Digital Electronics

Question 1

What are transistors?

Answer 1

In simple terms they can be considered to be switches.

They can open and close to let or stop current flowing through it.

Question 2

Moore’s Law

Answer 2

Every two years the transistor count in an integrated circuit will double

Question 3

MOSFET’s

Answer 3

Metal-oxide semiconductor field-effect transistor
Made from silicon
Made using complementary metal-oxide semiconductor (CMOS)
By applying voltage to the gate, you can control the current flow between drain and source

Question 4

Types of transistors

Answer 4

n-type (n-FET)
n-types don’t let current through by default
p-type (p-FET)
p-types do let current through by default

Question 5

What’s an embedded system?

Answer 5

A device containing a computer that interacts with its environment using sensors, displays or actuators.
Usually only designed for a few or only one task

Question 6

What’s a microcontroller?

Answer 6

Can be thought of as a computer system in a single-chip

They are programmed with software that tells it how to interact with hardware and peripherals

Question 7

Definitions of sampling

Answer 7

Sampling: how many times per second an anologue signal is read
Measured in Hz

Question 8

Definition of quantisation

Answer 8

A method of discretising an analogue signal
This means the amplitude can only take on of several discrete values/levels
Number of values it can take is called bit depth
For n bits there 2^n levels

Question 9

AND, OR, NOT symbol and function

Answer 9

AND: D shape, only if both inputs are 1
OR: D with with curved back shape, if one or both inputs are 1
NOT: Triangle with a cirlce on pointy end. Inverts input

Question 10

Logic gate algebra: AND, OR, NOT, NAND, NOR

Answer 10

AND: y=a.b
OR: y=a+b
NOT: y=a(line above a)
NAND, NOR: line above AND/OR algebra

Question 11

Buffer, XOR, XNOR

Answer 11

Buffer: y=a, input=output, used to amplify current in circuit
XOR: y=a⊕b, one or the other but not both
XNOR: XOR equation with a line above it, none or both

Question 12

Logic levels

Answer 12

Get henry to explain these

Question 13

Creating XOR and XNOR logic circuits

Answer 13

Write the truth table, use SOP algebra to figure it out

Question 14

Multiplexer + De-Multiplexer

Answer 14

M: Connects multiple inputs to a single output. Use truth table to figure out logic circuit
DM: Connects multiple outputs to a single input

Question 15

Programmable Array Logic (PAL)

Answer 15

Has a programmable AND array and fixed OR array.

Question 16

Creating basic gates with NAND and NOT gates

Answer 16

Revise this

Question 17

Creating NOT and NAND gates from transistors

Answer 17

Revise this

Question 18

Max number that can be stored in a given number of binary and hexdecimal digits

Answer 18

n=number of digits
Binary: 2^n - 1
Hexadecimal: 16^n - 1

Question 19

Converting decimal to hex, vice versa

Answer 19

Convert to binary first. Then split into two sets of 4 digits

Question 20

Sum of products and boolean algebra

Answer 20

Revise this

Question 21

Writing binary numbers using sign-magnitude

How does this affect the range? What’s special about zero?

Answer 21

Most significant but represents sign
0 = positive number
1 = negative number
Remaining bits represent magnitude of number
Range is smaller as one bit is used for sign (-2^(n-1) + 1 to 2^(n-1) - 1)
Two ways to represent zero (1000 and 0000)

Question 22

1’s Complement definition and range

Answer 22

1’s: Positive numbers are represented like in sign-magnitude. Negative numbers have MSB of 1 but the magnitude bits are inversed (opposite to positive bits)
Range is same as sign-magnitude

Question 23

2’s Complement definition and range

Answer 23

Positive numbers are represented same as sign-magnitude.
Negative numbers have MSB of 1. Magnitude bits are the inverse of positive magnitude bits PLUS ONE
Range given by -2^(n-1) to 2^(n-1) - 1

Question 24

Adding bits: how to do it, how to create a half adder circuit, when to use half adder?

Answer 24

Use column adding method. 1+1 results in a carry
Half-adder: create a truth table with two inputs, a sum and a carry column. Use boolean algebra
Use only for single-bit addition

Question 25

Full-adder circuit creation | When to use a full-adder circuit?

Answer 25

Create a table with 5 columns: two inputs, a carry-in, a sum and a carryout. Remember: a 1 in one column is a sum, two is a carryout and three is sum and carryout. Use when adding multi-bit numbers

Question 26

How does a ripple-carry adder work?

Answer 26

Multiple full-adders (one for each bit) connected in parallel Carry-out of each FA is passed into the carry-in of the next FA First FA in chain is least-significant-bit with carry-in set to 0 Final FA is most-significant-bit

Question 27

Arithmetic Logic Unit: definition, how many input and output bits and how many operations can be performed

Answer 27

A combinational logic circuit that performs arihmetic and logical operations on binary numbers Two inputs, one output and 'n' number of control bits For n control bits, a maximum of 2^n operations can be selected from

Question 28

How does a binary adder do subtraction?

Answer 28

Y=A-B | Y=A+(-B)

Question 29

The four different flags that can occur from a ALU

Answer 29

C-carry: the final carry-out of the ripple carry adder. Not applicable to signed numbers V-overflow: signed numbers can only take value in a certain range. Overflow is when result goes out of this range. Occurs when the sign bit changes Not applicable to unsigned numbers N-negative: whether the MSB is negative. Not applicable to unsigned numbers Z-zero: Whether the output is zero

Question 30

Sequential logic circuits-how do they differ from combinational circuits?

Answer 30

Current output depends on both the current and previous inputs-output is funneled backed into the circuit. Forms basis of memory

Question 31

What's a synchronous circuit? | Whats a asynchronous circuit?

Answer 31

Sequential logic circuit where a clock signal is applied to the circuit. Clock signal is a periodic waveform of frequency f Circuit only updates output on a clock-edge Circuit can be positive or negative-edge triggered Asynchronous: sequential circuit without a clock signal

Question 32

Flip-flop definition, most common type and how it works

Answer 32

``` A bi-stable multivibrator circuit Bi-stable means it has two stable states Can store a single bit indefinitely D-type flip-flop Two inputs, D and E. D is data input, E is enable input which determines whether output will change or not. 0 locks it, 1 unlocks it Two outputs, Q and NotQ ```

Question 33

What's a register? What's it used for?

Answer 33

Multiple flip-flops grouped together to store nultiple bits. Very high-speed memory but very expensive Used to store very small amounts of data inside a CPU

Question 34

Two ways to read and write data to registers

Answer 34

Serial-in/Serial-out: bits are loaded in one at a time. Outputs from one flip-flop are passed into the input of the next. Takes n clock cycles for a n-bit register Parallel-in/Parallel-out: each bit from the data bus is fed into an individual flip-flop Data will be written to the register in a single clock cycle

Question 35

Shift register definition What weird thing can shifting do? Shift/Load in registers

Answer 35

Devices that do parallel-to-serial conversion Shifting to the left multiplys binary number by 2 (division to the right) Operation determined by multiplexers. Shift shifts a values in registers serially Load loads a new value

Question 36

Microcontroller definition and uses

Answer 36

A computer on a single chip Contains a CPU, memory and various peripherals Used in embedded systems to work with sensors

Question 37

Different ways to use the term architecture

Answer 37

Instruction set architecture (ISA): describes the instructions Microarchitecture: refers to how the ISA is implemented in hardware System architecture: describes the computer system Load-store architecture: type of processor design

Question 38

Definition of a computer

Answer 38

A fixed hardware platform that can carry out an almost infinite number of tasks

Question 39

Program counter, incrementer

Answer 39

``` Program counter (PC): contains address of next instruction to be executed Incrementer: increments value of stored in PC ```

Question 40

MAR, MBR

Answer 40

Memory access register (MAR): stores address of memory location currently being accessed Memory buffer register (MBR): stores data that has just been read from memory/data to be written to memory

Question 41

IR, control unit

Answer 41

``` Instruction register (IR): stores instruction currently being executed Control unit: subsystem that controls the CPU ```

Question 42

Definition of byte-organised memory | How many memory locations can a n-bit address hold?

Answer 42

Each location in memory can store an individual byte with a unique address n-bit address allows 2^n memory locations

Question 43

How many bytes are in a kb, Mb, Gb?

Answer 43

1 kb: 2^10 1 Mb: 2^20 1 GB: 2^30

Question 44

Describe the fetch part of the fetch-execute cycle

Answer 44

``` Value in the PC is copied to the MAR Location of value in MAR is found and read from memory Instruction is copied to MBR Contents of MBR is copied to IR Value of PC is incremented ```

Question 45

Decoding data: how types of instructions are there and what figures out how to use the instruction?

Answer 45

Data procressing (done by ALU) Loading data from memory Storing data in memory Control unit determines what kind of instruction is in the IR and how to proceed

Question 46

What's pipelining?

Answer 46

Increases performance While the first instruction is being decoded, the second instruction is fetched. While the first instruction is being executed, the third can be fetched etc.

Question 47

RISC and CISC: definition and pros/cons

Answer 47

Reduced/complex instruction set computer RISC has simpler architecture with fewer instructions CISC has complex architecture with lots of complex instructions CISC is simpler to write programs for but RISC instructions can be executed quicker

Question 48

Harvard vs Von Neumann architecture

Answer 48

Von Neumann: instructions and data both share the same bus Creates bottleneck tho Harvard: data and instructions are stored in seperate memories and have different buses

Question 49

Counters: definition and max count value

Answer 49

Sequential logic ciruict element Built using flip-flops Max count value is 2^n - 1 where n is the number of flip-flops

Question 50

JK flip-flop definition

Answer 50

Flip-flop that can be made to toggle the value it is currently storing

Question 51

Synchronous and asynchronous counters

Answer 51

Synchronous: all flip-flops share a common clock signal Asynchronous: not all flip-flops are clocked. First one is clocked and the others are clocked by an output from the previous flip-flop Known as ripple counters

Question 52

Asynchronous counters

Answer 52

Frequency dividers: each subsequent flip-flop toggles at the half the frequency compared to the last one Counters can therefore be used to scale down the original clock frequency Used in quartz clocks

Question 53

Resolution vs range in a counter, and how pre-scalers help

Answer 53

Resolution is how often a clock 'ticks', range is how long a clock can count for. Higher resolution means lower range Pre-scalers allow users to modify clock frequency and find the best balence

Question 54

Encoders and watchdog timers

Answer 54

Encoders count clock cycles between events WT: a counter that constantly increments and has to be reset before overflow happens. If software crashs and it isn't reset, whole system reboots