ENGR101: Lecture 10 - ADC and intro to RPi · Hardware you will use in this week lab ... iit is...

Lecture 10

ADCs on the Rpi

Arthur Roberts ENGR101: Lecture 10 March 2017 1 / 29

What we cover today?

Hardware you will use in this week lab - don’t memorize this part,iit is more of a referenceMore on ADC and how to process ADC output signalLoops in C - you will need this for the lab


Hardware: Raspberry Pi

Full Linux computer$5 - $50Programmable in C, PythonetcNo hard drive, SD card insteadNo on-board ADC

Display

SD card

MouseKeyboard


Hardware: Our addition to Raspberry Pi

ADC ADCinputs(A0-A8)

GROUND

+5V

POWER INON_OFF

digitalinputs/outputs

MOTORCONTROL

Additions:10 bits ADC, 8 channels (likewe have 8 ADCs).Power supply output +5V forsensors.Digital input/output, 8channels.Motor control circuitry - candrive 2 motors.


Hardware: RPI start screen

Figure: On RPi start

Connect display, keyboard,mouseSwitch power onIt takes about 30 sec for RPI tostartIf nothing is happening (blackscreen) it is likely that SD cardis not working. Ask forinstructions.


Hardware: Common programs

Figure: Applications

Utility programs (underAccessories):

File managerText editorcommand terminal


Hardware: Programming RPI

We will use "Geany" again (for the time being, anyway).

It works almost same way as on PC. There is one subtle differencethough.


Hardware: Geany tuning for our hardware

add -le101

add -le101

To work with our custom hardwarewe need to use C library (moreon what library is in next lecture)To connect the library please clickBuild− > SetBuildCommands inGeany menuAdd -le101 to Compile and Buildat the end of command strings(Use library e101 when compiling)Add sudo at the beginning ofExecute command string (Thisprogram will run withadministrator/super usercredentials)


Hardware: How to copy files?RPi is connected to Victoria WiFi (no or very limited Internet though). Itcan be used to transfer files to your usual folders.

SERVER: greta-pt.ecs.vuw.ac.nz

Your ECS user name and password

Port: 22( SFTP)

Files on RPi Files in your homefolder

Drag over to copy

Under Intenet startFileZilaEnter server name,your ECS user nameand passwordPort 22Click Quick connectOK in pop-up dialogYou can copy files

SFTP? Wait until network lecture...Arthur Roberts ENGR101: Lecture 10 March 2017 9 / 29

Hardware: Where our analog signal is coming from?

We use infra-red distancesensorNarrow Field Of ViewOutput voltage changes ifdistance to obstacle changes.Manufacturer and modelnumber - printed on outer caseHave a look at sensorspecifications: Always helps toknow the answer in advanceSignal is changing very slowSignal is noisy

SENSOR GROUND

+5 V

SIGNAL


Reminder on ADC

There is an input signal.It can take any value ony-axis(amplitude) and is continousin time.


Reminder on ADC

We take measurements of thesignal at certain moments of time.We ignore what is happening withthe signal between measurements.OK as long as our measurementsare done fast enough.Measured values still can take anyvalue.Signal becomes discrete in time.


Reminder on ADC

We round the signal to nearestvalue which can represented asbinary number. Signal is discretein time and amplitude.

Figure:


More on ADCs: Noisy?

We measure voltage, i.e. howmany electrons are in signalwireMore electrons on wire -higher measured voltage.Electrons can not staystill(unless it is really cold).Changes in voltage are calledthermal noise - if you put wirein liquid helium then there willbe no more noise.Electrons do not conspireagainst you and have nomemory - thermal noise israndom.


More on ADCs: Noise?

Signal is there but we can notsee pure signal - only withnoise addedSignal is changing slowNoise is truly random:chances of one noise samplebeing positive(or negative) are50%If that is the case we can addour measurements togetherand hope that positive oneswill cancel eachother


More on ADCs: Averaging!

True value is 10. Randomnoise is added.Add all measurement resulttogether:11 + 9.2 + 9.2 + 10.5 + 10.5 =50.4... divide by number ofmeasurements to get average:50.4/5 = 10.0810.08 is closer to true value(10) than any specificmeasurement:11,9,2,9.2,10.5,10.5


More on ADCs: Filter

What we just did is called digital filtering.Algorithm for our filter is simple:

1 Take N measurements of the signal.2 Window is N samples wide.3 Calculate average value4 Average becomes new (hopefully improved) measurement

Of course, it is slower. It takes N times more as compared with singlemeasurement.


Losing information in filter

Signal changes fast - stepIn window2 two samples are ofold signal value and one is ofnew valueFilter output for window2 doesnot reproduce input signal

input signal

9

11

10

WINDOW 1

output signal

WINDOW 2WINDOW 3

time

Our filter is not really smart. It removes noise but it distorts the signalas well.


C programming: To read from ADC

Listing 1: Ex1.c#include <stdio.h>#include <time.h>#include "E101.h" \\this is our custom ENGR101 C library.

int main(){//This sets up the RPi hardware and ensures//everything is working correctlyinit();//We declare an integer variable to store the ADC dataint adc_reading;//Reads from Analog Pin 0 (A0)adc_reading = read_analog(0);//Prints read analog valueprintf("%d\n",adc_reading);//Waits for 0.5 seconds (500000 microseconds)sleep1(0,500000);

return 0;}


Remainder on C programming...

1 main() - starting point of the program. Program starts executionthere.

2 #include - it includes. Includes libraries.3 stdio.h - standard input output. Standard means display and

keyboard (Mouse is a pest). h stands for header - list of this libraryfunctions available. Let’s have a look...

4 "E101.h" - library for our hardware


You will need to run some code several times whencreating filter. Repetitions in CSometimes we want to repeat execution of some part of the codeseveral times. If we know in advance how many times we want torepeat we can use for operator.

int i;for (i=0; i<5;i=i+1){ printf("%d\n",i);}

1 2TRUE

3

4

FALSE

There are 3 parts in operatorfor(Part1 ; Part2 ; Part3)Part1 is executed whenoperator starts - alwaysPart2 is condition. If conditionis:

I TRUE - do whatever isbetween curly brackets andgo to Part3

I FALSE - abort for, go toclosing curly bracket andcontinue


Repetitions

int i;for (i=0; i<5;i=i+1){ printf("%d\n",i);}

1 2TRUE

3

4

FALSEExecute Part3Go to Part2, againRinse and repeat

It goes around and around, so it is called looping


Simple repetition

Listing 2: Ex1.c#include <stdio.h>int main(){int i;for (i=0; i<5;i=i+1){printf("%d\n",i);

}}

And result is...01234


Quiz time (0.5 minutes)

What is result of that?

Listing 3: Ex1.c#include <stdio.h>int main(){int i;for (i=0; i<5;i=i+2){printf("%d\n",i);

}}

1 0 1 2 3 42 0 2 43 0 1 3


Quiz time again(0.5 minutes)

What is result of that?

Listing 4: Ex1.c#include <stdio.h>

int main(){

int i;for (i=10; i<5;i=i+1){

printf("%d\n",i);}

}

1 10 9 8 7 6 52

3 5 6 7 8 9


Another way to loop

Using for() is good if you know in advance how many times you wantto loop. There is another way, can be simpler.

Program checks condition inthe brackets of while().If condition estimates to TRUEthen code between openingcurly bracket andcorresponding closing one isexecuted.And this is it.

int i = 0;while ( i<10){ // ...}

TRUE

FALSE

Too simple, isn’t it?


Yet another quiz!!! What is an output?

0 1 2 3 4 5 6 7 8 90 0 0 0 0 0 0 0 0 0 0 0...10 10 10 101 10 10 10

Listing 5: while.c#include <stdio.h>

int main(){

int i = 0;while (i<10){

printf("%d\n",i);}

}


Corrected

Listing 6: correct while#include <stdio.h>

int main(){int i = 0;while (i<10){printf("%d\n",i);i = i +1;

}}

It is surprisingly common error.


That is all for today.Next lecture - more on C programming.Questions?


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ENGR101: Lecture 10 - ADC and intro to RPi · Hardware you will use in this week lab ... iit is...

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