Digital Electronics Flashcards
What are transistors?
In simple terms they can be considered to be switches.
They can open and close to let or stop current flowing through it.
Moore’s Law
Every two years the transistor count in an integrated circuit will double
MOSFET’s
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
Types of transistors
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
What’s an embedded system?
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
What’s a microcontroller?
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
Definitions of sampling
Sampling: how many times per second an anologue signal is read
Measured in Hz
Definition of quantisation
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
AND, OR, NOT symbol and function
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
Logic gate algebra: AND, OR, NOT, NAND, NOR
AND: y=a.b
OR: y=a+b
NOT: y=a(line above a)
NAND, NOR: line above AND/OR algebra
Buffer, XOR, XNOR
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
Logic levels
Get henry to explain these
Creating XOR and XNOR logic circuits
Write the truth table, use SOP algebra to figure it out
Multiplexer + De-Multiplexer
M: Connects multiple inputs to a single output. Use truth table to figure out logic circuit
DM: Connects multiple outputs to a single input
Programmable Array Logic (PAL)
Has a programmable AND array and fixed OR array.
Creating basic gates with NAND and NOT gates
Revise this
Creating NOT and NAND gates from transistors
Revise this
Max number that can be stored in a given number of binary and hexdecimal digits
n=number of digits
Binary: 2^n - 1
Hexadecimal: 16^n - 1
Converting decimal to hex, vice versa
Convert to binary first. Then split into two sets of 4 digits
Sum of products and boolean algebra
Revise this
Writing binary numbers using sign-magnitude
How does this affect the range? What’s special about zero?
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)
1’s Complement definition and range
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
2’s Complement definition and range
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
Adding bits: how to do it, how to create a half adder circuit, when to use half adder?
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
Full-adder circuit creation
When to use a full-adder circuit?
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
How does a ripple-carry adder work?
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
Arithmetic Logic Unit: definition, how many input and output bits and how many operations can be performed
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
How does a binary adder do subtraction?
Y=A-B
Y=A+(-B)
The four different flags that can occur from a ALU
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
Sequential logic circuits-how do they differ from combinational circuits?
Current output depends on both the current and previous inputs-output is funneled backed into the circuit. Forms basis of memory
What’s a synchronous circuit?
Whats a asynchronous circuit?
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
Flip-flop definition, most common type and how it works
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
What’s a register? What’s it used for?
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
Two ways to read and write data to registers
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
Shift register definition
What weird thing can shifting do?
Shift/Load in registers
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
Microcontroller definition and uses
A computer on a single chip
Contains a CPU, memory and various peripherals
Used in embedded systems to work with sensors
Different ways to use the term architecture
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
Definition of a computer
A fixed hardware platform that can carry out an almost infinite number of tasks
Program counter, incrementer
Program counter (PC): contains address of next instruction to be executed Incrementer: increments value of stored in PC
MAR, MBR
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
IR, control unit
Instruction register (IR): stores instruction currently being executed Control unit: subsystem that controls the CPU
Definition of byte-organised memory
How many memory locations can a n-bit address hold?
Each location in memory can store an individual byte with a unique address
n-bit address allows 2^n memory locations
How many bytes are in a kb, Mb, Gb?
1 kb: 2^10
1 Mb: 2^20
1 GB: 2^30
Describe the fetch part of the fetch-execute cycle
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
Decoding data: how types of instructions are there and what figures out how to use the instruction?
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
What’s pipelining?
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.
RISC and CISC: definition and pros/cons
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
Harvard vs Von Neumann architecture
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
Counters: definition and max count value
Sequential logic ciruict element
Built using flip-flops
Max count value is 2^n - 1 where n is the number of flip-flops
JK flip-flop definition
Flip-flop that can be made to toggle the value it is currently storing
Synchronous and asynchronous counters
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
Asynchronous counters
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
Resolution vs range in a counter, and how pre-scalers help
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
Encoders and watchdog timers
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
