EMBEDDED SYSTEMS:

SPECIALIST SENSORS

Programming the mbed

Nucleo Boards

I/O SHIELD - REVIEW

In the lab we used a specially designed I/O shield

It has a range of I/O devices

Switches, LEDs, LDR are all ‘two terminal’ sensors

More complex sensors have multiple connections to them

In this final session, we look at a Digital Temperature sensor and a 3-axis Accelerometer

USING SENSORS

Using a sensor requires you to understand a little bit about the quantity you are measuring, and the sensor you are using

Although this is not software, it is still the responsibility of the embedded systems programmer

Today we will look at measuring acceleration and temperature – a feature of both these quantities is that they can take on negative values

Some physical quantities:

change very quickly – acceleration is a good example

change relatively slowly – temperature is a good example

Our measuring system will need to take account of this if it is to be useful

Temperature is a ‘scalar quantity’ – it has no direction

Acceleration is a ‘vector quantity’ – it has direction (in fact it has 3 directions, x, y, and z)

DEVICE SUMMARYDevice Nucleo/

Arduino

Pin(s)

ARM Pins Comment

4 LEDs D10–D13 PB6, PA7,

6, 5

Can output 4 bits, 1 ‘hex’ digit

Nucleo Board ‘LED1’ is D13

4 switches D6-D9 PB10, PA8,

PA9, PC7

Can input 4 bits, one ‘hex’ digit

Push Switch USER_BUTTON PC13 Interrupt, Polling

I2C sensor D14 = SDA

D15 = SCL

PB9

PB8

DS1621 temperature sensor

PWM D13 PA5 Brightness of LED1 will vary

Multi-coloured

LED

D2 = Red

D3 = Blue

D4 = Green

PA10

PB3

PB5

3 colour LED, allowing (almost)

any colour to be produced by

addition

Potentiometer A4 PC1 Adjustable Analogue input

Light dep.

Resistor

A5 PC0 Voltage Vin varies inversely with

light level

Analogue

Accelerometer

A0 = X

A1 = Y

A2 = Z

PA0

PA1

PA4

ADX335 3 axis analogue

accelerometer

Axes are marked on module

(A2 can be configured as DAC –

untested)

Serial ‘TX’, ‘RX’

(on main

Nucleo board,

top left, CN3)

PC10 = ‘RX’

PC11 = ‘TX’

(labels are

misleading)

On

USART3:

PC10 = TX

PC11 = RX

Used by USB-serial system. Note

functions of RX/TX swap (it’s an

age-old problem)

The table summarises all of the device connections on the shield

The DS1621 is an I2C digital temperature sensor

The ADX335 is a 3 axis analogue accelerometer

The table is a summary of all the devices studied during the course

The exam will include questions on how to use some of these

DIGITAL SENSORS

Digital sensors measure an analogue quantity (e.g. temperature) and convert it into a digital signal (a series of 1’s and 0’s)

This is part of a trend in modern technology – most traditionally analogue devices are becoming ‘digital’:

Telephones are digital

Music and other forms of entertainment are stored digitally

Control systems are becoming digital

Our lives are increasingly reliant on digital devices

Computers have almost always been digital (although analogue computers were built, based on operational amplifiers)

Working ‘digitally’ has advantages and disadvantages

DW IO GR IK TI AN LG LY

Advantages:

Digital signals very rarely degrade - a logic 1 is always a logic 1 – it (almost) never gets mistaken for a logic 0

Digital systems are generally ‘noise-free’

Digital information is very easy to store in cheap digital memory

Digital memory does not degrade (have a look at an old ‘analogue’ photograph – the colours fade)

Digital quantities can be manipulated mathematically (they are already numbers) in processor-based systems – this allows all kinds of audio and video ‘effects’ to be applied (including restoring old analogue material)

Disadvantages:

The real world is analogue (and always will be ?)

To work digitally, all analogue signals have to be converted to digital in an ‘ADC’ – this is often where information is lost

Often, digital quantities have to be converted back to analogue before being used in the real world – another source of error

Working digitally often requires a different way of thinking

DS1621 TEMPERATURE SENSOR

Measures analogue temperature to within 0.5 °C and converts it to digital within the sensor

Measures temperatures in the range -55 °C to +125 °C

Provides digital output – easy to record, store, analyse, etc.

Actually contains an inbuilt processor, and multiple internal data registers

Requires communication using a specific protocol, I2C

Requires special hardware at the receiving end

Requires special software at the receiving end

Data can be transferred at 400,000 bits per second

COMPLEX INTERNAL SYSTEM

The DS1621 are addressable (you can have multiple sensors all connected to the same microcontroller)

Note 3 address pins, A0 – A2

There is a one bit digital programmable ‘overtemperature’alarm output, TOUT

Two pins handle the digital communication:

SCL is a clock signal (400 kHz)

SDA is the data signal

Taken from DS1621

data sheet

I2C PROTOCOL

I2C stands for ‘Inter-Integrated Circuit’ (I.I.C.), developed by PHILIPS in 1980’s

One of two main protocols used with sensors – some sensors have both (the other is SPI – Serial Peripheral Interface)

In the diagram above illustrates an exchange of signals between a sensor and an ARM processor

the processor asks for the sensor at address 49 to send a temperature

The sensor responds by sending the value 17.5 The last byte of data has a special meaning: 1000 0000 = 0.5 0000 0000 = 0.0

SCL:

SDA:

USING I2C WITH MBED

mbed provides libraries for communicating with I2C devices, but we need to customise them for the DS1621

The process requires relatively complex code

You will be provided with a file containing a ‘C++ function’ which can interface with the DS1621 (you may want to look at the code inside the file to see how it works)

You will need to build this file into your project, and call the function when you want to read the DS1621 sensor

You will write a program that can output temperature in degrees centigrade to Terminal

Your program will switch on a warning LED when the temperature exceeds a limit of 25 °C

Think abut how a temperature of -10 °C will be stored digitally

REPRESENTING TEMPERATURE

The manufacturers of the DS1621 illustrate how they use 2-byte digital values to represent a floating-point temperature value

Right-hand byte indicates ‘.5’ or ‘.0’ – a single decimal figure

Left hand byte is an 8-bit two’s complement value (which in theory can represent numbers in the range -128 +127)

To convert from a negative (MSB set) 2’s complement 8 bit value:e.g. 1110 1100

Invert all the bits 0001 0011

add 1 0000 0001

0001 0100

convert to decimal 20

add minus sign -20

EXAMPLES – CONVERT THE FOLLOWING TO FLOATING POINT TEMPERATURES

Convert the following 16 bit values to floating point temperatures:

16 bit binary Floating point decimal

0000 1010 1000 0000

1111 0001 0000 0000

1100 0100 1000 0000

0100 1011 0000 0000

RETURNING ARRAYS FROM FUNCTIONS

The DS1621 produces 2 items of data – the first is the ‘integer’ part of the temperature, the second describes the value after the decimal point

The ReadDS1621() function returns these two values in an integer array (a function can only return one ‘item’)

The function returns a ‘pointer’ to (address of) the array containing the temperature:

temp[0] is the integer value of temperaturetemp[1] is 0 or 128 (see earlier, ‘128’ or ‘1000 0000’ means 0.5)

To get access to this data, we need to tell the function where to store it – so we provide a pointer as return type for the function when we call it:

int* temp; //the address to store temperature valuetemp = ReadDS1621(); //call the function to get the temperature

PROGRAM TO READ TEMPERATURE

We need to tell the program about the ReadDS1621() function because it lives in another file (use ‘extern’)

We need to declare an address for the function to store the data from the sensor

This program then prints out: Temperature = x:yx = integer temperaturey = 0 or 128

We will need to do additional processing in order to convert to a proper ‘floating point’ value of temperature

ACCELEROMETER SENSOR

An ANALOGUE sensor which measures acceleration in three dimensions, x, y and z

ACCELERATION is a change in velocity – always caused by a force

ACCELERATION may be positive or negative (see graph).

When the sensor and board are laid flat on the table, the z axis points upwards

Because of the force of gravity, z will already be experiencing an acceleration when placed in this position

Acceleration is often measured in ‘g’, where 1g = 9.81 metre per second squared, m/s2

Whenever we move the accelerometer, it will record an acceleration in one or more of the directions

ACCELEROMETERS

Accelerometer sensors are ‘MEMS’ devices (micro electro mechanical systems)

They are small (very, very small). 1 micron is 1/1000 mm or 10-6 m

They are based on capacitance changes between plates that move under the influence of a force – the diagram indicates the scale of these devices

The change of capacitance is converted into an analogue voltage

ANALOGUE OUTPUT

The ADX335 has three analogue outputs, x, y, and z

The outputs consist an a voltage between 0 and 3.3 V

Because acceleration can be positive or negative, the manufactures decided that 3.3/2 = 1.65 V means zero g

Full scale reading is +/- 3g, so:

1.0 = 3.3 V = +3g

0.5 = 1.65 V = 0g

0.0 = 0 V = -3g

‘g’ reading = (AnalogIn.read – 0.5) * 6

Complete the table:

1.0 = 3.3V = +3g

0.5 = 1.65V = 0g

0.0 = 0V = -3g

AnalogIn

.read()

Output voltage

(V)

Acceleration (g)

0.1 0.33 -2.4

0.4

0.8

USING THE ACCELEROMETERNote from slide 4 that the accelerometer is connected to A0, A1 and

A2 analogue inputs

The code below shows how to read the three acceleration values into Terminal. As you move the board around, the values will change

What does ‘/t’ mean in a printf() statement?

ACCELEROMETER APPLICATIONS

Accelerometers react to movement, sudden mechanical shocks or ‘jolts’. They know which way is ‘up’ because they measure gravity

they are used in cars to sense an impact and set off safety systems like airbags

They can be used to sense earthquakes

They are used on mobile phones to keep the picture the right way up (how?)

They can be used as ‘burglar alarms’ because they can sense if something has been picked up and moved

And (obviously) they can be used to measure acceleration of a vehicle

ACCELEROMETER OUTPUTS

The changes in readings (0 – 1) from the accelerometer tend to be small – they can be mathematically amplified by multiplying by a scaling factor

We can build ‘graphing’ functions on Terminal by printing multiple asterisks (*) indicating the size of the reading (this technique can be used with the temperature sensor also)

The following code produces a line of asterisks proportional to the value of acceleration:

(note use of a ‘cast’ on line 13)

CONCLUSION

mbed provides an easy, low-cost way to implement embedded systems

We have only looked a some of the mbed features

You are encouraged to look at the mbed website which has a searchable reference for all the commands + a forum where you can get help with your programming

The website details over 50 ‘mbed compatible’ boards

The Nucleo boards we use come in 3 sizes, and with a wide choice of ARM processors on them: