tkt-3500 microcontroller systems 8 – external modules and sensors for mcu applications ... 1 2 3...

TKT-3500

Microcontroller

systems

Lec 8 – External Modules and Sensors

for MCU applications

Teemu Laukkarinen

Department of Computer Systems

Tampere University of Technology

Fall 2011

Copyright Tampere University of Technology Department of Computer Systems

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Contents

What sort of components exist and can be

used with a MCU

Through serial interfaces, buses, or GPIO

Sensors, flash-memories, ethernet/usb

connectivity, displays, user interfaces


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Introduction

MCU itself is useless in any embedded system

In many applications MCUs use sensors to

inspect physical world

MCUs interact with physical world through

actuators

Many embedded systems have user interfaces

Displays, buttons, detectors etc.

In modern world, many applications require

communication from the MCU: so called M2M

Ethernet, wireless, USB, BT


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Basic paradigm for (MCU) applications


INPUT OUTPUT TEMPUT

”BACKPUT” (Feedback)

Disclaimer: Vuoden 2011 paras

luentokalvo –

palkintokandidaatti

Sensors

User inputs

Other machines

Actuators: motors,

relays, switches

Displays

Other machines

MCU with macig software

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EXAMPLE SENSORS


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Temperature Sensor Maxim DS620

Operating voltage: 1.7 – 3.5 V

Communication: I2C

Selectable resolution:

LSB 0.5°C, 0.25°C, 0.125°C or 0.0625°C

Conversion time depends on accuracy

10 bit 25 ms, 13 bit 200 ms

Continuos conversion and one shot modes

Image: www.maxim-ic.com


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Humidity: Sensirion SHT1x

Humidity AND temperature sensor


Communication: ”almost I2C” Not compatible with standard I2C interface

Conversion times: 11 ms (8 bit)

210 ms (14 bit)

Accuracy: Humidity 2 %, temperature 0.3 °K

Current consumption: Sleep 0,3 uA, measuring 550 uA


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Accelerometer: VTI SCA3000

3D accelerometer


Communication: SPI

Modes:

Free fall detection

Motion detection

Normal mode

Current consumption 120 uA in active mode

Image: www.vti.fi


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Compass: Hitachi HM55B


Communication: SPI

Transmission length 4 bits (normal SPI 8 bits)

Current consumption: Sleep 1 uA, Measuring

9 mA

Measuring time 30 ms

Resolution: 11 bits

Measures magnetic field strength

Min. value -180 uT max. 180 uT

Image: www.parallax.com


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Luminance: Agilent APDS-9002

Photosensor: Luminance is relative to current


Connected to uC’s analog-to-digital converter

Resolution equals AD-converter’s resolution

10 bit in PIC18LF8722

Sample rate = ADC’s sample rate

In PIC user selectable, but depends on clock

frequency. About 1 Mhz is theoretical maximum

PIC’s AD-converter operates also in idle

mode

Completion of conversion can cause an interrupt

Image: www.farnell.com


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PIR MS300

PIR = Passive Infra Red

Connected to uC’s I/O, but needs an operating

voltage

Used as motion detector

Operating voltage: 2.6 V – 5.5 V

Current consumption: ~35 uA


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VGA C328

Max. 640x480 pixels, 16 bit

Frames/s depends on image quality

JPEG still picture with best quality 0.75 fps

Camera + compression module + EEPROM

No extra components needed

Connection: RS-232

JPEG picture format


Operation current: 60 mA

Suspend current: 100 uA

Image: www.electronics123.net


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GPS: iTrax03-s+GeoHelix–S

Fasttrax iTrax03-s

Includes lots of functionality: only few

extra components needed

Communication: USART with NMEA

(National Marine Electronics Association)

messages


Max power dissipation 500 mW

Sarantel GeoHelix-S

Active GPS antenna

Operating voltage: 2-3.5 V

Typical current 15 mA Copyright Tampere University of Technology Department of Computer Systems

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Microphone AOM-6746P-R

Capacitor microphone

Omnidirectional

Picks up sound evenly from all directions

Connection: ADC

PIC18LF8722 has 16 ADC channels, but one converter

Nyqvist frequency: to record 20kHz frequency, sampling

must be done at 40kHz rate

Image: www.farnell.com

Image: www.farnell.com Copyright Tampere University of Technology Department of Computer Systems

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Capacitive sensor

Measures capacitance between two inputs

Outputs raw values through a serial interface

(typically I2C)

or interrupts via output pins

Can be used for user interfaces, detect changes in

near by environment (e.g. something is removed or

brought near by)

In my experience, both conductors and insulators can be

detected (but cannot be necessarily classified)


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Capacitive sensor - Application

Everyone knows the iPod click wheel?

Wheel is an X input capacitive sensor

The user input direction can be figured out by following

the interrupts associated with the X inputs of the sensor

Example: 2 pieces of Analog Devices AD7156


MCU

Sensor

Interrupts

1

2 3

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More..

1-wire sensors

Acoustic proximity sensors

Gases

HALL –sensor

Pressure

More?


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ACTUATORS


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Motors (1)

MCUs control motors in many applications

Robotics, CNC machinery, air conditioning, cars…

Two types: ”fixed” position motors and speed

controlled motors

Fixed position motors can be instructed to stay standstill

in one position

Step motors, servo motors, linear actuators

Speed controlled continue rotating at some speed


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Motors (2) - Common problems/challenges

Motors require more current than MCU can provide -> an

external motor controller is needed, which can drive high

current to the motor according to the MCU’s commands

Motors have limited torque

If load is constant, some motors require more power to get

moving from standstill than when running

This requires actions in certain applications (more info on coming

slides)

Variable load can cause trouble, thus limit switches are

needed

calibration must be done, if limit is not reached when supposed to,

or if limit is reached unexpectedly


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Step motors (1)

Constant power, torque decreases when speed increases

are moved in steps, eg. 360 steps per round

A controller is needed

when step motor is kept standstill, it basically is a short circuit and

draws all the current it can (holding torque)

MCU can command motor to go one step (or half-step) through the

controller

A phase is associated to running/rotating motor (self-study more)

Calibrate motor to a position with a switch or a potentiometer,

then calculate steps and motor position is known relatively to

the calibration point

E.g. CNC machines use this method (Google DIY CNC machine)

Beware of skipping steps, thus always use limit switches or

potentiometers


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Step motors (2)

Can skip steps, if the load is too high (or resonance)

More torque is needed to move one step from standstill

than many steps on a running motor

There are maximum start and stop frequencies

Step motors have resonance frequency, where the

motor will loose its torque and might stall

You must know this resonance frequency and avoid it!

Google for more information, it is important to

acknowledge these hazards, thus, they are mentioned

on this course

http://www.sapiensman.com/step_motor/stepping%20motors.

htm


http://www.sapiensman.com/step_motor/stepping motors.htm

http://www.sapiensman.com/step_motor/stepping motors.htm

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(”RC”) Servo motors (1)

are controlled with PWM, the length of the pulse

decides the position of the motor

NOTE: a DC –motor with a potentiometer is considered

as a servo: motor can be driven to a position by

measuring the resistance of the potentiometer. These

use some encoder/controller to drive the motor. Also,

these can be used to determine the speed of the DC

motor

Once again, calibrate starting position and then

relative position is known


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(”RC”) Servo motors (2)

If the motor is not powerful enough for the load, MCU might

have to cheat to get precise position change of the servo on

short distances

1. Instruct motor to go beyond desired position

2. the controller might push more power to the servo, since the

distance to move is longer

3. Then, change the position to the desired before servo reaches it.

Controller will be able to stop it precisely where it should be

This is a hack and requires sometimes trial&error –method.. Not all

servos and applications require such actions and there can be

servos, which do not push more power for longer distance

Protip: by a stronger servo


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RC Servo motor

A RC servo motor does revolve freely but

between two extreme positions

Here they are marked as

CW - clock-wise

CCW -counter clk-wise mid

CW CCW

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clock-wise

counter-

clock-

wise

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Motors (4)

Typical AC/DC –motors

Run on variable speed to a direction

A speed controller is used with MCUs

Add potentiometer and you have a servo motor

Linear actuators

Construct of a motor and a screw (other methods exist

also)

The screw creates linear motion of the rotary motion of

the motor

Typically an extending/retracting shaft

Controlling is similar to servo motors


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Relays, switches, pneumatics,

hydraulics..

skip


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MCP41xxx digital potentiometer

Potentiometer provides variable resistance

Controlled via parallel or serial interface Analog potentiometer is wiped mechanically

Three terminals full resistance between PA0 and PB0

variable resistance from wiper, PW0-PA0 and PW0-PB0

PIC is the

master

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PID Controller

Proportional Integral Derivative Controller

Control loop feedback

Tries to minimize error between measured and ”asked” setpoint

value

Kx are tuning parameters, which are used to find best behavior for

the controller

Affect on rise time, overshoot, settling time, steady-state error and stability


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DISPLAYS


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LCD displays

2x16, 2x32, 2x64, 4x16, 4x32 etc. ASCII character matrixes Typically 4-8bit parallel bus, R/W, Cmd/DATA lines

Commands like clear screen, cursor position controlling (home, shift), program own characters..

”Real” LCD/OLED displays E.g. those in mobile phones

Controlled via serial bus, very similar to SPI or I2C, in addition of Cmd/DATA line

Use some standard graphic controllers such as PHILIPS PCF8833


#34/67 Copyright Tampere University of Technology Department of Computer Systems

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Flash memories

Typically SPI or I2C interface SD memory card supports SPI directly (and one-bit

SD bus)

Typically^2 only transition from ’1’ -> ’0’ possible Memory block can be written once, then it must be

erased back to the ’1’s for another write You cannot write again before erasing

Typically^4 flash memories have page / block structure Random access may be possible, but not over page /

block border

Erasing can be done only per page / block (or whole memory) You cannot erase one single byte


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COMMUNICATION MODULES


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Machine-to-machine – M2M

Machines communicate with each other more

and more every day

M2M used to mean ”large” machines

communication over GSM/SMS/GPRS

Communication may take place over

Wired bus (e.g. CAN)

Wireless ”Personal” Area Network

802.14.5, RFID etc.

Over internet (ethernet, WLAN, 3G/GPRS..)


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Radio: nRF24L01


Frequency: 2.400 – 2.4835GHz

Communication: SPI

Fully automated packet handling

Current consumption:

Power down 900 nA

Standby 22 uA or 320 uA (Two standby modes)

Transmission < 11.3 mA, Reception < 12.3 mA

Range about 15-30 meters indoors

Antenna dependable

Image: www.nordicsemi.com


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Communications – RS-232 / 485 / 422

etc.

Maxim MAX220 – MAX249 drivers an example

With RS-232 pretty much works as voltage

converter

RS-485 for example requires differential

signaling, thus chips change serial data format to

the differential signal (longer wires possible)

Similar for CAN bus for example


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Communications - USB

Small MCUs typically connect through USB-to-serial

adapter to a computer

These adapters appear as an RS-232 interface in

PC/Mac with standard drivers

SPI or UART used to connect adapter to the MCU

Some MCUs provide USB connectivity

The MCU can implement one of the standard USB

communication payloads, such as HID, audio device, etc.

Or the MCU can use proprietary USB communication

packet payload definition

Then, for sure, a driver is needed to the host device


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Communications – Internet –

Ethernet/WLAN/3G/GPRS

There are UART or SPI connected modules for basic

internet communication

Ethernet, WLAN, 3G/GPRS

These are either totally autonomous (e.g. the user must

configure Internet connection parameters) or controlled from

the MCU

Typically AT command specification is used to set configuration

Some controllers have a built in Ethernet connectivity HW

Then, the TCP/UDP, IP and so on protocol stack must be

implemented on the MCU

Luckily there are plenty of either free or open source stacks

Google: uIP, IwIP, Microchip provides one for their own ethernet

capable MCUs


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Ethernet and 3G examples

Ethernet: Lantronix XPORT Uart-to-ethernet ”bridge”

Up to 921kbps

AES-256 encryption

Configurable to establish TCP/IP connection with server

WLAN model exist also

Microchip ENC624J600 implement SPI to Ethernet

Microchip provides a TCP/IP stack for their Ethernet capable MCUs

With others you may use uIP


3G

Either modules (SPI/UART) or RS-232-to-3G modems

May be programmable some how

E.g. Telit

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Communications - BT

Similar to Ethernet / WLAN / 3G adapters, SPI and

UART conneted BT modules exist

AT commands are used to establish connection

between BT module and BT devices

Must be configured from the MCU

Similar to USB-to-serial –modules, BT has the same

functionality, however, the configuration and

connection establisment must be made from the

MCU

External RS232-to-BT adapters do exist, which can be

configured from the adapter itself


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BT example

http://www.sparkfun.co

m/categories/16

Serial over BT

Human Interface

Devices (BT dongle

provides interface to

directly mimic HID, e.g.

mouse/joystick/keyboa

rbd)


http://www.sparkfun.com/categories/16

http://www.sparkfun.com/categories/16

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Communications – wireless - ZigBee /

Zwave

External modules, which form a ZigBee (or similar)

network and provide easy wireless communication

between devices

Typically connected through serial interface

(I2C/SPI)

May require some configuration from the MCU

The network is autonomously kept alive by the

modules

Applications: wireless remote controls, wireless

sensor networks


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801.15.4 / ZigBee example

Implements 802.15.4

protocol stack

Add ZigBee routing etc.

and you have a ZigBee

Understanding stacks is

out of scope

Simple star and peer-to-

peer networks between

machines (MCUs, or

MCU <-> Computer etc.)


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RFID

RFID uses active

reader device

and passive

RFID tag

The tag receives

power over

induction when

brought close

proximity

Applications in

logistics,

security, key

cards, time cards


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More and conclusion

There is plenty of small cheap IC’s which can be

used with MCUs

Sensors, displays, communication units

Typically UART, SPI, I2C or parallel connection

Motors need some external circuitry since MCU do

not provide enough power

Different motors require different ICs

Explore more:

Google sensors, drivers, chips etc.

Exploring SparkFun gives a good idea what sort of things

you can do with ICs http://www.sparkfun.com/


http://www.sparkfun.com/

tkt-3500 microcontroller systems 8 – external modules and sensors for mcu applications ... 1 2 3...

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