synchronous circuits

Question 1

what is a gate delay?

Answer 1

when an input to a logic gate changes, the get takes some time to respond and make its output valid.

this time is the gate delay

Question 2

what is the validity of the signal before the gate delay?

Answer 2

if the input to a logic gate changes, you cannot rely on the output until the gate delay has elapsed

Question 3

if the signal is invalid, does it mean it is wrong?

Answer 3

it means that it is not guaranteed to be correct

Question 4

what is a glitch?

Answer 4

for a short time, the output of the circuit is incorrect, but if you wait a short time, it will reach the correct value.

Question 5

what is a dff (delay flip flop)

Answer 5

a dff is a circuit that remembers one bit of data (the “state”)

there is a data input x carrying a value to be remembered, and a data output y conveying the state value

as well as a clock tick

Question 6

what is special about the clock tick?

Answer 6

there is one global click signal, which is sent to all flip flops of the circuit. every flip flop updates its state at the same time

Question 7

flip flop versus registers

Answer 7

a flip flop remembers a bit for a very short time, just one clock cycle
a register remembers a bit until you tell it to load a new value - controllable memory element

Question 8

describe the behaviour of a 1-bit register

Answer 8

remebers a bit

receives a control input ld and a data input x, and outputs a 1-bit state

initial value is zero

at each clock tick, the register loads the data value x if ld ==1

if ld==0 the register ignores the value of x and retains previous state

Question 9

what does the dff do: 1 bit reg?

Answer 9

hold state and load a new state at every clock tick (if ld == 1 new if 0 old state)

Question 10

how is the dff input finalised?

Answer 10

mux 1

Question 11

what is the intial state of a simulation table?

Answer 11

”?” - the circuit should never use this value

Question 12

cycle 0 : ld = 1 x= 1
cycle 1: ld = 1 x = 0
2: 0 1
3 1 1

complete this simulation table

Answer 12

Question 13

what is the simultaneous update of state?

Answer 13

The state of a circuit consists of the states of all the flip flops

In a digital circuit, all the flip flops change their state simultaneously at the clock tick compared languages like python change one variable and leave all the others unchanged