Lecture 3 - Combinatorial and Sequential Circuits Flashcards
NOTE: NEED TO REFER TO SLIDES AS IT IS MAINLY IMAGES
What are combinational circuits?
- Everything is a combination of AND, OR, NOT gates (or just NAND gates, or just NOR gates)
- There are no feedback loops!
- There is no memory, every output line is a simple function of its inputs
- Arbitrarily complex circuits can be built from AND/OR/NOT gates
- The output will only depend on the input lines (after some propagation delay)
REFER TO SLIDES FOR TRUTH TABLES
What is De Morgans Law for NOT (A OR B) and NOT(A AND B)
NOT (A OR B): NOT A AND NOT B
NOT(A AND B): NOT A OR NOT B
What is a decoder?
A decoder takes two inputs and splits it into four outputs where one is set to high (set to 1) based on the combination of two inputs. This is tyically when x1 = 0 and x0 = 0. REFER TO SLDIES
What is an encoder?
Takes four inputs and outputs 2 outputs, the idea is to take one of the four inputs which are mutually exclusive and convert it to binary code on the output lines
What is a multiplexer?
It can take to two of more inputs based on what it is connected to (ie for a cpu you can have load, add, sub, clear, etc) and then output a value based on that
What is a demultiplexer?
It takes one input and has several outputs
What is a half-adder?
It essentially adds two single bit binary numbers together, where it produces a sum bit and a carry bit. Think of it this was if I wanted 1 in binary that is just 1 so the sum is 1 and the carry is 0. Now if i had both a = 1 and b = 1, this is 2, therefore the sum = 0 and there is a carry of 1
What is an adder?
Addition of 3 bit inputs results in a 2 bit output
What needs to be considered when doing a full adder with n stages?
You get something called a ripple carry, which is the carry-out from each adder “ripples” into the carry-in of the next adder, hence the name “ripple carry adder. REFER TO SLIDES - but if i have 4 full adders you first need to consider the delay, the longest path is 3 and there are 4 adders hence a delay of 12 is necessary. But you dont need a carry for the lowest bit at 0 typically and so instead you ground it.
What is a tri-state (tri-state buffer)?
A tri state (also known as a tri state buffer is used to allow multiple devices to share the same output line without interfere using either the high or low states to determine what can and cannot pass. It takes a signal (or a high on the circuit) to change what is and isnt allowed through
What is a feedback loop?
- This will allow to “trap” a single bit of information
- It is the smallest instance of a memory cell
- Required functions:
– Change the bit (set to 1 or reset to 0)
– Read the current bit value
Understanding memory…
REFER TO SLIDES
Memory - What happens if R,S are both set to 1, then reset to 0?
*Uncontrolled oscillation
*That’s is why this state is illegal
What is an RS latch/flip flop
Essentially it takes two input S and R and outputs Q and NOT Q, based on the inputs it will do 1 of 4 things: hold previous state, reset Q to 0, set Q to 1 or be invalid. REFER TO SLDIES AND NOTE THAT A LATCH AND FLIP-FLOP DIFFER IN THE WAY ITS TRIGGERED
What is a D latch/flip flop
Is a simplified version of an RS flip-flop, which takes a clock input a d input and will output either Qn (previous state), 0 or 1
