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How should I understand FPGA architecture?

The green boxes are IO pins, the blue lines are wires, the red boxes are configuration bits, and the grey boxes are logic blocks. The red boxes can supply a constant logic 0 or logic 1 to whatever ...
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19 votes
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check if an unsigned binary number is divisible by 15

Do you know how to check for divisibility by 9 in base 10? Add all the digits using base 10 arithmetic. If the result has multiple digits, repeat the process. Stop when you have one digit. If the ...
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16 votes
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I am unable to figure out how the answer to following question is 70ns

You have to check the longest path through the design. In this case it involves the carry, because that goes through all the adders. The question isn't clear whether the output that is considered in ...
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12 votes

Which gate is better in building the Full Adder ? XOR or OR

If I were doing a silicon implementation, I would use an XOR because of the symmetric properties. Symmetric circuits use much less power because the stack size is the same that does a few useful ...
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8 votes

Is it important to add a transistor for the output of a 4 bit adder

What you are really asking is whether you can drive a LED directly from a digital signal, or whether it should be buffered by a transistor. Think about it. This has nothing to do with a adder or ...
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8 votes

How should I understand FPGA architecture?

You posted your own explanation. Take a closer look at your own image: The red box is meant as a label box for you to write into with a value or signal, and represents the signal that controls the ...
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7 votes

How can I add three AC signals?

Use an op-amp mixer circuit. This will add three signals into one: - Alternatively, if you did the math you would find that three independent voltage sources connected together thru 3 resistors of ...
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7 votes
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Building a full adder with NPN BJT transistors

It's probably most straight forward using a NOR form as the basis of a BJT adder. It's quite simple to form the basic gate, this way. Nothing crazy, at all. Just simplicity. The following schematic ...
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6 votes
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Binary Adder and Parallel Adder

There is a distinction between parallel adder vs serial adder. Both are binary adders, of course, since are used on bit-represented numbers. Parallel adder is a combinatorial circuit (not clocked, ...
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6 votes
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What is the purpose of a "carry in"?

It's important to understand there's a difference between a half-adder and a full-adder. A half-adder is the simplest. It has 2 inputs and 2 outputs. It's basically a XOR. The primary output is only ...
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6 votes
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How much voltage does a green LED need to be supplied? Will it handle 5V?

Some logic chips have definite current limits built into their output structure, but many do not. Depending on the logic chips being used you may well be able to direct connect the LEDs from output ...
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6 votes
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Why does a full adder need an OR gate?

You're making a couple of assumptions: Multiple outputs can be connected directly together When any such output is "high", then the wire is "high". This is known as "wired-OR", and indeed, such ...
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6 votes

Efficiently calculate \$-x-y\$ using a single adder

Premature optimization is the root of all evil. --Donald Knuth If you want to do it efficiently, let the synthesis tools choose the best implementation. Just use an HDL to describe the behavior you ...
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6 votes

VHDL: difference between using "+" or writing our own adder

Both have their uses: The second gives potentially better hardware in a real-life scenario, the first can be given to students as coursework for a course named "history of hardware" or something ...
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5 votes
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What would be the output of A = 0100 and B = 0111 with S = 1 in a 4-bit binary adder-subtractor?

The blocks labeled "FA" are called full adders, and they are fundamental to how binary arithmetic is done at the gate level. The circuit you have presented here is an implementation of a 4-bit adder/...
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5 votes
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Why is a half adder implemented with XOR gates instead of OR gates?

0 + 0 = 0 0 0 + 1 = 0 1 1 + 0 = 0 1 1 + 1 = 1 0 The ones place of a single-bit addition is equivalent to the exclusive OR operation, not the OR operation. Hence ...
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5 votes

What is the purpose of a "carry in"?

Besides what is said by those who replied earlier, Carry can also mean an input coming from the flag register that allows linking one sum on a ALU to the next one. By linking I mean that, if your ...
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5 votes

How should I understand FPGA architecture?

The green lines are wires, the red boxes are connections, you can connect a green wire to a block with a switch. The switch is in the red block and it can connect two wires together if enabled. This ...
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5 votes

Current adder or voltage adder

Neither, if the inputs come from voltage sources. The voltage at resistor output would equal to whichever of the two input voltages is higher, minus the voltage drop of the diode. Of course, if the ...
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5 votes

Simplify the boolean function $$Z=A\bar B \bar{C_i} + \bar A B \bar{C_i} + \bar A\bar B {C_i} + A B {C_i}$$

Observe that \begin{align}\overline{\overline{A} \cdot\overline{B} + A\cdot B} = \overline{(\overline{A}\cdot\overline{B})}\cdot\overline{(A \cdot B)} = (A + B)\cdot(\overline{A}+\overline{B}) = A \...
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4 votes

What is the purpose of a "carry in"?

The purpose of the "carry" in is to accept the "carry" out of the previous adder. Carry works just like normal arithmetic.
4 votes

VHDL - Cheapest-Fastest unsigned to signed binary number converter

It really isn't worth worrying too much about this detail. Adding 1 to a number is such a common idiom in HDL that synthesis tools have highly-evolved methods for dealing wtih it. Also, most modern ...
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4 votes
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calculate clock cycle of ripple Carry and lookahead adder

Ripple carry and carry lookahead adders are combinatorial circuits - they do not hold state and they are not clocked. So counting clock cycles is meaningless, unless perhaps you are talking about ...
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4 votes

check if an unsigned binary number is divisible by 15

The technique is similar to what you would do to check if a number is divisible by 9 in decimal. We need split the number into four bit digits and then add the digits together repeatedly until we have ...
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4 votes

What does "2 digit BCD number" mean?

The idea here is that you can treat a BCD digit as a single entity. Look at the components you have: 1-digit adder Multiplexer (presumably for BCD digits) 9's complement unit All of these use full ...
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4 votes
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Simplify the boolean function $$Z=A\bar B \bar{C_i} + \bar A B \bar{C_i} + \bar A\bar B {C_i} + A B {C_i}$$

When in doubt about booleans, just build a truth table. Truth tables for XOR (\$\oplus\$): | 0 | 1 ---+---+--- 0 | 0 | 1 ---+---+--- 1 | 1 | 0 For "is ...
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4 votes

How do I create a 1-bit full adder that outputs a 2-bit sum?

So I looked up what constitutes a full adder and found its truth table. Taken from wikipedia: So you can see that if the adder has the correct output, your sum and carry bits are simply the 2 bit ...
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3 votes

Which 4-bit Binary Number Is Greater?

Actually in some applications your lookup table idea would be how it's done. It uses a fair amount of read-only memory to accomplish the task, but has the advantage of all solutions being found ...
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3 votes
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Which 4-bit Binary Number Is Greater?

If you want to find the largest number between two unsigned numbers A and B, all you need to look at is the most significant bit where the bits in A and B are not the same, the larger number is the ...
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3 votes

Design of a basic half-adder

For a half adder with inputs A and B SUM = A XOR B CARRY = A AND B (from All About Circuits)
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