT X AND Z) OR (Y AND Z)


Full-Subtractor Circuit



Binary Adder-Subtractor Circuit

Take a good look at the circuit:


Here the [+] are the full adders. 

Use the clock as M to control whether it adds or not.
That is, when m=0 it acts as an ADDER
And when m=1 it acts as a SUBTRACTOR

To get the B' + 1 (2's complement) we use the XOR Gate.
B (+) 0 = B
B (+) 1 = B'

Since making each of the four full adder circuits is gonna take time, we use ICs.
7483A ( 4-BIT BINARY FULL-ADDER CIRCUIT)

Incrementer & Decrementer

We can do these in two ways.

Option 1:
Use a 4-Bit Half-Adder circuit where the Carry(in) = 1
(but we'll have to create each half-adder circuit separately = tedious)



Option 2: 
Use the 4-Bit Full-Adder IC (7483A)


For Incrementer: B = 0001
For Decrementer: B= 1111


01 Basic Stuff

posted Nov 3, 2012, 11:42 PM by Neil Mathew   [ updated Nov 3, 2012, 11:45 PM ]


I had some problems pasting images in the CA Lab during the first few classes. 
So, instructions to start a project are unfortunately not aided with screenshots.

Softwares Used: ORCAD Capture & PSpice



Without pictures, I really don't see the point in explaining how to create a project. 
So, I'll just mention a few mistakes I made which I hope I won't make again.



Caution #1

When you create a project, make a 'combined' 'analog' project.

My memory is a bit faulty but I do recall facing problems in the simulation
if the above is not properly specified.



Caution #2

We are using a Trial Version of OrCad. Hence, the software repeatedly checks online 
and continues to remind the user to validate the software. 

This causes all kinds of problems like it not starting up, 
random closing of unsaved projects, and a stupid dialog box 
which will not disappear until you click TRY AGAIN a countless number of times.

Solution: Disconnect your system from the internet. Trust me, it helps. 



Caution #3

When you place the various components (Gates, ICs, etc) onto the page,
make sure you take them from the EVAL Library. 

If you take them from elsewhere, a green circle is seen next to each gate.
See it as a sign of doom. Your circuit will not simulate properly.




Caution #4

This, not so important. But something I've repeatedly faced.
Make sure your connecting wires are... well.. properly connected.
Often the wires seem like they are connected, but they're not. 

Check that in the following way.
Move the gate or component around and if the wires move with it, It's connected.




Only Screenshots I could manage. 

The Main Toolbar.



The PSpice Toolbar


(nmnm is the name of my simulation profile)


And Full Screen Screenshots:

          


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