MICROPROCESSOR INTERFACING - HASANUDDIN … · If you examine the data sheet for many microprocessors, you will find that a given pin can sometimes be an input, ... Microprocessor

Microprocessor Interfacing - v1.05 - J R Smith

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MICROPROCESSOR INTERFACING

1 INTRODUCTION

2 BINARY LOGIC AND ELECTRONICS

2.1 From voltages to logic2.2 TRI-STATE logic2.3 Binary inputs and outputs

3 BINARY INPUT TRANSDUCERS

3.1 Mechanical switches.3.2 Multiplexed inputs3.3 Switch debouncing.3.4 Some other switches3.5 Non-mechanical switches.3.6 Pseudo-binary inputs

4 BINARY OUTPUT TRANSDUCERS

4.1 Solenoids4.2 Pseudo-binary outputs

5 ENCODING INFORMATION BY VARIATIONS WITH TIME

5.1 Introduction5.2 Elapsed Time5.3 Frequency Modulation (FM)5.4 Pulse Width Modulation (PWM)5.5 Bitstream Modulation (BSM)5.6 Coding5.7 Information coding in Biology (not required for exam)

6 BASIC ANALOGUE COMPONENTS

6.1 Amplifiers6.2 Comparators6.3 Using analogue transducers as binary transducers (not required for exam)

7 DIGITAL TO ANALOGUE CONVERSION

7.1 How many bits?7.2 Bitstream7.3 Binary-weighted resistors7.4 R-2R Ladder

8 ANALOGUE TO DIGITAL CONVERSION

8.1 Parallel or Flash8.2 Successive Aproximation8.3 Integrating8.4 Delta - Sigma


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9 TRANSDUCERS

9.1 TRANSDUCERS FOR TEMPERATURE9.1.1 Thermocouple9.1.2 Thermistor9.1.3 Semiconductor junction9.1.4 Temperature dependent oscillator9.1.5 Resistor9.1.6 Peltier (thermoelectric) module.

9.2 TRANSDUCERS FOR LIGHT9.2.1 Light Dependent Resistor (LDR)9.2.2 Photodiode9.2.3 Phototransistor9.2.4 Solar cell9.2.5 Incandescent lamp (Light Emitting Resistor)9.2.6 Light Emitting Diode

9.3 TRANSDUCERS FOR SOUND9.3.1. Dynamic microphones9.3.2. Elecret, capacitor and condensor microphones9.3.3 Dynamic Speaker9.3.5 Electrostatic Loudspeaker9.3.6 Magnetostrictive transducer

9.4 TRANSDUCERS FOR CHEMICAL CONCENTRATIONS

10 INTERACTION SCHEMES

10.1 Programmed interaction or polling10.2 Interrupts10.3 Direct Memory Access (DMA)

11 SOME ASPECTS OF COMPUTER ARCHITECTURE

11.1 Types of memory

These notes are written with specific reference to the 'ATOM' microcontroller. Howevermuch of the information is also applicable to the 'BASIC STAMP' microcontroller or othermicrocomputers.

Copyright J R Smith 2003


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1 INTRODUCTION

You should now be familiar with the BASIC MICRO 'ATOM' microcontroller. It is based onthe 16F876 PICMicro MCU.

It has 8K of FLASH memory used for storing programs, 384 bytes of RAM for storing thevariables used in programs and 256 bytes of EEPROM that can store data when the power isremoved.

What can a device like this do?

It turns out that it can do almost anything. However it does have two fundamental limitations- speed and complexity.

The internal cycle time (200 ns) and the time taken to execute instructions (of the order of 30µs) both limit how rapidly the ATOM can respond to external events. This limitation can beovercome for short periods by using external circuitry with a faster response. However forcontinuous operation the speed is ultimately limited by the instruction execution time.Consequently the ATOM is simply too slow for some tasks (e.g. real time, high fidelity, audioprocessing).

The limited space available for program and variables also imposes an eventual upper limit onthe complexity of tasks that the ATOM can reasonably handle. However you are unlikely toapproach this limit. I have written large programs (>20 pages of code) that still fit into the 8Kmemory.

Physicists are interested in the behaviour of the real world, however the parameters of interestdon't occur in the form of binary signals with voltage levels compatible with the binary logicof microcomputers. Consequently transducers are used to convert various physicalparameters to and from suitable electrical signals.

These notes aim to provide an introduction to the

• interfacing computers to the real world• some common transducers• various techniques used to convert between analogue and digital variables• techniques for synchronising a microcomputer with real world events

Suitable circuits and programming examples will be presented wherever possible. They willoften be specifically for the ATOM28, but the basic principles are applicable to mostmicrocomputers or microcontrollers.


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2 BINARY LOGIC AND ELECTRONICS

2.1 From voltages to logic

Although you have now been using an electronic implementation of digital logic for severalweeks, until now you have not needed to concern yourself with what voltages were used torepresent binary logic. However before you can interact with signals from the real world youneed to which voltage levels correspond to a logical 0 (often called LOW) and which belongto a logical 1 (often called HIGH).

Currently there are 2 main families of electronic logic that you are likely to meet.

The first is 'TTL' - short for Transistor Transistor Logic. They are almost always poweredfrom a 5 V supply. Although there are many subspecies with slightly different characteristics,their logic levels are approximately

logical 0 corresponds to a voltage between 0 and 1.5 voltslogical 1 corresponds to a voltage between 1.5 and 5 volts

The second main family is CMOS. - Complementary Metal Oxide Silicon. These are usuallypowered with a voltage (VDD) in the range 3 to 18V - the particular maximum value of VDDdepends upon the particular species. Their logic levels are approximately

logical 0 corresponds to a voltage between 0 and VDD/2logical 1 corresponds to a voltage between VDD/2 and VDD

There also exist species of CMOS that have inputs that are compatible with TTL voltagelevels when VDD = 5V.

2.2 TRI-STATE logic

If you examine the data sheet for many microprocessors, you will find that a given pin cansometimes be an input, and sometimes be an output! To understand how this is possible weneed to examine a slightly different form of electronic logic - TRI-STATE logic.

This has the normal digital logic levels of LOW and HIGH plus an extra output state wherethe output is essentially disconnected from the internal logic circuitry. An additional 'enable'input determines whether the output behaves normally or is disconnected.

QABE

A B E Q

0 0 0 00 1 0 01 0 0 01 1 0 1x x 1 z

x = doesn't matter

z = high impedance or 'disconnected'

Fig. 2.2.1 A 2-input AND gate with tri-state output. The ’enable’ input is denoted by E.

Bus structures

TRI-STATE logic is very useful in bus structures where it enables multiple outputs to beconnected together. This will not give rise to logical contradictions providing only one outputis enabled at any given time.


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Multiplexed inputs

Many devices designed for interfacing with microcomputers come with TRI-STATE outputs.The outputs of multiple devices can thus be connected to a common set of inputs, and thedesired device selected via its enable input.

P3P2P1P0

DEVICE AD3 D2 D1 D0

DEVICE BD3 D2 D1 D0

P6P7

E

E

ATOM

Fig. 2.2.2 Schematic diagram showing how two devices with tri-state outputs can bemultiplexed. The state of the outputs P6 and P7 will determine whether the data (D3-D0)from either device A or B will be present at the inputs P3-P0.

Bidirectional ports

These allow the transfer of data in either direction - consequently it is possible for a given pinto be either an input or an output at different times.

A E Q

0 0 01 0 1x 1 z

x = doesn't matter

z = high impedance or 'disconnected'

QA

E

Fig. 2.2.3 A buffer with tri-state output.

direction

input/output

input (direction = 1)

output (direction = 0)

Fig. 2.2.4 Schematic diagram of a circuit that allows a single connection to be used as eitheran input or an output according to the state of an input that specifies the direction of datatransfer.


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2.3 Binary inputs and outputs

The circuit shown below can be used to determine the range of input voltages that correspondto a logical ‘0’ and a logical ‘1’.

20k

+5V

V

P0

ATOM

Fig. 2.3.1 Circuit for measuring input characteristics of digital inputs.

The ATOM has 2 types of input.

Inputs 0 to 7 have conventional TTL levels.

Schmitt trigger inputs.

Sometimes the input characteristics are deliberately given a degree of hysteresis - the logiclevels are different for increasing and decreasing voltages. This feature gives the logic adegree of immunity against 'noise' on the input signal - often very useful when interfacing tothe real world.

Inputs 8-15 are Schmitt trigger inputs.

The 0->1 transition occurs around 3V, whereas the 1->0 transition occurs around 1.5V

input voltage0 1 2 3 4 5

0

1binaryoutput

input voltage0 1 2 3 4 5

0

1binaryoutput

Fig. 2.3.2 The input characteristics for conventional TTL (left) and TTL with Schmitt triggerinputs (right).


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3 BINARY INPUT TRANSDUCERS

Some input transducers or devices inherently have only two possible states.

3.1 Mechanical switches.

These usually involve contact between two or more pieces of conductive material - usuallymetal. There are many possibilities and we will limit discussion to some simple basic types.The diagrams below indicate how they could be connected directly to a digital input. The 10kresistor connected between an input and +5V is often known as a 'pull-up resistor' - it 'pulls'the input up to +5V when nothing is connected to that input.

On the ATOM the command SETPULLUPS can be used to connect a set of 10k internal pull-up resistors on inputs 1 to 7 if required.

P0

+5V10k

P0

+5V

10k

P0

+5V

SPST SPST SPDTSingle Pole, Single Throw Single Pole, Single Throw Single Pole, Double Throw switch open: P0 = 1 switch open: P0 = 0 switch up: P0 = 1 switch closed: P0 = 0 switch closed: P0 = 1 switch down: P0 = 0

Fig. 3.1.1 Circuits indicating how switches can be connected to a digital input, in this case P0of the ATOM.

SPDT switches are sometimes called 'changeover'.

All of the pins on the ATOM are inputs by default when a program starts. A pin can be madean input via the INPUT command, its value can be determined by examining the variable In0.

Some switches only change their state momentarily, e.g. push buttons. They are oftenavailable as 'normally closed' (nc) or 'normally open' (no). They can be wired using the samecircuitry as conventional switches. Often the transition between states is important. Thefollowing code repeatedly tests if the input = 1, and proceeds to the next instruction once theinput = 0, i.e. the button has been pushed. It thus effectively detects the 1-> 0 transition.

Loop1:IF In0 = 1 THEN Loop1'program reaches this point when In0 = 0


8

10k

P0

+5V

Fig 3.1.2 Circuit indicating how a push button can be connected to a digital input.

3.2 Multiplexed inputs

The 16 switches on your BS2 development board are connected to the ATOM via a 16 inputmultiplexer. A 4 bit address determines which of the 16 inputs is connected to the multiplexeroutput which can then be connected to an ATOM input. Determining the state of a switch isslower with this technique because the address of the desired switch must be supplied to themultiplexer before reading. Multiplexing also means that you can only determine the state ofa single switch at a time - without multiplexing the ATOM could simultaneously determinethe state of 16 switches. However it has the great advantage of needing only 5 input/outputpins rather than the 16 required if the switches were connected directly.

Question: What is the maximum number of switches that your ATOM could monitor withsuitable external multiplexing hardware?

3.3 Switch debouncing.

Mechanical switch contacts usually 'bounce' for a few milliseconds after they make initialcontact. Consequently in the circuits given above for SPST switches you will get a series ofmultiple rapid transitions between 1 and 0 as the contacts close. In most applications you willneed to ignore these initial bounces.

1

0

Idealised Real world

Fig. 3.3 Diagram illustrating the multiple transition between states that occur when a switchchanges states.

The ATOM has a BUTTON instruction is designed to take care of switch debouncing. I can't get it to work!


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3.4 Some other switches

ThermostatsThese are switches that change state at a given temperature, usually based on a bi-metallicstrip. Transition is either predefined or adjustable. Quite cheap (e.g. JAYCAR ST-3821/3/5/6= $4.45).The bi-metallic strip is composed of two metals with different coefficients of thermalexpansion. When heated the lower layer of metal expands more than the upper layer, the stripbends, and the circuit is is broken.

high temperaturenormal temperature

Thermal cutouts and thermal fusesThese are switches that go open circuit (usually permanently) above a predefined temperature.Cheap (e.g. JAYCAR ST-3800/4/8 = $2.80). Two springy wires are held together by a wax-like material. At a sufficiently high temperature the wax melts and the wires spring apart.

high temperaturenormal temperature

Mercury switchesA small drop of mercury maintains electrical contact between two wires when the switch is inthe upright position. If the switch is rotated, the mercury drop moves and an open circuitresults. Useful for detecting when an object is moved from the vertical position.Compact and relatively cheap ( e.g. JAYCAR SM-1035 = $2.80)

2 wires

bead ofmercury

glasscontainer

Magnetic Reed SwitchConstructed from magnetic wire sealed inside an evacuated glass enclosure. Contacts arenormally open, but close when a magnet is near. Can also get n.c. or changeover contacts.Compact and cheap (e.g. JAYCAR SM-1002 = $2.80). Widely used for burglar alarms. Haveseveral advantages including:

(i) no mechanical contact required(ii) metal contacts are in an isolated environment – no corrosion

Trembler switchThe end of a thin springy wire is surrounded by a ring of wire. Vibration or motion will makethe springy wire vibrate and thus make momentary contact with the wire ring. Useful fordetecting motion, or <TILT> on a pinball machine.


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3.5 Non-mechanical switches

Switches can operate without moving mechanical parts. Some examples use the strength of amagnetic field (Hall Effect switches) or the presence or absence of light (various opticalswitches).

3.6 Pseudo-binary inputs

Many analogue transducers are often used in a pseudo-binary fashion. This is because it isoften sufficient to know if an analogue parameter is greater or less than a certain value. Theinputs 0 to 7 of the ATOM assume that Vin<1.5 V is a logical 0 and Vin > 1.5 V is a logical 1.The following code sets an output high or low depending upon the input voltage.

LOOP:IF In6 = 0 THEN

LOW P8 ELSE

HIGH P8ENDIFGOTO LOOP

A potential divider can be used to increase Vin above 1.5V. Similarly an amplifier (see later)can be used for Vin <1.5.


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4. BINARY OUTPUT TRANSDUCERS

4.1 Solenoids

The most common bistable output transducers are variants of the solenoid. These operate bypassing a current through a coil of wire. The resultant magnetic field then attracts amagnetizable material into the solenoid. The mechanics of the device are usually designed toensure that the operation is bistable.

Solenoids are used to move mechanical components between two positions. They are oftenconnected to a hydraulic valve to control water flow in domestic appliances such asdishwashers and washing machines. They can also be used to operate levers.

Relays are a solenoid in which the mechanical action is connected to a switch. It is thenpossible for the small current from a microprocessor to switch much larger currents. Theyalso provide electrical isolation between the microprocessor and the controlled circuit.

Solenoids often require relatively large currents for operation. The ATOM can only supply asmall current from its outputs, and so a single transistor, or even a Darlington pair, will oftenbe used to amplify the current available from the outputs of the ATOM. The value chosen forR will depend upon the particular relay and transistor used. The transistor effectivelyamplifies the current by a factor known as b; typically b = 100. The function of the diode inthe following circuit is to 'short out' any transient high voltages that can occur when thecurrent through the inductance of the solenoid is removed.

+V

R

solenoidor relay

P0

4.2 Pseudo-binary outputs

Many analogue transducers are often used as pseudo-binary fashion. Examples include LEDS,heating elements, etc.

The intensity of light emitted by a LED depends upon the current flowing through it, and thisdepends upon the voltage across it.

R

LEDP0

The value of the resistor R can be changed to suit the particular LED. The voltage across atypical LED is about 2 V. Consequently the current I flowing through the LED will be givenby I = (5 - 2) / R. If R = 220 ohms, then I = 14 mA.


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5. ENCODING INFORMATION BY VARIATIONS WITH TIME

Up until now we have discussed simple binary operations - inputs and outputs have only twopossible states. Surely a microprocessor can do more than this.

The answer is to consider the past history of binary operations -information can be encodedinto the time variation of a binary signal. The encoding process is often called 'modulation'.We will look at a couple of possibilities.

5.1 Introduction

Elapsed TimeThe information is encoded in the elapsed time between some events.

Frequency Modulation.The frequency of the binary signal carries the information

Pulse Width Modulation (PWM)The frequency is kept constant, but the width of the pulse carries the information.

BitstreamThe running average value of a stream of pulses carries the information. The frequency andpulse widths will usually change to allow the desired waveform to be as precisely as possible.

Digital CodingCan use the presence or absence of pulses in a specific sequence to carry the information.Some electronic examples are RS232, USB, Firewire (IEEE1394), I2C, etc. Some non-electronic examples include Morse code, semaphore, etc.


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5. 2 Elapsed Time

One common technique involves measuring the time taken to charge or discharge a capacitorthrough a series resistor.

The ATOM has a useful instruction that allows it to measure the resistance and/or capacitanceof an external element - the instruction RCTIME

RCTIME Pin, State, Variable

Pin - specifies the pin to be usedState - specifies the desired state for measurement. A counter is started once RCTIME starts

to execute. Once Pin is not in State, the instruction terminates and the value of thecounter is stored in Variable.

Variable - used to store the measurement of ELAPSED time. For your ATOM each unitstored in Variable corresponds to 1 µs. If Variable is a WORD, the maximum time thatcan be measured is 216 µs = 65.535 ms.

This instruction can be used to measure the time take to charge or discharge an externalresistor / capacitor (RC) circuit. This can be very useful - the ATOM can measure the settingof potentiometer. It can also measure the output of many transducers that change theirresistance or capacitance in response to changes in the parameter of interest. It can be used tomeasure the duration of short pulses. It can even be used to measure voltages.

When RCTIME commences execution it starts an internal counter. This counter is stoppedonce the specified Pin is no longer has the value = State.

The time constant t = RC can be calculated from the time taken for the capacitor to charge ordischarge. Then R can be calculated from C, or C can be calculated from R.

Before RCTIME is used, it is essential to use an OUT command to charge or discharge thecapacitor to either 0 or 5 V. This level must be maintained until the capacitor is effectivelycharged or discharged - typically 4t will be sufficient - see manual.

Charging circuit

V(t) = V [1 - exp(-t/t)] charging

t = - t / ln[(V-VFinal)/V] = - t / ln[(5-1.5)/5] = 2.8 t

For the circuit shown below, VFinal = 1.5V (the transition from 0->1 occurs around 1.5 V oninputs 0-7 of the ATOM).

volta

ge

0

1.5

timeRCTIME

P0

+5V

R

C

Demonstration code for RC charging.


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Temp var TIMELOW 0 ; start to discharge capacitorPAUSE 10 ; the length of the pause should be ≈ 4RC

; you should calculate it for your circuitRCTIME 0,0,TIME ; now measure time to recharge

Discharging circuit

A more accurate approach is to measure the time it takes to discharge a capacitor. Thisbecause then VInitial = 5 V and VFinal = 1.5V, a difference of 3.5 V.

V(t) = V exp(-t/t) discharging

t = - t / ln[VFinal/V] = - t / ln[1.5/5] = 0.83 t

volta

ge

01.5

timeRCTIME

P0

+5V

R

C5

Demonstration code for RC discharging.

Temp var TIMEHIGH 1 ; start to discharge capacitor

; i.e. both plates at +5VPAUSE 10 ; the length of the pause should be ≈ 4RC

; you should calculate it for your circuitRCTIME 0,1,TIME ; now measure time to recharge

Measuring voltage

It's also possible to use the RCTIME command to measure an unknown voltage by connectingcomponents with known values of R and C to the unknown voltage. The process of chargingthe capacitor from the voltage source can drain a significant current, and so this approach isonly suitable for voltages with a low source impedance.


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5.3 Frequency Modulation (FM)

The frequency of the binary signal carries the information. The example below shows howthree different frequencies might be encoded.

low freq high freq medium freq

Its often convenient to keep the duty cycle constant and equal to 0.5. Because random noiseand interference will generally affect only the amplitude of a signal, FM is very tolerant ofnoise and interference. For example, compare FM radio with AM (amplitude modulated)radio.

The ATOM has an instruction for measuring the frequency of an input signal.

COUNT pin, period, variable

This instruction makes the selected pin an input, then counts the number of complete cycles(i.e a 0->1->0 or 1->0->1 sequence) during the defined period (in ms). The count is stored in‘variable’. It can measure square wave inputs with frequency < 125 kHz (the pulse widthmust be ≥ 4 µs).

Sometimes the frequency itself of an input signal is important. One example would be a guitartuning meter.

An analogue voltage can be measured by connecting it to a voltage-controlled oscillator(VCO). A VCO produces an output signal with a frequency that depends upon the inputvoltage. The ATOM can measure the VCO frequency, and thus determine the analoguevoltage.

Voltage controlled oscillator

ATOM(COUNT instruction)

unknownvoltage

There is no single instruction for producing a square wave output at a defined frequency. Youwill have to write your own if needed using PULSOUT or OUT. (The instructionsDTMFOUT and FREQOUT generate pseudo-sine waves using a bit-stream technique).

FM is sometimes used to send purely binary information in noisy environments – for examplemodems.


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5.4 Pulse Width Modulation (PWM)

The frequency is kept constant, but width of the pulse (tH) carries the information. The timetH + tL is kept constant. The duty cycle is defined as tH / (tH + tL).

tH tL tH tL

duty cycle = 0.5 duty cycle = 0.75

tH tL

duty cycle = 0.25

The ATOM has no simple instruction for producing a ‘correct’ PWM signal with constantfrequency. (The PWM instruction uses what we will call bitstream modulation).The interpreter is slow, each instruction takes a few ms for execution.The following example can produce pulses of different width depending upon the value of'Num'. The frequency remains constant, but is hard to predict accurately because extra time istaken by the For loops and the BRANCH instruction.

Demonstration code‘ Program to demonstrate PWMVar VAR WORDNum VAR WORDDenom VAR WORDNum = 2Denom = 5Loop:For Var = 1 to NumHIGH 0NEXTFor Var = Num+1 to DenomLOW 0NEXTBRANCH Loop

You will later use a sort of PWM signal to control the angle of servomotors.

The average value of a PWM signal can also be used to carry information. However it is notparticularly suitable for rapidly varying signals because it takes many complete cycles for theaverage value to change. A better approach involves allowing the frequency to vary as well asthe pulse width - see bitstream modulation.


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5.5 Bitstream Modulation (BSM)

The 'running average' value of a stream of pulses carries the information. The frequency andpulse widths will usually change to allow the desired waveform to be as precisely as possible.

The example below shows roughly how a triangle wave could be encoded.

The PWM instruction effectively performs BSM. The output string of pulses can be filtered toproduce different analogue voltages. It thus can act as a Digital to Analogue converter (DAC).The component pulses are quite short (often approx. 4µs and so are easier to filter out thantrue PWM).

Demonstration code.

' Program to demonstrate BSM of LEDs‘ Throbbing LEDsLoop VAR WORDLED VAR BYTEFOR Loop = 0 TO 255FOR LED = 0 TO 255PWM 0, LED, 5 // gradually increase brightnessNEXTFOR LED = 0 TO 255PWM 0, 255-LED, 5 // gradually decrease brightnessNEXTNEXTEND

The FREQOUT instruction uses BSM to produce an output containing one or two pseudo-sinusoidal signals.

FREQOUT Pin, Period, Freq1, Freq2


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Sound generation

The stream of pulses produced by PWM or FREQOUT needs to be filtered to remove thehigh frequency components associated with the rapid 0->1 and 1-> 0 transitions. A simple RCfilter is usually OK (see manual). This instruction is very useful for generating audio outputthat is useful for:• Warning ‘beeps’ or ‘pings’• Provideing feedback on program status, - e.g. an indicator that program has finished somepart of program• Providing feedback (usually via frequency) about some measurement that does not require

visual interaction.• Fun !! (Why not program your ATOM to play ‘Stairway to Heaven’ ? )

390R

SPEAKER

P0

5.6 Coding

Can use the prescence or absence of pulses in a specific sequence to carry the information.Some examples are RS232, USB, Firewire (or IEE...).

The ATOM has some instructions designed to extract information from pulse sequences.

SERIN – used to read in asynchronous serial dataSEROUT – used to send asynchronous serial dataSHIFTIN – used to read in synchronous serial data (i.e. when a clock is involved)SHIFTOUT – used to send synchronous serial data (i.e. when a clock is involved)

5.7 Information coding in Biology (not required for exam)

Frequency modulation is used in the human nervous system. Nerve impulses are binary pulsesthat switch from approx - 60 mV to approx + 50 mV and back to - 60 mV in a few ms.Information about external stimuli is generally carried by the rate at which these nerveimpulses occur (usually the rate is approximately proportional to the logarithm of the strengthof the stimulus - physiologists call this Fechner's law).

The biological equivalents of logic gates in the nervous system are implemented by a cunningtemporary transition to analogue chemical transmission. Nerves connect to each other atspecialised junctions called synapses. When the nerve impulse reaches the end of an ‘input’nerve, it releases small amounts of a chemical messenger into a small region between thenerves – the synaptic cleft. The rate at which nerve impulses occur at the ‘output’ nerve isdetermined by the concentration of chemical messengers in the synaptic cleft. There are twotypes of chemical messenger – agonists (these tend to increase the firing rate) and antagonists(these tend to decrease the firing rate). The binary signal produced by the output nerve thusdepends upon the nett effect of the various agonists and antagonists that are released by the‘input’ nerves. Try PHYS2410 if you would like to know more about this 'Biologic'.


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6 BASIC ANALOGUE COMPONENTS

6.1 Amplifiers

+

_Vout

VA

VB

Vout = Gain (VA - VB)

This is a differential amplifier, i.e. it amplifies the difference between the two inputs.

6.2 Comparators

Basically a high gain amplifier with analogue inputs and a digital output, i.e. the outputvoltage levels are compatible with standard logic.

Used to determine whether VA is greater or less than VB

+

_Vout

VA

VB

If VA < VB Vout = 0 If VA > VB Vout = 1

6.3 Using analogue transducers as binary transducers

Often we just want to know if an analogue signal is greater or less than a certain value. Theinputs 0-7 of the ATOM assume that Vin<1.5 V is a logical 0 and Vin > 1.5 V is a logical 1.So by amplifying the input voltage Vin with a gain of G, and subtracting a reference voltageVREF it is possible for the 0"–>"1 transition at an ATOM input to correspond to any desiredinput voltage.

+_ P0

+5V

+_

VREF

VIN

Comparator

Amplifier gain = G

GVIN GVIN VREF_


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7 DIGITAL TO ANALOGUE CONVERSION

We have already seen how an ATOM can output an analogue voltage using PWM orFREQOUT. However in many situations we require higher precision, faster response andmultiple outputs. These will require extra components external to the ATOM.

7.1 How many bits?

How many bits are required to produce a voltage with sufficient precision?

n bits can encode 2^n possible states

n = 8 10 16 20 24

# of states 256 1024 65,536 ~10^6 ~16x10^6

precision ~0.5% 0.1% 16 ppm 1 ppm 0.06 ppm

ppm = parts per million

24 bit precision is incredibly precise. A plane journey from Sydney to Rome is about 16,000km and takes perhaps 20 hours flying time. 24 bit precision would require knowing yourabsolute position with respect to Sydney with an error of only 1 m, only 20% more than thedistance between seats in economy class.

7.2 Bitstream

Could use the PWM instruction to produce a desired DC voltage.PWM Pin, Duty, Cycle

If output is smoothed with a suitable filter, Vout = (Duty / 255) x 5 V

For time varying output voltages, very sophisticated algorithms can be used to produce theclosest match to the desired waveform, whilst minimising noise in the output signal.Bitstream can be very precise because there are essentially no components that need to becalibrated (see later). However very precise values require averaging a large number ofpulses, so they are relatively slow. Some examples can reach 23 bit precision with samplerates below 100Hz.


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7.3 Binary-weighted resistors

8 mA

4 mA

2 mA

1 mA

Load

R1 = 1.25k

R2 = 2.5k

R1 = 5k

R0 = 10k

S0

S1

S2

S3

Fig. 7.3 An idealized circuit in which the load resistance is assumed to be small incomparison with the resistors R0 to R3. A functional DAC would generally use someadditional transistors or op-amps.

By closing the appropriate switches we can get any current from 0 to 15 mA.If each switch were controlled by an appropriate bit, the current I could be described by

I = value of 4-bit word in mA.

The current can easily be converted into a voltage if required.

The problem: for an n-bit DAC we need n resistors that cover a range from R to 2nR, where Ris the value of the resistor with the lowest value, each resistor must have a tolerance of at least± R. For n > 10 this is very difficult to achieve. Even if discrete resistors are initiallysufficiently precise, they will slowly experience different shifts in value with temperaturevariations and aging. Laser trimming of resistors on the same substrate can achieve suitableprecision for n <= 16.

Another approach generates binary weighted currents by using digital logic to produce pulsesequences with different average values. In this situation there is nothing to go out ofcalibration.


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7.4 R-2R Ladder

A cunning method of obtaining binary weighted currents that only needs one or two values ofresistor.

i

i

i

2i

2i

2i

4i

4i

4i 2i

8i

8i

8i 4i16i R 2R

2R2R2R2R

R R

The resistors can be manufactured on the same substrate, and consequently should havesimilar variations with temperature and time.


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8 ANALOGUE TO DIGITAL CONVERSION

Our ATOM now needs a method of measuring an analogue voltage using of externalcircuitry. We now discuss Analogue to Digital converters (ADC).

8.1 Parallel or Flash

This involves simultaneous comparison with multiple voltage standards.

+_

+_

+_

Vin

3V

2V

1V

Priorityencoder

D3

D2

D1

D0

A0

A1

+5V

The above circuit is a 2-bit flash converter.

The output of the priority encoder (A0,A1) is the binary address of the highest asserted input(D0-D3).

Advantages: very fast, only requires the time for a comparison plus some digital logic.Disadvantages: expensive - an n-bit converter requires 2n-1 comparators. Becomes prohibitivefor n>10.

They are usually used for low resolution, very high speed situations (e.g. digitising video ordigital oscilloscopes). The oscilloscopes in the lab have 8 bit, 1 GHz converters.


24

8.2 Successive ApproximationThis involves a single comparator in sequential comparisons with a voltage standard that isadjusted according to prior comparisons.

+_

Vin

Successiveapproximation register

Start

VDAC

DACn

end of conversionEOC

The 'start' command initiates a binary search sequence controlled by the SuccessiveApproximation Register (SAR)

(1) The most significant bit (msb) of the DAC = 1, all other bits = 0.(2) the comparator output now indicates whether VDAC<Vin or VDAC>Vin.(3) if VDAC<Vin then msb = 0 or if VDAC>Vin msb = 1 for the rest of the conversion(4) now set the next significant bit = 1. Again the comparator output will indicate if this bitshould remain set at 0 or 1.(5) repeat for all the bits of the DAC.(6) assert the EOC output (End Of Conversion)(6) the digital approximation to Vin is then the final code sent to the DAC.

Advantages; relatively cheap, only requires one comparator + DACDisadvantages: slower than flash because an n-bit conversion requires n comparison steps.

Can operate at speeds approaching 10 MHz at 12 bits.Slower versions (200 kHz) can have 16-bit precision.


25

8.3 IntegratingThis measures the time taken for a known capacitor to charge using a known current until itsvoltage is equal to the unknown voltage.

+_

Vin

Binary counterstart

stop

Start

C

IVC

(1) Initially the counter is set to zero and the switch is closed, so VC = 0.(2) The first step is to open the switch and start the counter.(3) The capacitor then starts to charge up. The current I is constant, so VC increases linearlywith time.(4) Eventually VC = Vin. The comparator output then changes state and stops the counter.(5) The count will then be proportional to Vin.

Advantages: very cheap, capable of very high precision - 20 bits or more, the integrationprocess can remove noise and interference.

Disadvantages: very slow, conversion rates are typically a few Hz.


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8.4 Delta - Sigma

+_

Vin +_

Integrator ComparatorDifferentialamplifier

1-bit DAC

bitstreamoutputVDAC

V1 V2 V3

The output of the differential amplifier , V1 = Vin - VDAC

The integrator output V2 = sum of V1 over many cycles.

V3 is the output of a comparator: if V2>=0 V3 = 1 else V3 = 0

VDAC = +1.00 if V3 = 1 or VDAC = -1.00 if V3 = 0

The circuit operates at a very fast clock rate. If the value in the integrator is positive the DACoutput will be positive and this will be subtracted from Vin, reducing the output of theintegrator. Similarly if V2 is negative, then VDAC will be negative and the output of theintegrator will be increased.

Consequently a stream of pulses is produced whose average value reflects the value of Vin.This then undergoes sophisticated filtering via a decimation filter to become the final serialoutput.

Advantages: cheap, very precise (a one bit DAC has nothing to go out of calibration), canperform very sophisticated filtering to remove noise and interference.

Disadvantages: The clock rate must be very much higher than any changes in Vin.

These ADC are increasingly used for digital audio (20 bit resolution at 96 kHz) and very lowfrequency measurements (23 bits at 20 Hz).


27

9 TRANSDUCERSTransducers are devices that convert one form of energy into another. Usually we use them tocovert various physical parameters to and from electrical signals. This is because amicroprocessor or microcontroller can conveniently measure electrical signals via an ADCand output electrical signals via a DAC.

9.1 TRANSDUCERS FOR TEMPERATURE

9.1.1 Thermocouple

A thermocouple is made when two dissimilar metals are placed in contact. A voltage isgenerated between them that varies with the temperature of the junction.The voltage is quite small <100 µV/oC, so amplification will often be required. To reduce thenumber of temperature dependent junctions between different metals, the unknowntemperature is often measured with respect to another thermocouple that is kept at a constantreference temperature (usually at 0oC). A special integrated circuit can also be used toprovide the correct reference voltage.

ATOM(ADCIN instruction)ADC

+_

Reference

Unknown

9.1.2 ThermistorThese use semiconductor materials or oxides with a resistance that varies with temperature. Itis possible to get NTC (negative temperature coefficient) or PTC (positive temperaturecoefficient) devices. Their resistance is usually a non-linear function of temperature. A typicalvalue might be 10 kW with a variation of - 1kW/oC.

!!!!!!ATOM(RCTIME instruction)

+5V

Thermistor

C

!!!!!!ATOM(SHIFTIN instruction)ADCR

+5V

Thermistor


28

9.1.3 Semiconductor junctionThe voltage across a semiconductor junction varies with temperature. (For a silicon pnjunction it is approx. 2.2 mV/oC). In theory one can use any diode, but calibration is difficult.Consequently it is easier to use specialised integrated circuits that provide either a voltage or acurrent that is linearly proportional to the temperature.

!!!!!!ATOM(SHIFTIN instruction)ADCR

+5V

9.1.4 Temperature dependent oscillatorThis transducer uses an oscillator that is sensitive to temperature changes. Temperature ismeasured by counting its output frequency.

Temperature controlled oscillator

!!!!!!ATOM(COUNT instruction)

9.1.5 ResistorWhen a current I passes through a resistor R, the Joule heating produced is given by I2R.When a voltage V is maintained across a resistor R, the Joule heating produced is given byV2/R. (Because the value of a resistor changes slightly with temperature, they are alsosometimes used to measure very low temperatures).

9.1.6 Peltier (thermoelectric) module.In these modules a flow of electric current provides the necessary energy for a flow of heatagainst its thermal gradient (i.e. heat is extracted from a cold surface and passed to a hotsurface). These allow a computer to control the amount of cooling. If the polarity of thecurrent is reversed, the direction of current flow is also reversed.

!!!!!!ATOM(PWM instruction)

R

C

amplifier

load

The load in the above circuit could be a resistive heating element or a thermoelectric module.


29

9.2 TRANSDUCERS FOR LIGHT

9.2.1 Light Dependent Resistor (LDR)When a photon of light of a suitable wavelength falls on a semiconductor, an electron can beexcited from the valence band into the conduction band. The resistance of the material thendecreases. This leaves the absence of an electron in the valence band - usually referred to apositively charged 'hole'. Eventually an electron and a hole will meet and recombine, and theoriginal resistance will be restored. There are some materials, particularly CdS or CdSe,where the decrease in resistance can be greatly amplified. An LDR usually consists of a thin,folded, strip of such material. Their response time is quite slow (typically hundreds ofmilliseconds). Typical values might be 1-10k in dim light and 100 ohms in bright light. Theycan be connected in the same fashion as a thermistor (see section 9.1.2)

9.2.2 PhotodiodeThese are made by placing p- and n- type semiconductor materials in contact and so forminga pn diode. The n-type has an excess of mobile negative carriers (electrons) and the p-type anexcess of mobile positive carriers (holes - really the absence of electrons). Electrons at theedge of the n-region will be attracted to the holes at the edge of the p-region, and vice versa.They combine, and a high resistance 'depletion layer', with a very low concentration of mobilecharge carriers is formed across the region of contact. A strong electric field is producedacross this depletion layer because of the charge movement associated with its formation.

When photons strike a depletion layer they can generate electron-hole pairs. These wouldnormally quickly recombine, but the strong electric field across the depletion layer rapidlyseparates the electron-hole pairs so that they can't recombine. This produces a shift in thevoltage-current characteristics of the diode, and can be used to produce a current or a voltagedepending upon the external circuitry.

The current increases linearly with illumination, but generally requires some amplification.

!!!!!!ATOM(SHIFTIN instruction)ADC

R

+

_

9.2.3 PhototransistorThe current generated by the electron-hole pairs is now amplified via normal bipolartransistor action. Used mainly to detect the presence/absence of light.

9.2.4 Solar cellThese are p-n junctions that are designed to optimise power production.

!!!!!!ATOM(SHIFTIN instruction)ADC

Solar cell


30

9.2.5 Incandescent lamp (Light Emitting Resistor)If a material is made sufficiently hot it will start to emit visible light. Usually made from acoiled piece of resistance wire in an evacuated bulb. (Air would allow the hot wire tooxidise).Can be used in a similar fashion to resistive heating elements and thermoelectric modules.(see section 9.1.6). Because of thermal inertia they have a relatively poor frequency response.

9.2.6 Light Emitting DiodeThese are semiconductor diodes that emit approx. monochromatic light when current flowsthrough them. The corresponding voltage across the diode is usually in the range 1.5 to 2.5volts. (Shorter wavelengths require higher voltages).

9.3 TRANSDUCERS FOR SOUND

9.3.1. Dynamic microphones

The sound pressure wave acts on a diaphragm that moves a coil of wire in a magnetic field,thus producing a voltage. The voltage is small (mV or less) and will require amplificationbefore the ADC.

Diaphragm Voltage

Magnet

Coil of wire

9.3.2. Elecret, capacitor and condensor microphones

Sound pressure moves one plate of a very delicate charged capacitor. Because the chargeremains constant, while the plate separation varies, the voltage across the plates will change.Elecret microphones use permanently charged plastic electrodes (electrets). Capacitor andcondensor microphones usually require an external source of charge (48V on standard audiosystems).

Voltage

+ -

9.3.3 Dynamic SpeakerCurrent is sent through a coil of wire (the voice coil) that is placed in a magnetic field. Adiaphragm is connected to the voice coil so that a varying current causes motion of thediaphragm that moves the surrounding air.

Speaker coneVoltage

Magnet

Speaker coil


31

9.3.4 Piezoelectric speakerA voltage across a piece of piezoelectric material causes it to flex, and thus set air in motion.

Speaker cone

voltage

piezo-electricmaterial

9.3.5 Electrostatic LoudspeakerA high voltage is place between two conducting plates. Causes relative motion of plates thatin turn set air in motion. basically a capacitor microphone in reverse.

9.3.6 Magnetostrictive transducer

Magnetostrictive materials contract in a magnetic field. One example is the 'Soundbug'. Arod of magnetostrictive material (Terfenol - made from terbium, iron and dysprosium) issurrounded by a wire coil. When a current flows through the coil, the resulting magnetic fieldcauses the Terfenol to change shape. This in turn vibrates the object to which it is attachedand that produces the sound.

Voltage

magnetostrictive material

suction cap

9.4 TRANSDUCERS FOR CHEMICAL CONCENTRATION

9.4.1 pH electrode

The electrode tip is made from special glass that is permeable to protons. A voltage is thengenerated between the inside and outside that depends upon the internal and external protonconcentrations. Their output impedance is very high, usually > 109 ohms.

proton permeable glass

+_

unknown [H]

known [H]

high inputimpedanceamplifier


32

10 INTERACTION SCHEMES

10.1 Programmed interaction or polling

In this approach the computer program continually monitors various status flags that indicatethe state of real world variables.

Advantages: Simple to program, requires minimal hardwareGives the fastest response to an external event, i.e. minimum latency.

Disadvantages:Fastest response requires that the microprocessor continually monitor the status flags leavingno time for other tasks.

If a very fast response is not required, for example checking if a key has been pressed on acomputer keyboard, then the microprocessor can do other tasks whilst only checking thestatus flags occasionally.

10.2 Interrupts

This involves the external hardware having access to the internal workings of themicroprocessor via specific ‘interrupt’ logic lines.

When a specific condition occurs on one of these lines, the microprocessor acts as if the nextinstruction in your program was

Gosub address

This can be thought of as a ‘pseudo-instruction’. The subroutine that you wish to be executed(the interrupt handler) should be located at this address. When this subroutine has finishedprocessing, the program continues operating from the line in your code after the one it wasexecution when the interrupt occurred.

Instruction are available that tell the microprocessor to either ignore or to pay attention tointerrupts. Serious microprocessors also have methods of specifying different priorities fordifferent interrupts.

Advantages: Microprocessor can perform useful tasks whilst waiting for interrupt.

Disadvantages: Requires special hardware Increased latency Harder to program

RESETS

All microprocessors have another type of interrupt - the RESET.

The microprocessor can then always start execution from a well-defined state.


33

The ATOM has three types of RESET

Power on reset = ONPWR: This occurs when power is first applied to the ATOM, or whenpower is removed and restored

Brown out reset – ONBOR: This occurs when the power supply voltage falls below 4.1V.This can be useful for situations that derive their power from batteries. A ‘LOW BATTERY’warning can be initiated, and any important parameters saved in EEPROM. Also useful fordetecting situations when the mains power fails.

Reset pin reset – ONMOR: This occurs when a 0->1 transition occurs at the ATN/RES pin.Often triggered by a ‘RESET’ button.

10.3 Direct Memory Access (DMA)

The device that wishes to perform DMA must first undergo an arbitration process with themicroprocessor to become the ‘bus master’. When ready the microprocessor can relinquishcontrol of the busses. The device can then take control of the busses and rapidly transfer data.

Advantages: The microprocessor is free to perform other tasks Very fast data transfers, particularly when large blocks of data are moved.

Disadvantages: High latency can make it inefficient for transferring small amounts of data. Requires more complex hardware – the device must be smart enough to become ‘busmaster’ Requires more complex programming.

The ATOM28 has no DMA capability (there are no external busses that it can release toanother device).


34

11 SOME ASPECTS OF COMPUTER ARCHITECTURE

11.1 Types of memory

ROM = Read Only Memory: The desired truth table is permanently programmed by themanufacturer of the integrated circuit.

PROM = Programmable ROM: The desired truth table is programmed by the user. Thisinvolves using an externally applied current to melt small internal wire links. Cannot bereprogrammed.

EPROM = Erasable PROM: The desired truth table is programmed electrically by the user asthe presence of charge on internal ‘capacitors’. The data can be erased by exposure tophotons of sufficiently high energy, i.e. ultraviolet light. A quartz window is provided for thispurpose.

EEPROM = Electrically EEPROM: The charge on the internal ‘capacitors’, and hence thestored truth table can be altered by an applied voltage.

FLASH = a type of EEPROM that can be easily reprogrammed.

RAM = Random Access Memory: nowadays this refers to read/write memory that ‘forgets’when the power is removed.

There are two main species: Dynamic and Static.SRAM = Static RAM: This memory is implemented using flip-flops.

DRAM = Dynamic RAM: This memory is implemented via charge on tiny ‘capacitors’. The charge on these capacitors leaks away quite rapidly – in a couple of ms. Consequentlythe memory is ‘refreshed’ by internal logic every ms or so.

Static vs Dynamic

Static memory requires more transistors per bit than dynamic, and so is more expensive andoccupies more space. However it is significantly faster.

An ideal computer would use only SRAM, however this would be too expensive for mostsituations. The common approach is to use SRAM within the microprocessor and DRAM forthe main memory. A copy of sections of the main memory is then kept in caches built usingSRAM that can be accessed rapidly by the microprocessor.

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