embedded c lecture 3 2008

8/6/2019 Embedded C Lecture 3 2008

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embedded

Part 3

C and the PIC Microcontroller


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PIC PORTS

Port A Digital I/O and Analogue in

Port B Digital I/O and Analogue in

Port C Serial (RS-232 / SPI/ I2C) Port D Digital I/O

Port E More Analogue


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Analogue to Digital Convertor

(ADC) Converts an analogue voltage to a digital

number.

Resolution the number of bits used to hold theresulting number. 8 bit, 10 bit, 12 bit, 16 bit, 24 bit.

(fixed for a particular ADC)

Sampling rate how frequently the analogue

voltage is sampled. (set by micro) The ADC on the PIC has a resolution of 10 bits

(how many possible states)


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ADC examples

1. your input voltage that can vary in the range

0-5 volts. your 10-bit ADC reads the value

512. What voltage is it reading?

2. you have a voltage that can vary in the range

0-5 volts. your 10-bit ADC reads the value 12.

What voltage is it reading?

3. What number would you expect to come out

of the ADC in the above examples if you put

in a d.c. value of 1.00 volts?


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Teds ADC codevoid configure_analogue_ports(void)

{ // Configure analog inputs

TRISA = 0b11111111; //Pins 2,3,8,9,10 as AN0,AN1,AN5,AN6,AN7ADCON1 = 0b00000111; // 00 for unimplemented bits, 0 for Vref- //= Vss,

0 for Vref+ = Vdd, 0111 for AD0-7 enabled as analog //inputs

}

unsigned int read_analog_channel(unsigned int n)

{ ADCON0 = n


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Connecting a resistive sensor

5 V

10k

220


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Input voltage clamping using

Zener diode+12V

-12V

Zener

diode

+5V

0V


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QRB 1134 Light Sensor

The phototransistor

responds to radiation

from the emitting diode

only when a reflectiveobject passes within its

field of view.

The area of the optimum

response approximates a

circle .200 in diameter

Reflective surface (tape)

infrared

emitting

diode

NPN silicon

phototransistor

Optimum

distance

5mm


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Port C

5v

220 10k

C

QRB1134 Light Sensor

coloured

material


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Port C

switches

3v

10k


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Sharp range finder

0 10 20 30 40 50 60 70 80 90 100 110 120 130

3.0

2.5

2.0

1.5

1.0

0.5

0

Output voltage vs distance

Distance [cm]

Output

Voltage

[v]


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Sharp Rangefinders


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Presentations

Weight , mass centre of balance / gravity (Jonathan, Conor)

Wheel positioning and steering (Liam, stephen reilly)

Combining batteries (John Brady)

Measuring speed (John Byrne, John Oneill)

Pressure / touch sensor ideas (Aidan, Martin)

Finding centre of arena (Paul Devitt, Mark McConnell)

Measuring load on motor (Illa, Paul)

Tilt switches and interfacing them to PIC (Ali, Alex)

Finding your opponent (Gary, Darren)

Gearing - Billy Purcell, Yumi

Non destructive weapons (Gra???


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joystick interface

1 potentiometer common (Joy A)

2 button 1 (Joy A)

3 X coordinate potentiometer (Joy A)

4 button common (Joy A)

5 button common (Joy B)6 Y coordinate potentiometer (Joy A)

7 button 2 (Joy A)

8 unused

9 potentiometer common (Joy B)

10 button 1 (Joy B)

11 X coordinate potentiometer (Joy B)12 MIDI TXD (transmit) (computer ->

midi)

13 Y coordinate potentiometer (Joy B)

14 button 2 (Joy B)

15 MIDI RXD (midi -> computer) J


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Multiple ADC inputs

You can wire as many analog inputs to the PIC but you may notice some strange effects when

you do so. The ADC circuits will affect each others' readings, because all the circuits are

drawing from the same +5V source. One thing you can do to minimize this is by using

decoupling capacitors ( 0.1F to 1F) for each analog input, to smooth out dips and surges in

the current caused by the other ADC's. Place the capacitors between power and ground, as

physically close to the ADC input as you can get.

The effect is also reduced if you increase the sampling time and the delay between ADCIN

commands

Finally, it helps to sample at a lower resolution. Sampling at 8 bits instead of 10 will improve

the speed and stability of a multiple ADC program calculate the resolution that you need.

You can also (if you have a voltage regulator powering the PIC which you dont for the robo

sumo but you do for instrumentation) decouple the power supply using the circuit shown

below on your 7805 regulator.


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Ranges of different rangefinder

sensors in Sharp RangeThe GP2XX detectors come in several derivatives. The table below helps tocharacterize each type by minimum and maximum ranges, as well as whether the

sensor returns a varying distance value or a boolean detection signal:

GPDP12

GPDPD12D

GP2Y0A02YK

GP2Y0A21YK

GP2Y0A700K

GP2D15

GP2Y0D02YK


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Pulse Width Modulation

Can be used to control

power to a motor

By adding an

appropriate low passfilter can be used to

produce slowly varying

analogue output from

a digital output pin.

Duty cycle (%)

This filtered

Signal would provide

An analogue value which is an

average of the original PWM signal

20% duty cycle-> V =20% of full scale etc

This signal has been filtered but either the LPF cutoff

needs to b reduced or the period of PWM increased


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Pin setup for PWM example5 V

10k


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Teds analogue example with PWMmain() {

configure_pins();while(1) {

n=read_analogue_channel(0);

set_pwm(1,n);

set_pwm(2,255-n);

}}

void configure_pins(void)

{configure pins code here}

unsigned char read_analogue_channel(unsigned int channel)

{code to do conversion and get analogue value here}

set_pwm(unsigned char channel, unsigned char value)

{stuff to change PWM duty cycle and send out to pins here}

forever

embedded c lecture 3 2008

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