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ADC (digital ramp)

Converts analogue data to digital data
  • slower
  • cheaper
  • useful for digitising audio and slowly changing signals from nature like temperature
  • few components needed

ADC (flash)

Useful Site

Converts analogue data to digital data
  • very fast
  • expensive
  • useful for digitising video
  • very simple in concept
  • huge number of components needed or a complex expensive IC

Address Decoder

A logic circuit that responds to or recognises a single address.

RAM locations and memory mapped devices each have an address decoder. The RAM location or device becomes active when the matching address is on the address bus.

ALU

Arithmetic and Logic Unit
  • this Sub-system is found inside a computer processor (CPU).
  • this sub-system carries out arithmetic operations
  • this sub-system makes decisions (logic)

AM

Amplitude Modulation

Bandwidth = 2 x Fmax

Fmax is the highest audio frequency to be sent.

Analogue

Infinitly variable signals that occur in nature.

Temperature, pressure, speed ...

Angular Resolution

The smallest anglular movement that can be measured. This is measured in degrees.

 

Binary

Base 2 number system

  • Uses 0 and 1
  • Computers use binary

Bit Mask

Select or ignore selected bits.
  • using an AND gate, bits are ignored by closing the AND gate.
  • A zero on the mask input closes the gate. The input is ignored.
  • A one on the mask input opens the gate. The input is copied to the output.
  • Bit masking can be done in software too.

Bus

A wire or collection of wires shared by two or more devices.

Having a shared bus reduces the number of interconnecting wires needed.

  • simplifies the interconnection of devices because many devices can share the same bus.
  • Tristate logic is used so inacitve devices can be disconnected from the bus.

Bus (address bus)

The processor sets the address bus to determine which memory location or I/O port will be active.

This bus is unidirectional. The processor sets the addresses.

  • every memory location has its own unique address
  • every memory mapped device has its own unique address
  • the number of wires in the address bus determines how many addresses can be used.
  • if there are N wires in the address bus then 2N addresses will be available
  • if there are 8 wires in the address bus then 28 addresses will be available = 256. They are numbered from 0 to 255.

Bus (control bus)

Control bus wires carry control information from the processor to all the devices that need this information.

This bus is unidirectional. The processor sets the control bus lines.

  • R/W - this wire determines whether the processor is reading or writing.
  • I/O - this wire determines whether memory is active or the I/O ports are active. In this way, the same addresses can be used for memory and I/O.
  • Clock - this wire carries the clock signal used to synchronise all operations

Bus (data bus)

This bus is bidirectional. Data travels both to and from the processor along the data bus.

Data can be read or written from or to RAM

Data can be read or written from or to the I/O ports

For bidirectional operation, tristate logic is needed.
  • data can be copied from the processor to memory (Write)
  • data can be copied from memory to the processor (Read)
  • the number of wires in the data bus determines the biggest number that can be sent along the bus.
  • if there are N wires in the data bus, the biggest possible number = 2N - 1
  • if there are 8 wires in the data bus, the biggest number is 28 - 1 = 255.

Bus Contention

More than one device is attempting to write to the bus at the same time. This must never happen.
  • Use tristate logic to prevent this problem.
  • The processor uses the address and control buses to determine which device gets access to the bus.

Clock

The clock keeps all parts of a computer synchronised.
  • determines the number of instructions the processor can perform per second

Closed Loop

Closed loop control systems measure the output . This signal is fed back to the input.
  • Example - vehicle cruise control
  • set the wanted speed (input)
  • measure the actual speed (the output)
  • calculate the difference between the wanted and actual speed (process)
  • use this difference signal to control the throttle

Communication

Communication is the transfer of meaningful
information from one location to another.

Control System

Input, Process, Output plus Feedback in closed loop systems.

Programmed va Hard Wired

CPU

Central Processing Unit - often just called the processor.
  • writes addresses onto the address bus
  • writes control data to the control bus
  • reads AND writes data from and to the data bus
  • performs arithmetic
  • makes decisions (logic)

DAC

Digital to Analogue Converter

Used to convert digital data to analogue.

  • CD/MP3 music
  • DVD and MPEG video

Dipole Antenna

This is half a wavelength long

C = F λ

  • C is the speed of light 3x108 m/s
  • F is the frequency
  • λ is the wavelength

DO ...
LOOP UNTIL

Used in BASIC programming

Repeat the code between the DO and LOOP keywords. The repetition stops when the condition after UNTIL is true.

Fibre Optics

Data communication is achieved by sending flashes of light down optical fibres.
  • better security
  • no electromagnetic interference
  • higher bandwidth
  • longer cable runs possible

Flow Charts

Create and Interpret

  

FM

Frequency Modulation

Bandwidth = 2 x (deviation + fmax)

  • deviation is the amount by which the carrier deviates either side of its centre frequency
  • fmax is the highest frequency transmitted (music or voice)

Frequency

The number of complete cycles per second measured in Hertz.

C = F λ

C = 3 x 108 (speed of light)
F = Frequency
λ = Wavelength (Greek lambda)

Hard Wired

Non-programmable control system.

  

Hardware

Physical devices with a power supply.

  

Hunting

Systems which include feedback can suffer from hunting. This is an unwanted oscillation of the output caused by positive feedback. A lack of damping may cause hunting that dies away. Audio feedback is an example of this type of oscillation.

  

I/O

Input Output Ports
  • used for interfacing the CPU to the outside world.

Impedance

Impedance is the AC equivalent of DC resistance. Connected devices work best if their impedamces are the same (matched).

Devices like RF amplifiers, antennas and coaxial feeder cables all have an impedance (often 50 ohms). If device impedances are not matched, RF signals are reflected. This reduces efficiency and can cause overheating.

Interrupts

A message to the processor from a hardware device or from a software process
  • normal processing is suspended
  • code is run to handle the event that caused the interrupt
  • normal processing is resumed
  • interrupts can be interrupted by other interrupts
  • caused by key presses, mouse movements, input data arrivimg from hardware devices, hardware errors, software errors

Label

Used in BASIC programming

foo:
    goto foo
  • programs can jump to labels
  • jumps are used to repeat code
  • polling uses repetition
  • jumps are used so skip code

Latch

A memory circuit used to store one or more bits. It is needed because the data to be stored may only be available or valid for a very short time.

D Type Flip Flops can be used to latch data.

Memory

Temporary (volatile) amd permanent (non-volatile) data storage.

Registers, RAM, ROM, Flash, Disk, Tape ...

Memory Mapping

Specific RAM addresses are reserved for interfacing to the outside world and can not be used for normal data storage.
  • RAM locations are connected (via latches) to the outside world.
  • these RAM locations can not be used to store normal program data because they have an alternative use.

Microprocessor

A central processing IC.

These chips mostly use the Von Neumann Architecture with three buses.

Microprocessor chips need several supporting chips to control memory, the buses and I/O. Their main advantages are speed, processing power, instruction set and the ability to deal with very large amounts of memory.

Mobile Phone

Frequency and time division multiplex digital communication system.

  

Modulation

Endoding information for transmission.

A carrier wave or a stream of pulses is modified in order to encode the information.

Multiplexing

Sending two or more signals across a shared medium (channel).

Neural Network

Compare this with a typical PC (personal computer).
  • has many simple processors
  • the processors work in parallel
  • has many inputs
  • stores data in the weightings in multiple locations within the network
  • solves fuzzy (not well understood) problems
  • are not programmed by a programmer
  • must be trained - this process changes the stored weightings
  • are good for image recognition
  • can do weather forecasting
  • good at predictive tasks / problem solving

Noise

Unwanted random fluctuations added to the a clean signal.

Open Loop

Open loop control systems do not have feedback
  • light dimmer (does not measure the light level)
  • volume control (does not measure how loud the sound is)

Optical Shaft Encoder

A method for measuring the rotation of a shaft.
  • Measure shaft position
  • Measure shaft speed of rotation

PAM

Pulse Amplitude Modulation

The amplitude of a stream of pulses is varied to encode the music, voice or data.

Parallel Port

8 Bit Data Input/Output Port

 

PC

Personal computer
  • has one complex processor
  • has few inputs
  • stores temporary data in one location (RAM).
  • solves well understood problems
  • must be programmed by a programmer
  • solve big but simple tasks like accounts or payroll.

PCM

Pulse Code Modulation

Data is transmitted as a stream of pulses that represent binary numbers. Analogue data (music etc) must be converted to digital before transmission.

Photodiodes

The leakage current through a reverse biased photodiode depends on the incident light level. This current can be used to detect light levels.

Faster and more sensitive than LDRs.

 

PIC / AVR

Programmable IC - A single integrated circuit that contains most of the functionality of a small microprocessor system.

These chips use the Harvard Architecture. There is an extra bus used to fetch instructions. This is a good idea because instructions and data can be fetched at the same time.

The single chip contains ...

  • a processor
  • four buses for address, data, instructions and control
  • flash memory (keeps its data with the power off)
  • RAM
  • I/O controls and pins
  • may also contain ADC and DAC circuitry

Polarisation

The orientation of the electric field component of an electromagnetic wave.

 

Polling

Repeated data input from a port.
  • Action is taken when the data input matches a value or range of values. (See Interrupts)

Ports

Used for Input and Output.

Ports have addresses.

The address bus is used to select the active port.

A control bus line activates either the I/O ports or RAM and memory mapped devices.

Input

  • keyboard
  • mouse
  • sensor with ADC

Output

  • printer
  • actuator
  • transducer with DAC

PPM

Pulse Position Modulation.

Pulses are transmitted slightly early or late compared with a reference clock signal. This is used to encode the music, voice or data.

PWM

Pulse Width Moculation

  

Radio Reveiver

The antenna receives radio waves and converts them into an alternating current. The tuned circuit selects the wanted station and rejects the others. The demodulator extracts the audio information from the carrier wave. The audio amplifier increases the power to be delivered to the loudspeaker. The speaker converts an alternating current into sound.

Radio Transmitter

The transmitter produces a carrier wave. An input transducer picks up information (eg microphone and sound). The modulator encodes the information onto the carrier. The modulated carrier is sent to the antenna where radio waves are radieated into free space.
  • input transducer (eg microphone)
  • carrier generator
  • modulator
  • RF power amplifier
  • antenna

RAM

Random Access Memory - provides temporary high speed storage.
  • forgets its data when the power is turned off (volatile)
  • is used to store temporary data
  • can be re-programmed
  • RAM locations have a unique address

Regenerator

A noise reduction technique that uses a Schmitt Trigger to clean up digital signals.

  

Resonance

Resonant systems respond most strongly at the resonant frequency.

 

ROM

Read Only Memory - provides permanent storage used for control and I/O code needed by most programs.

  • remembers its data when the power is off (non volatile)
  • can not be re-programmed
  • contains input/output code needed by other programs
  • contains control code needed by other programs
  • contains code to perform basic system management

Sample and Hold

Store an analogue voltage for long enough for it to be processed

This is used with ADC circuits to allow the conversion to complete without the input changing during the conversion.

Software

Programs and Data

 

Superhet

A better radio receiver design

  • Antenna
  • Tuned Circuit
  • RF Amplifier
  • Local Oscillator
  • Mixer
  • IF Amplifier and Filter
  • Demodulator
  • Audio Ampligier

Stepper Motor

A motor that provides precise angular position control.

  • Provide precise angular position control.
  • Useful for robotics, printers and disk drives.
  • Do not work well at high speeds.
  • Tricky to make this type of motor accelerate and slow down.
  • High power stepper motors are difficult to make.

Transmitter

Microphone, RF Oscillator, Amplitude Modulator, RF Amplifier, Antenna.

  • input transducer
  • carrier generator
  • modulator
  • antenna

Tristate

Logic that can disconnect itself from a shared bus
  • Outputs can be 0 or 1 or disconnected.
  • The chip-enable pin activates the device
  • Memory chips are activated by the correct address and signal on the R/W control bus wire

Tuned Circuit

The resonant frequency is calculated using the formula (left)

  • L - inductance in Henries
  • C - capacitance in Farads
  • f - frequency in Hertz

Q factor is the quality factor of the tuned circuit. A high Q gives a better ability to tune in radio stations and reject other nearby atations.

Wave Terminology

 
  • period
  • frequency
  • amplitude
  • cycle
  • polarisation

Wavelength

The length of a complete cycle of a wave. See frequency

C = F λ

C = 3 x 108 (speed of light)
F = Frequency
λ = Wavelength (Greek lambda)