DATA LOGGING

26 ❙ February 09 - Electronics World

TEMPERATURE is basically themeasurement of how hot or cold somethingis, and it is one of the fundamental entities inany kind of process control. Temperature isalso a measure of the average kinetic energyof particles in a system. Adding heat to asystem increases the kinetic energy of itsparticles and hence an increase in thetemperature of the system.

Temperature is easily measured using athermometer. Nowadays, thermometers comein all shapes and sizes. In computer-basedautomated process control applications, whereit may be required to control the temperatureof a plant, it is necessary to interface thethermometer to the computer. This is usuallydone either by using analogue temperaturesensor devices (such as thermistors,thermocouples, RTD devices etc) and thenconverting the outputs into digital formatusing A/D converters, or digital sensors (suchas semiconductor sensor chips) are used withdirect interface to the computer’s.

A Selection of Temperature Data Loggers

There are many applications where oneneeds to store the variation of temperaturewith time continuously over a long period oftime. In such applications it is common to usetemperature data logging devices. There aremany types of temperature data loggingdevices in the market with prices ranging from

tens of dollars to over thousands of dollars.Examples include Squirrel OQ610(www.tempcon.co.uk), which is a 6-channeltemperature data logger using thermocoupleprobes. The device is battery-operated and upto 260,000 temperature readings can bestored in its non-volatile memory.

The TH-03 temperature data logger(www.picotech.com) is a 3-channel resistivelogger with PC interface and ±0.3˚C accuracy.The device is connected to the serial port of aPC and sends the temperature readings to thePC where the data can be analysed using thesupplied software.

The USB-5201 (www.microdaq.com) is an8-channel temperature data logger withCompact Flash card for data storage.Temperature is measured using thermocoupleprobes. The device is supplied with a numberof software packages for data analyses and itcan also provide high/low alarms. In addition,8-bits of digital I/O pins are provided.Although the device is very powerful its cost isover $600, typically only afforded byprofessional organisations.

OM-EL-USB (www.omega.co.uk) is asmall, low-cost, USB type temperature datalogger which plugs into the USB port of a PCto collect temperature data. The device hasbuilt-in memory that can store 16,000temperature readings over a single channel.The logging interval can be changes from 12seconds to 12 hours. Although the device is

low-cost, its memory is limited. Interested readers should note that other

devices in the same family have highermemory capacities. For example OM-EL-USB4(www.winling.com.tw/OM-EL-USB.pdf) issupplied with a memory that can store up to32,000 temperature readings.

TL-500 (www.audon.co.uk) is a wirelesstemperature logging device with a USB basedreceiver and up to 50 remotely locatedtransmitters with a range of 20-40 metres.The receiver has built-in memory and canoperate standalone. The measurement intervalis 45 seconds and data can be collected forover 110 days using one channel only.Wireless sensors are useful in remoteapplications which may be difficult to reacheasily.

MCU-Based DesignThis article describes the design of a

microcontroller-based multichanneltemperature data logger device with real-timeclock chip and an SD card for the storage ofvery large amount of data. One of the nicefeatures of the device is that it is based on the1-Wire bus where a large number oftemperature sensors can all be connected onthe same bus.

A battery-operated RTC chip keeps the real-time clock information and the temperature iswritten on the SD card with real-time clockstamp. Each sensor on the bus has a unique64-bit ID and a sensor responds only when itsown ID is on the bus, thus enabling the datato be read selectively.

The RTC data, data logging interval and theID of each sensor can be set by connectingthe device to the serial port of a PC andrunning terminal emulation software on it.Once the device is configured it can beoperated standalone to collect and save thetemperature from a large number of sensors.The collected data can then be importedoffline into a spreadsheet package such asExcel, and statistical analysis can be carriedFigure 1: 1-Wire bus structure

Design of a multichannel temperaturedata logger with SD card storage

DATA LOGGING

out, or graphs of the collected data can easilybe drawn.

The device also sends out from its serialport the same data as written to the SD card.Thus, it is also possible to capture this data ona PC and perform online analysis of the datain real-time.

1-Wire Bus1-Wire bus is a serial protocol using a single

data line (plus ground reference) forcommunication. 1-Wire bus provides low-speed data rate and is similar in concept toI2C bus, but with lower data rates and longerrange. It was developed by DallasSemiconductors (www.maxim-ic.com) as ameans of connecting a large number ofsensors over a long distance to a single buscomplete with integrated power source.

There are many 1-Wire supported devices,such as temperature sensors, humiditysensors, pressure sensors, switches, counters,real-time clock chips, EEPROM memories, A/Dconverters and so on.

Depending on the 1-Wire device used, there

are two ways to power a device on the bus:parasitic mode and external power mode. Inparasitic mode, a capacitor is charged in eachdevice during the logic high state of the dataline. The capacitor then provides power to thedevice during its normal operation and is re-charged whenever the data line is at logichigh. In this configuration, the 1-Wire busconsists of only two wires, data and ground. Ifthere are many devices on the bus then acurrent source may be required to feed thebus to charge the capacitor of each device.Parasitic mode is normally used in smallapplications with only a few devices on thebus.

In external power mode, a third wire is usedto supply the 5V directly to each device andthis mode is recommended in large 1-Wirenetworks.

Dallas recommends that unshielded CAT5cables should be used as the 1-Wire bus.Shielded cables should not be used as thenetwork may be upset as a result of theincreased capacitance. For small networkapplications and short runs, standard

telephone cables can be used for the bus. Thedesign and layout of the cabling is alsoimportant to avoid any timing, loading, orreflection problems. In small networks, star orradial connections can be used where eachdevice is connected to a central point with itsown cable. In large networks, therecommended method is to use daisy chainingwhere a single continuous cable is used toloop from sensor to sensor, as shown inFigure 1.

In a typical application, a 1-Wire masterinitiates communication on the bus andcontrols the bus with one or more slavedevices present on the bus. Each device onthe bus has a unique, factory programmedand unalterable 64-bit identification number(ID). When there is communication on thebus, only the device with the referenced IDresponds to the request. When there is onlyone slave device on the bus the ID can be“skipped” and the device will respond to allrequests on the bus.

The 64-bit ID consists of 1-byte family code,6 bytes serial number and 1 byte CRC code.

Each device type has a unique family code.For example, the family code of DS1820temperature sensor devices is 0x10. The serialnumber uniquely identifies the device. TheCRC byte is used to detect any errors insending the ID.

The ID of a device is not supplied by themanufacturer, but it can be found easily whenthere is only one device on the bus, forexample by writing a program to read anddisplay the ID on an LCD. When there is morethan one device on the bus it is much moredifficult to find the ID of each device andspecial algorithms are needed to determinethe ID of each device. Alternatively, a 1-WireEvaluation Kit, such as the DS9090K(www.maximic.com/quick_view2_cfm/qv_pk/4135) can be used to find the ID of alldevices on the bus. The evaluation kit isplugged into the USB port of a PC and issupplied with a Java program namedOneWireViewer.

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Professor Dr Dogan Ibrahim from the Department of Computer Engineeringat the Near East University in Cyprus describes the design of a microcontroller-based multichannel temperature data logger device with SD card storage andreal time clock interface

Figure 2: Block diagram of the temperature data logger

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28 ❙ February 09 - Electronics World

DS1820 1-Wire Temperature SensorAlthough there are several types of 1-Wire

temperature sensors, the DS1820 chip is usedin this design. DS1820 is a 3-pin sensor withthe following specifications:● 3-pin chip● Direct 1-Wire interface with no external

components● Measures temperatures from -55˚C to

+125˚C in 0.5˚C increments● User definable, non-volatile temperature

alarm settings● 9-bit digital output● 200ms conversion time.

In addition to the normal temperaturemeasuring functions, the DS1820(www.systronix.com/Resource/ds1820.pdf)can be used in thermostatic controlapplications in industrial systems, for exampleto set temperature limits with alarm outputs.

The output of DS1820 is in 16-bits signextended two’s complement format, but forpractical applications only 8-bits define thetemperature and the ninth bit is used toindicate the sign, as shown in Table 1. It ispossible to obtain much more accuratetemperature readings from the device byusing the on-chip counter values, but thisapplication is beyond the scope of this paper.

The conversion of the read digital value toactual temperature in degrees Celsius isstraight forward; for positive readings, simplydivide the value read by 2. For example, if the9-bit reading is “0 00110010” (i.e. decimal50), dividing by 2 gives the temperature as+25˚C.

For negative readings, take the complementof the reading, add 1 and then divide by 2.For example, if the 9-bit reading is “111001110”, taking the complement andadding 1 gives “0 00110010”, dividing by 2we get the temperature as -25˚C.

DS1820 contains an 8-byte scratchpadregister where the converted temperature isstored in the first two bytes. A typicaltemperature conversion process consists of

the following steps.For a single DS1820 on the bus:

● Send “SKIP ROM” command (0xCC).● Send convert temperature command (0x44)● Wait for the conversion (it can take up to

400ms)● Send the read temperature command. First

of all, send the “SKIP ROM” command(0x55), followed by the READSCRATCHPAD command (0xBE).

● Read the low-byte and high-byte of thetemperature from the scratchpad register.For more than one DS1820 on the bus:

● Send the “MATCH ROM” command (0x55),followed by the 64-bit device ID to therequired device.

● Send convert temperature command (0x44)● Wait for the conversion (it can take up to

400ms)● Send the read temperature command. First

of all, send the “MATCH ROM” command(0x55), followed by the 64-bit device ID,and then the READ SCRATCHPADcommand (0xBE).

● Read the low-byte and high-byte of thetemperature from the scratchpad register.

Design of the Temperature LoggerThe data logger is designed around a

PIC18F4520 (www.microchip.com) type

microcontroller. This is a highly powerfulPIC18 series microcontroller having thefollowing basic features:● 40-pin package● 32 K-byte flash program memory● 1536 byte RAM memory● 256 byte EEPROM memory● 36 I/O pins● 13 channel, 10-bit A/D converter● Analogue comparators● 4 Timer/counters● Prioritized interrupts● SPI, I2C, USART support● Operation up to 40MHz● 8 x 8 hardware multiplier● C compiler optimized● Sleep/Idle mode.

The block diagram of the temperature datalogger device is shown in Figure 2. The usercontrols the device with two push-buttonswitches: Reset and Configure/Run. Resetsimply resets the device to a known state.

When in Configuration mode the date andtime of the RTC and various loggerparameters can be set through the RS232 portusing a terminal emulation program on a PC.

In Run mode, the device reads thetemperature from all the sensors attached tothe 1-Wire bus and stores the date/time andthe temperature in degrees Celsius in a file on

Figure 3: Circuit diagram of the temperature data logger device

TEMPERATURE DIGITAL OUTPUT

0 11111010

+25˚C 0 00110010

+0˚C +125˚C

-25˚C 1 11001110

-55˚C 1 10010010

Table 1: DS1820 output data format

DATA LOGGING

the SD card. The data is stored in a formatcompatible with the Excel spreadsheet.

The HardwareThe circuit diagram of the temperature data

logger device is shown in Figure 3. At thecentre of the circuit is a PIC18F4520microcontroller, operated with an 8MHzcrystal. A PCF8583 (www.nxp.com) typereal-time clock (RTC) chip is connected to portpins RD3 and RD4 using pull-up resistors. TheRTC chip is operated in I2C bus mode and asmall battery is used to power the chip toretain the date and time information whenexternal power is not available.

The SD card is connected to port pins RC2through RC5 and is operated in SPI mode. Acard holder is used to physically makeconnections to card pins. The voltage atoutput pins of the microcontroller is too highand can damage the input circuitry of the SDcard. A pair of potential divider resistors (using2.2K and 3.3K resistors) is used to lower themicrocontroller output voltages to a levelacceptable by the SD card inputs. The SD cardis powered using a 3.3V regulated supply,obtained using a MC33269DT-3.3(www.volkin.com/MC33269DT-3.3.html)type regulator.

A MAX232 (www.maxim-ic.com/quick_view2_cfm/qv_pk/1798) levelconverted chip is used to obtain RS232compatible signals from the device. This serialport is used to set the RTC date and time and

also configure various parameters used in thedesign. The date/time and the temperaturedata are sent to the serial port at therequested intervals, as well as being stored onthe SD card.

The OperationThe device is operated by the Reset and

Configure/Run buttons. There are twooperating modes: Configuration mode andRun mode.

Configuration mode: This mode is enteredby pressing and releasing the Reset buttonwhile holding down the Configure/Runbutton. In this mode, the RS232 port shouldbe connected to a PC and a terminalemulation program (e.g. HyperTerm) shouldbe used on the PC to establish communicationwith the logger device. The defaultcommunication parameters are: 2400 Baud, 8bits and No parity bit.

In the Configuration mode the user isdisplayed a MENU with the options to eitherset the configuration, or to display the currentconfiguration. Figure 4 shows a typicaldisplay in Configuration mode. The followingparameters can be set or displayed in thismode:● RTC date and time● Data logging interval (in minutes)● Number of sensors used● 64-bit ID of each sensor (must be entered

as 16 hexadecimal numbers).The red LED is turned on when the device is

in the Configuration mode.Run mode: The Run mode is entered after

either pressing the Reset button, or applyingpower to the device. In this mode the devicechecks the presence of the SD card and, if thecard is missing, both red and green LEDs areturned on to indicate an error condition andthe logger is halted.

If, on the other hand, a card is detected inthe card holder, the green LED is turned on toindicate that data logging has started. Thedevice reads the temperature from all sensorson the 1-Wire bus at the requested intervals.The temperature, together with the currentRTC date and time are stored in a file on theSD card, as well as sent to the RS232 port.Data logging stops when the Configure/Runbutton is kept pressed for about five seconds.The SD card should only be removed after thedata logging is stopped.

A new file is created on the SD card eachtime a new logging session starts. Thefilename is unique and is made up of thecurrent day, month, hours and minutes and isgiven the extension “.TXT”. For example, ifthe logging session is started on the 12th ofMarch, at 5 minutes past 10 o’clock, thefilename will be “12031005.TXT”.

Data is stored on the SD card (and also sentto the RS232 port) in the following format:dd/mm/yy hh:mm:ss pp.p qq.q rr.r ss.s ...........where dd/mm/yy/ hh:mm:ss is the current dateand time, and pp.p qq.q rr.r ss.s are thetemperature read from the sensors.

Figure 5 shows the data stored on the SDcard in a typical session. In this example therewere only two sensors on the 1-Wire bus andthe sensors were initially read on the29/11/2008 at 14:29:01. The temperatures ofboth sensors were initially 22.5˚C.

29/11/08 14:29:01,22.5,22.529/11/08 14:30:01,31.5,23.529/11/08 14:31:01,31.0,24.029/11/08 14:32:01,25.5,23.529/11/08 14:33:01,26.0,23.529/11/08 14:34:01,28.0,23.529/11/08 14:35:01,28.5,28.029/11/08 14:36:01,24.5,24.029/11/08 14:37:01,23.5,28.029/11/08 14:38:01,28.5,24.029/11/08 14:39:01,27.0,27.529/11/08 14:40:01,24.0,29.029/11/08 14:41:01,23.5,24.029/11/08 14:42:01,23.5,23.5

Figure 5: Typical data stored on the SD card

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Figure 4: The Configuration mode is entered by pressing and releasing the Resetbutton while holding down the Configure/Run button

DATA LOGGING

30 ❙ February 09 - Electronics World

ConstructionThe hardware was constructed on a PIC-

Ready (www.mikroe.com) developmentboard (see Figure 6), manufactured bymikroElektronika. This is a low-cost ($24.00)powerful development board with thefollowing features:● Socket for 40-pin PIC microcontrollers● 8MHz crystal● +5V regulator (an external 9-12V power

supply is required)● In-circuit debugger (ICD) and PIC

programmer interface● Reset button● Easy access to port pins via 10-way IDC

connectors● Plug-in compatible with most

mikroElektronika development modules● Small development solder area.

One of the nice things about PIC-Readydevelopment board is the built-in ICD and theprogrammer. This requires the use of aPICFlash2 ICD device, manufactured bymikroElektronika. During programdevelopment one can easily insert breakpointsor single step a program with the help of theICD. The ICD can also program most of thePIC chips on-board, without having to removethe chip from its socket. This feature isextremely useful during programdevelopment.

The RTC module is available frommikroElektronika as a plug-in module and thiswas connected to the PIC-Ready board.

Similarly, the SD card module and RS232modules were connected to PIC-Ready bymaking the necessary wirings. Two externalLEDs and a push-button switch were connectedto the board. Figure 7 shows the completedata logger development system, built aroundthe PIC-Ready development board.

The SoftwareThe software was developed using the

mikroC compiler (www.mikroe.com)developed by mikroElektronika. This is a verypowerful C language compiler developed forPIC microcontrollers and supports both PIC16and PIC18 series. The compiler provides a veryrich library of routines for developingapplications for SD cards, Compact Flashcards, RS232/RS485 devices, USB, CAN bus,I2C, 1-Wire bus and much more.

Figure 8 shows operation of the softwareas a Program Description Language (PDL). Theprogram is modular and consists of a numberof functions and procedures for easymodification, update, or maintenance of thecode. The following functions and proceduresare used:

Initialize_RTC: Initialises the I2C busRead_RTC: Reads the RTC date and

time and stores in arrayRTCData

Write_RTC: Updates the RTC date andtime from array USART

Send_Text: Sends a text message toUSART

Read_Usart: Reads text from USARTuntil carriage-return isdetected

Send_Newline: Sends a carriage-returnand new-line to USART

Display_Setup: Display configurationparameters on serial port

Edit_Setup: Modifies the configurationparameters from serial port

MENU: Displays MENU on serialport

Initialize_SD: Initialises the SPI busHex_Byte: Converts a single character

into decimalConv_Hex: Converts hex bytes into

decimalRead_Temps: Reads temperature data

from sensorsTrim: Removes spaces from a

textFormat_Temp: Formats the temperature

data, writes to the SDcard, and also sends to theRS232 port.

Figure 6: PIC-Ready development board

Figure 7: Data logger development system

DATA LOGGING

At the beginning of the program portdirections are configured, USART is initialisedand so is the I2C bus. The I2C bus iscontrolled using the Software I2C Libraryroutines of mikroC compiler.

The program then checks to find outwhether or not the Configuration/Run buttonis down, and if so, enters the configurationmode. In this mode, a MENU is displayed (seeFigure 4) and the user has the option of eitherdisplaying the current configurationparameters or to modify these parameters.

The configuration parameters are stored inthe EEPROM memory of the microcontroller sothat they are not lost after the removal of thepower. The layout of the configurationparameters in the EEPROM is shown in Table2. Location 0 stores the year field of the RTC.Location 1 stores the data logging interval inminutes. Location 2 stores the number ofDS1820 type sensors connected to the 1-Wirebus. The remaining locations of the EEPROMare used to store the 64-bit ID of each sensor.

Each sensor ID consists of 8 bytes (16hexadecimal letters), occupying 16 locations ofthe EEPROM memory. Thus, the first sensor IDstarts at location 3 and terminates at location18. Second sensor ID starts at location 19 andterminates at location 34, and so on.

When the device is in Run mode, a new fileis opened on the SD card, RTC date and timeare read, temperature data are read fromsensors and then all of this data are written tothe opened file. The same data are also sentto the serial port of the microcontroller.

The data logging activity is terminatedwhen the Configure/Run button is keptpressed down for about five seconds. At thispoint the Run LED is turned off and the SDcard can be removed from its adapter safely.

Main Program:BEGINInitialise port directions

Initialise USARTInitialise I2C busDeclare and initialise program

variables

IF Configuration/Run pressedCall Configuration Mode

ELSECall Run Mode

END IFEND

Configuration Mode:BEGINTurn on Configuration LED (red)DO FOREVER

Display MENU1. Display Configuration2. Edit Configuration3. EditChoice:

IF Choice = “1”Display Configuration

ELSE IF Choice = “2”Edit Configuration

ELSE IF Choice = “3”Wait to enter Run mode

END IFEND

Display Configuration:BEGINDisplay date and timeDisplay logging intervalDisplay number of sensorsDisplay ID of each sensorEND

Edit Configuration:BEGINRead Date and TimeUpdate RTC chipRead logging intervalRead number of sensorsRead ID of each sensorStore configuration in EEPROMEND

Run Mode:BEGINIF SD card missing

Turn off both LEDsError - Halt

END IFTurn on Run LED (green)Initialise SPI busCreate a new file on SD card

DORead temperature from sensorsRead date and time from RTC chipSave RTC data and temperatures onSD cardSend RTC data and temperatures toserial portUNTIL Configuration/Run button iskept pressed for 5 secondsEND

Figure 8: Operation of the software

Importing to ExcelThe data stored on the SD card is

compatible with the Excel spreadsheet andcan easily be imported into Excel for eitherstatistical calculations or for drawing graphs ofthe change of temperature with time. Theanalysis can either be carried out Offline,where the data stored on the SD card is used,or Online, where the data is analysed in real-time as it is captured from the data logger.Both methods are described here.

‘Offline’ AnalysisThe steps for drawing a graph of the

change of temperature for sensor 1 are givenbelow:● Start the Excel package● Insert the SD card into an adaptor so that it

can be read on a PC.● Open the required temperature “.TXT” file

on the SD card. Select File -> Open ● Select Delimited and click Next● Select Delimeters as Space and Comma,

and click Next● Click Finish. You should see the collected

data in an Excel sheet● Highlight the time column (column B ) and

sensor 1 output column (column C)● Click Chart Wizard icon in top menu● Click Finish to select the default graph type

(Bar chart)● Click on the graph. Select Chart → Chart

Options to enter the axes titles● The graph of the change of temperature

will be drawn as in Figure 9 (this data wascollected on the 29th of November at14:28.The graphs of other sensors, or multiple

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EEPROM ADDRESS EEPROM CONTENTS

0 RTC year

1 Logging interval (mins)

2 Number of sensors used

3 Sensor 1 ID

................................................................

................................................................

18 Sensor 1 ID

19 Sensor 2 ID

................................................................

................................................................

34 Sensor 2 ID

................ ...................

Table 2: Microcontroller EEPROM layout

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32 ❙ February 09 - Electronics World

sensors on the same graph, can easily bedrawn with the Excel package. In addition,other types of graphs can be drawn, or thedata can be analysed statistically, for exampleto find the maximum and minimumtemperature in a given date and time interval,or the average, or the standard deviation ofthe temperature in a given interval, and soon.

‘Online’ AnalysisThe data logger also sends data to its

RS232 serial port at the requested intervals(see Figure 10) and it is possible to develop aprogram on the PC (e.g. using Visual Basic) tocapture this data from the serial portdynamically and then draw the graph of thechange of temperature with time as ithappens. Such an application would be veryuseful in real-time temperature monitoringand control applications where it may berequired for example to know thetemperature of an oven at any instant oftime, or to know the trend of change in thetemperature.

1-Wire Bus DeviceThe design of a microcontroller-based

multichannel temperature data logger device,based on the 1-Wire bus has been described.The program code of the device is generaland allows any number of temperaturesensors to be connected to the 1-Wire bus,with a maximum bus length of severalhundred meters.

Using one channel only, and logging everyminute, the device writes about 36K of datato the SD card every day. Thus, with a 1GBSD card and providing the device is operatedfrom a continuous power source, the datacollection period is many years.

One of the problems with portable SD cardbased projects is that writing to the carddraws high current from the battery and thushigh capacity batteries (or a suitable mainsadaptor if possible) should be used inapplications where data is to be collected forlong periods of time.

There are many types of 1-Wire devices inthe market and the device described here canbe expanded by the addition of other 1-Wiredevices such as, humidity and pressuresensors, switches, counters, A/D convertersEEPROM memories etc. Such an expansion willrequire modifications to the existing softwarecode to read, format, and then store therelevant data on the SD card. ■

Figure 10: Data sent to the microcontroller serial port

Figure 9: Offline analysis of the collected temperature data