Digital Logic Design

What is the BCD representation of 124?

0001 0010 0100

What are examples of combinational circuits?

half-adders, full-adders, multiplexers, decoders

What is the hold time constraint?

t_cd ≥ t_hold - t_ccq

What is the setup time constraint?

t_pd ≤ T_c - (t_pcq + t_setup)

What is clock skew?

the difference in the arrival time of a clock signal to any two flip-flops due to wire/routing delay; important when output from one ff is sampled by another

What is the sequencing overhead?

- It is the delay that comes from sequential elements (flip-flops) in a circuit. - t_pcq + t_setup

What is the difference between a latch and a flip-flop?

- Latches and flip-flops both hold data. - The content of a flip-flop only changes on a clock-edge, whereas the content of a latch can change as long as the enable (clock) signal is asserted.

How many 2:1 multiplexers are necessary to construct a 2{^n}:1 multiplexer?

2{^n} - 1

What is the difference between a decoder and an encoder?

A decoder yields an n-bit output for a given m-bit input, where n>m. An encoder yields an m-bit output for a given n-bit input, where n>m.

What is the difference between a multiplexer and a demultiplexer?

A multiplexer chooses 1 output from n inputs, while a demultiplexer directs 1 input to n outputs.

How is an encoder different from a multiplexer?

A multiplexer has just one output (one bit) whereas an encoder has n outputs (n bits) for 2{^n}-1 or fewer inputs.

How is a priority encoder different from a simple encoder?

A simple encoder only accepts a 2{^n}-bit one-hot vector input. A priority encoder is not limited to one-hot vector inputs, but only examines the MSB of the vector to determine the output.

How does the value of the parity bit depend on whether "even" or "odd" parity is used?

If the number of "ones" in the string is odd and even parity is used, the parity bit will be "one" to even the count. If odd parity were used, the parity bit would be zero to maintain the odd count, and visa versa for a string with an even number of "ones"

What are two universal logic gates?

NAND and NOR

What are advantages of using gray code over binary code?

Only one digit is changing between successive values in gray code, so any ambiguity from digits which have different latencies in switching is avoided. Also, since only one digit is changing at a time, gray code has power savings over binary.

What is hold time?

The minimum amount of time after a clock edge during which the data signal must remain stable

What is the difference between synchronous and asynchronous circuits?

The output and state of a synchronous circuit changes only on clock edges. The output and state of asynchronous circuits change as soon as the primary inputs change.

What is the difference between a combinational circuit and a sequential circuit?

The output of a combination circuit depends only on the current inputs at any instant in time. The output of a sequential circuit depends both on the current inputs as well as past inputs, so sequential circuits have memory.

What is a parity bit?

a bit appended to the end of a binary string to indicate whether the number of bits having a value of "one" (1) is even or odd

What is a universal logic gate?

a gate which can implement any Boolean function without the need for a different gate

What is a ring oscillator?

an odd-numbered sequence of NOT gates whose output alternates between two voltage levels; the period is two times the sum of the delays of each NOT gate

What is a race condition?

an unexpected dependency on the relative ordering of events

What is binary coded decimal?

binary coded decimal (BCD) is a code that uses four binary digits to represent each digit in a decimal number

What are examples of sequential circuits?

flip-flops, counters, registers

What is setup time?

minimum amount of time before a clock edge during which the data signal must remain stable.