Development of a Temperature Controller for the Order
Contents
1. physical mapping of the printer port pins to the register ports inverter to converts TTL signals to IC open drain standard The open drain outputs of inverters are pulled high using external pull up resistors 10 kQ each connected to the power supply Input pins of hex inverter are also connected through 10 kQ each pull up resister so that inverter remains insensitive to PC to PC variations of the printer port voltage levels The hex inverter is also meant to provide some degree of protection to printer port from external circuitry An 0 1uF capacitor inserted between supply voltage and ground serves as noise filter A summary of the interface circuit is given in the following e The serial data SDA from computer to DS1621 is transmitted from Pin 2 which is DO bit of the Data register For the correct logic the Pin 2 signal goes through two inverting stages Y1 amp Y2 before feeding the SDA line of the DS1621 e Pin 11 which is S7 bit note the inverted logic of the Status register is configured to read the serial data SDA from DS1621 via Y4 inverter Before reading data from 9 DS1621 the software should release the SDA line by forcing the pin 2 D0 bit to high e The clock SCL signal is generated at pin 14 which is C1 bit note the inverted logic again of the Control register It is routed through Y3 inverter to generate the clock SCL signal for DS1621 e Inputs of unused inverter Y6 is grounded to prevent its output2. National Large Solar Telescope IIA Technical Report Series No 1 ILA TRS 0110 21 Available at http prints iiap res in handle 2248 5094 M Freed M Kriiger C J Spanos and K Poolla Autonomous On Wafer Sensors for Process Modeling Diagnosis and Control IEEE Trans Semicond Manuf 14 255 264 2001 J H Moore C C Davis M A Coplan and S Greer Building Scientific Appratus Chap 8 Cambridge University Press 4th Edition 2009 P R N Childs J R Greenwood and C A Long Review of Temperature Measurements Rev Sci Instrum 71 2959 2978 2000 R J Widlar New Developments in IC Voltage Regulators IEEE J Solid St Circ 6 2 7 1971 P Brokaw A simple three terminal IC bandgap reference IEEE J Solid St Circ 9 388 393 1974 Available online from http datasheets maxim ic com en ds DS1621 pdf High Resolution Temperature Measurement With Dallas Direct to Digital Temperature Sensors Application Note AN105 accessed online from http glotov pp ru filebase cpu APP105 PDF 2 Wire Communication Using LabVIEW Application Note 215 Sep 2002 Available at http pdfserv maxim ic com en an AN215 pdf How to Use a PCs Parallel Port to Communicate with 2 Wire Devices Application Note 3230 May 2004 Available at http www maxim ic com app notes index mvp id 3230 E Lunca A Salceanu and M Cretu Implementing the I2C Communication Protocol in LabVIEW accessed o
3. R W bit For DS1621 the address has a fixed and a programmable part as illustrated in the Figure 6 a The fixed part consists of 4 bit control code 1001 and the programmable part has three device select bits A2 A1 AO allowing up to 8 similar sensors to share the I2C bus The last bit R W indicates the data direction with LOW 0 implying WRITE to slave and HIGH 1 indicating READ from slave The 12 LabVIEW code outlines in Figure 6 b is built using For loop to transmit an 8 bit address byte over the SDA line Three frame stacked sequence structure is used to transmit each Control code Device select SCL UUUUUUUUUL Wins 0 0 Of Start Nike Byte AKN N Self index FOR loop to write 8 bits control byte Ref Out Printer Port Write the Write the data bits to DO bits to DO Out Port vi Address Byte k Delay ms FIG 6 a Address byte format b Address vi code to transfer address byte Delay out ms bit The Oth frame sequence inside the For loop pulls the SCL low where the Base 2 is wired to the address input and 0 to the write value terminal of the Out Port vi In the lst frame shown in Figure 6 b the first address bit MSB is transferred to the SDA line by writing to the Base register A Boolean TRUE and FLASE array of address byte is converted to corresponding 1s and Os using Boolean To 0 1 f
4. from floating e The configured device address e g 000 is obtained by connecting the address select pins A2 Al and AO to the ground For configuring multiple devices pins A2 Al and AO should be accordingly connected to ground and 5VDC power supply e Tou drives the gate input of the power MOSFET to switch ON OFF the current through heating mat The choice of using one pin from each register port is not absolutely essential but it has an added advantage As we will see in the next section the digital read or write operations are performed on the entire register 8 bit at once As long the status of the selected bit of a register is affected in the desired way we don t need to keep track of what happens to other bits in the same register On the contrary if we want to configure two pins of a same register port for I2C operation then additional programming steps will be required to ensure that read write operation only affect the status of the selected bit and not the other IV IC BUS PROTOCOLS IC protocols specify a common standard for signal timings bit format addressing scheme clock synchronization and hand shaking mechanism to foster a reliable data ex change For a detailed overview and understanding the reader is directed to the IgC bus specification document from Philips 13 Here we outline some salient features and gen eral considerations that must be borne in mind while writing the LabVIEW programme to control the
5. or resistance of the sensor bears a certain relationship with tempera ture that forms the basis of measurements 4 The choice of a particular sensor depends on the temperature range linearity precision and desired accuracy level in the specific experiment 3 For instance thermocouple are capable of making measurements over a wide temperature range 260 C to 1600 C The RTDs have low drift errors and excellent re peatability over 200 C to 850 C Thermistors are preferred for their high sensitivity to detect minute changes in temperature within 100 C to 150 C range With the exception of Si semiconductors the sensor output is nonlinear and require lookup table to obtain correct temperature readout Additionally analog sensors also need extra signal conditioning calibration A D convertor logic comparator and a voltage or a current amplifier to make their output useful in many practical applications Basic theory behind different types of temperature measurements construction techniques accuracy and range can be found in references 3 4 _ Henceforth our discussion will be confined to IC based digital sensors for simple laboratory requirements where temperature resolution is limited to about 0 5 1 C and a large number of experiments can be performed within their temperature range The success of digital thermal sensor can be attributed to two key factors First being silicon which is most commonly used semicond
6. 1 0 5 C i 9 bit temperature readout e J eee eee nnn sf FIG 2 An example of two byte temperature readout of DS1621 II PRINTER PORT TO ILC HARDWARE INTERFACE The data and command transfer between DS1621 and control hardware take place serially over a 2 wire bus commonly known as the I C bus It was developed by Philips Semicon ductor in early 1980s to facilitates simple communication link between digital devices such as IC sensors LCD drivers general purpose I O lines single chip microcontrollers EEPROM A D and D A data convertors The IC bus has two lines namely a clock line SCL and a serial data line SDL Multiple devices can be connected to a single IC bus Each device is recognized by a software controlled unique address Communication between the devices is based on simple master slave relationship that exits during the data transaction The data transfer over 2 wire bus can vary from 100 kbit s in standard mode 400 kbit s in the fast mode and 3 4 Mbit s in high speed mode Microcontroller or PC is usually the master that generate clock signals and initiate and terminate the data transfer processes on the bus Master is also responsible for ensuring synchronization and arbitration for multi slave I C bus Besides microcontroller virtually any computer port e g serial parallel or USB with digital I O lines can be utilized to control an IC device Some electronics hardware is however necessary to convert the port s
7. A read write 8 bit Configuration Status register is used for setting the mode of operation and to read the status of different parameters of the sensor The least significant bit LSB bit 0 for example is used to set either one shot bit O 1 or continuous bit O 0 temperature conversion mode The bit 1 determines the output polarity 1 active high 0 active low of the thermostat output pin Tour Analog to Digital Convertor The in built analog to digital convertor ADC is used for converting the sensor output to binary format The sequence of binary digits 1 and 0 is passed onto the PC or microcontroller hardware which is then encoded to decimal equivalent of temperature values Temperature can be read either as a single byte 1 C resolution or a two byte 0 5 C resolution data as shown in Figure 2 The most significant bit MSB bit 7 of the 1st byte represents the temperature sign 1 for ve and 0 for ve while the decimal equivalent of the remaining bits bit 0 to bit 6 yields the temperature reading in C The leading digit bit 7 of the 2nd byte determines the 0 5 C resolution while all other bits are set to zero This means 0 5 C should be added to the temperature readout from the 1st byte when the bit 7 of the 2nd byte is high As an illustration the 9 bit temperature output shown in Figure 3 corresponds to 37 5 C IC Bus Interface The on chip interface for I2C bus that allows serial commu nication
8. DS1621 e The master in this case LabVIEW programme initiates a transaction on the I2C bus by issuing a START condition The slave DS1621 is responsible for monitoring the bus and identify itself if the address matches 10 e The clock is always generated by the master but if the slave is busy it can hold SCL line low to force a wait condition e Both master and slave can transmit Tx and receive Rx data e Each byte placed on the SDA line must be 8 bit long A byte transfer has to be followed by an Acknowledge bit from the receiver Data is transferred with the Most Significant Bit MSB first e Data is placed on SDA lines when the clock is LOW e The data on SDA line is valid for READ WRITE only during the HIGH period of the clock pulse e The master terminates the communication with slave by sending a STOP condition 2ByteRead vi DS1621 Library Functions FileHeader vi RSS 1ByteRead vi PrintPort vi ps2 PE FIG 4 A set of LabVIEW VIs written to control DS1621 V LABVIEW PROGRAMME The control software for DS1621 was written using LabVIEW Version 8 2 running on 11 Windows XP platform LabVIEW does not have high level functionality to control and communicate with IgC devices However it has In Port and Out Port VIs that allow the user to read from and write to hardware registers in the PC s I O space Three registers associated with the printer port can be directly acces
9. IHA Technical Report Series No 10 pp 1 22 2012 Report No IIA TRS 1012 Development of a Temperature Controller for the Order sorting Interference Filters Ravinder Kumar Banyal and B Ravindra Indian Institute of Astrophysics Bangalore INDIA Dated March 30 2012 Abstract A Fabry Perot narrow band imager NBI is being built for solar studies at IIA 1 The NBI has an order sorting pre filter that selects an optical beam within desired wavelength band A constant temperature is required to keep the transmission response of the interference filter stable during the observations This technical report describes the design and construction of a digital temperature sensor and control system for the pre filter assembly Temperature measurement and thermostat operations are performed by DS1621 a semiconductor IC from Mazim An IC serial interface also called 2 wire interface was developed using computer s parallel port The design of the I2C interface circuitry and LabVIEW programmes to implement serial communication between the temperature sensor and the PC are described in details The device operation is validated and results of laboratory test are presented banyal iiap res in I INTRODUCTION Semiconductor microfabrication techniques are now increasingly used to build on chip sensors for a variety of applications in engineering physics chemistry and biological sciences 2 These sensors contain all the necessary electr
10. aximum update rate is about 1Hz as each temperature conversion inside DS1621 takes about one second to complete Once user exits the programme from the front panel a STOP condition is generated to terminate the data transfer over the IC line A time stamped temperature data collected from both channel is written to a text file using DataSave vi It will be a straightforward task to integrate the current LabVIEW code with the main instrument control software for NBI which is also written in LabVIEW DEVICE APPLICATION AND LABORATORY TEST In our application we require a temperature controlled oven to house a narrow band optical filter meant to take images of the sun during the daytime The central wavelength of the filter changes as the ambient temperature departs from the specified operational temperature The filter therefore has to be mounted inside a thermally insulated box whose temperature is then maintained at a preset level about 3 5 C above the ambient 18 FIG 11 A single layer printed circuit board design for electronics during the observations This requirement is easier to fulfil with resistive heating without the need for external cooling A two channel digital temperature sensor was built following the electronic circuit shown in Figure 3 A single layer printed circuit board PCB design for the interface circuit shown in Figure 11 was prepared using ExpressPCB freeware A laboratory picture of the fa
11. bricated filter box and PCB is shown in Figure 12 FIG 12 Laboratory picture of the dual port filter box and electronic circuit board The inset image is of sensor 2 which is placed inside the filter box 19 Two self adhesive heating mats RS components of 10W each were connected in parallel and glued to the inner walls of the filter assembly box The heater was driven by a 24 VDC power supply The sensor electronics PCB was powered by 5 VDC obtained from the same 24 VDC source using LC7805 voltage regulator One of the DS1621 ICs was mounted on the PCB board to measure the ambient temperature while the other was placed inside the filter assembly The Tour pin of the assembly sensor was used to control the current flow through the heating mats A simple ON OFF control of the heater is sufficed to en sure a required level of temperature stability 0 5 C inside the box A simple circuit for switching the resistive load of the heater was made with IRFZ44 an enhanced mode power MOSFET 14 The IRFZ44 MOSFETs is compatible with the 5 volt power supply requirement of logic circuitry The maximum gate input voltage required to drive this MOS FET to saturation is 4 VDC The logic level thermostat Tour pin of DS1621 was therefore sufficient to drive the gate terminal of the MOSFET Figure 13 shows a screen captured Two Channel I2C Temperature Sensor DS1621 Printer Port Set Delay ms Temp Sensor 1 degC TempSensor2 degC S
12. control is an essential component in many experimental apparatuses Examples include cooled CCD for astronomical imaging ther mally stabilized cavities for high power lasers crystal growth ovens cold traps in vacuum line and thermal evaporation and coating chambers The apparatus temperature in these applications has to be maintained within a few degrees This technical report has two broad objectives The first objective is to introduce an open source electronic circuit to build an IC based digital temperature measurement and control TMC system The second objective is to develop a simple IC interface also called 2 wire interface programme using National Instruments LabVIEW to perform necessary tasks related to temperature measurements automatic data collection and thermostat operation A 2 wire serial interface for DS1621 IC is created using digital I O pins of PC s printer 2 port We illustrated how to manipulate the printer port pins using LabVIEW s In Port and Out Port VIs to implement device read write operations which otherwise require a microcontroller or some dedicated hardware Typically a digital temperature sensor IC requires a microcontroller to support device communication over the 2 wire bus An user is expected to have some level of familiarity with IC system protocols and microcontroller hardware to make temperature measurements with digital ICs Alternatively when device synchronization and speed are not absol
13. ddress of the parallel port is 0x378 or it could be 0x278 or 0xBD00 Which means the Data register is mapped to Base address 0x378 The Status register and Control register would then be at Base 1 and Base 2 respectively The DB 25 pin assignment to the corresponding registers is shown in Figure 3 b In next section we will use Inport and Outport VI of the LabVIEW to access these ports directly and see how to alter the ON OFF condition of individual pins to control the DS1621 temperature IC The printer port pins cannot be connected directly to SCL and SDA pins of the I C device despite the fact that both can operate on ideally 0 5VDC digital levels The main reason being the TTL logic of parallel port pins that can source and sink current typically 12 mA to drive the pin output low or high internally The SDA and SCL terminals on the other hand are open drain or open collector Which means I2C chip can drive its output low but it cannot drive it high The interface circuit shown in Figure 3 a uses a 74HC05 hex 8 57 DB 25 connector is ata register 0000000000000 0 2 5 AG 22 21 20 a9 QOO Sy Control register Wy 10kQ R 209 E T O 5 4 3 2 Data Base 0x278 0x378 0x3BC Pin Number gt 11 10 12 13 15 7 56 55 S4 3 Status Base 1 gt 17 16 14 1 b eyes Control Base 2 FIG 3 a Hardware circuit to convert printer port voltage signals to I2C standard and b
14. during the HIGH period of the clock After receiving the ACK the DS1621 transfers the 2nd byte of the temperature data which is read in the similar fashion using second For loop structure in 4th frame As per the DS1621 protocol master sends no acknowledge NACK signal by keeping the SDA line high to mark the end of 2nd byte data transfer with additional clock cycle After this master can generate a STOP condition or repeated START to initiate a new transfer 1ByteRead vi This VI can be used to read one byte data from configuration counter or slope register The code implementation is similar to reading one byte data in 2ByteRead vi SendACK vi After receiving a single byte of data from slave master sends an acknowl edgment by pulling the SDA line LOW that should remain stable during the HIGH period of the clock cycle DataSave vi DataSave vi saves the time stamped temperature data from DS1621 into txt file The file header is programmed to include user s name and set temperature value through interactive input The date and time are appended to the file header by reading the system clock using LabVIEW s Get Date Time String Function Stop vi The STOP condition is met by enforcing a LOW to HIGH transition on the SDA line while keeping the SCL in HIGH state as indicated in Figure 5 a Main vi Having created a Sub VI library to perform various operations in accordance with IC protocols using them in a specific application is intuit
15. efore writing to the configuration register e A programmable hysteresis can be introduced to prevent the frequent ON OFF be havior of the heater at the temperature set point This is achieved by writing a high temperature Ty Figure 9 b and low temperature Ty registers of DS1621 An in built 17 VI comparator monitors the deviations of Ty and T away from the measured temper ature The Toyr pin becomes active and turn the heater on when the temperature equals or drops below Ty It remains active until the temperature exceeds Ty In Figure 9 c the Start Convert command EEh initiates the temperature conversion process The LabVIEW code shown in Figure 10 has a while loop placed inside the frame 4 of the outermost sequence to perform continuous temperature readout from both channels Following the Read Temperature command AAh the two byte temperature data is read using 2ByteRead vi in the frame 0 of the innermost sequence shown in Figure 10 a High resolution temperature measurements are possible as explained in section III A Read Counter A8h command shown in frame 1 of Figure 10 b reads the counter byte Similarly the slope byte is read using Read Slope A9h command in frame 2 which is not shown in the Figure 10 Finally in frame 3 high resolution temperature is computed as per the expression given in Equation 1 Temperature data from both the channels is displayed in real time on the LabVIEW front panel The m
16. ensori AR Eoo f 22 75 27 75 Sensor 2 Select Tout Polarity Set Temperature DegC E Active High lt x J28 Sensor E Temperature Sensor 1 Temperature Sensor 2 T2C address Write T2C Address Read I2C Address Read 1 0 0 1 AZ Al ARW 1 0 O 1 A2 Al AD RIW File location for Temperature Log qc Users Lab Desktop D51621 Temperature_data STOP FIG 13 A screen captured image of LabVIEW front panel during the trial run image of the Main vi front panel The white curve shows the ambient temperature inside the lab sensor 1 while the red curve indicates the constant temperature sensor 2 main tained within the filter assembly The slow oscillations about the set temperature 30 C are not unexpected This is typical response of a simple ON OFF controller which injected 20 constant power into the system during the ON state The noticeable temperature overshoot and undershoot 0 25 C are caused by a delayed response occurring due to poor thermal conductivity of the air and the finite separation of heater from the sensor In other words temperature inside the oven is not uniform and always has a weak but changing thermal gradient In essence the thermal inertia of the system prevents the ON OFF controller from operating very close to the temperature set point For a better temperature stability the current through the heater should not be constant or zero but varied in such a way so as to reduce the
17. ive and straightforward The LabVIEW programme Main vi was built to control a two channel DS1621 sensors The graphical programming instructions to configure DS1621 for continuous temperature 15 HioMeMeoeNeeMeNeMeKeNeleeNeNeKeKe Mele Kel gt SOG WTRS 90 XrloeKeNedeleNeMeNedeeleNekeRedelele Ree fi 14 9 0 4 7P ji H MeHeKeKeR R KeleKeK ReReKeKeH H KeleKeK R MeleReNeN R ReReK h ReleReKeheR R Releh R Relelel Rem POONA OUT Wyo KeKsKsNsKsKoKoKsKsKeKsKeKsKsRsNsXoKsMeKeKel gt d z Oeu COUN SS SOE oOo 5 1 pMa etaa gt 3 Bennes E E isisisEsEsEsEeNsE Esie NsEsisisieEsEsEeEsEsEs OSeh l heheh h h ReSehenel i l i lt n n bsMeleNeMeNeReReleeReleleN Heheh h NeReHeNeheN l leNeN heN heleelelei l h R hehel ReHel lel t OCU OO OOOO OOOO OOTY 319 4 vh TISI TIO e a te eee ete iB i MoMouoHeHeNeKeHeHoNoMoHoKeKeReHooNoHeeKeReHoH MoHeHeReReMoHoRoHeHeReReNH KoHeHeHeReRek E R ReReleKoReReR ReMeleLeh ReR R Rekeleh h R heR heleh R R ReR hekeheK k R ReR heleheh hem eae nenen FIG 9 Block diagram a to configure the DS1621 for continuous temperature conversion and setting the polarity of the thermostat pin b writing Ty register to set the upper temperature limit c initiating continuous temperature conversion process measurements and thermostat operations are shown in Figure 9 and 10 The data sheet of DS1621 provides a complete list of hexadecimal commands to perform various tasks For clarity it is necessary to give a b
18. nline from www imeko org publications tc4 2007 IMEKO TC4 2007 112 pdf J Axelson Parallel Port Complete Programming Interfacing amp Using the PC s Parallel Printer Port Lakeview Research 1997 Avaibale online from http www nxp com documents user_manual UM10204 pdf Data sheet accessed from http www vishay com docs 91291 91291 pdf 22
19. onics and mechanical hardware on a single chip Other attractive features of IC sensors are miniaturized size reduced cost multi functionality and simple digital interface for device control and data transfer The main focus of this report however is to discuss the usage of semiconductor based digital temperature sensors for thermal control applications They can represent temperature in digital format that can be easily read by microprocessor or any other control hardware capable of scanning the digital lines Further the on chip functionality built to perform logical operations analog to digital conversion temperature comparator etc are extremely useful features for building low cost dynamic thermal control system For our application we have chosen DS1621 IC test samples supplied by Maxim Similar temperature ICs from other suppliers could have also served the purpose equally well In general the need for temperature control in laboratory experiments can manifest in several ways 3 There are applications where physical properties of material critically depend on temperature In such cases accurate temperature measurements and control becomes important to determine for example the specific heat of a substances estimation of semiconductor band gap determination of the heat transfer rates thermal expansion coefficients of materials and temperature induced wavelength changes in diode lasers Then there is another category where temperature
20. pecific voltage levels to those used for data com munication over the 2 wire bus The choice of the hardware would depend on the type of computer port that one selects For example an RS 232 to 2 wire interface requires an adapter to translate from 12 VDC serial port signal to 0 5VDC open drain IC standard 9 Given the steep rise in USB supported peripheral devices in recent years many variants of USB to IC adapters are commercially available Apart from added cost the internal working and in system programming of these propitiatory hardware remains unknown to end users One of the simplest ways to control an IC device is to use PC s parallel ports which is also known as LPT or printer port Several open source hardware options exits to exploit the printer port for IC communication Figure 3 a shows one such interface circuit built using off the shelf electronic components 10 11 There are three 8 bit registers associated with standard 25 pins printer port namely Data Register Control register and Status register 12 Data register transfers 8 bits of data to the external devices Read only Status register reports different aspects e g online out of paper error busy etc of the device while bidirectional Control register is meant to perform tasks of device selection initialization and linefeed etc What is important is that each of these register ports are uniquely mapped to I O address space of the PC For a typical PC the default Base a
21. resistive heating rate gradually as the set point temperature is approached In that case a more sophisticated technique such as PID controller are often used VII CONCLUSIONS An automated temperature measurement and control system is an essential part of many laboratory experiments In this work we have built an C based temperature sensors and control system suitable to meet modest requirements of temperature regulations in an as tronomy application Temperature measurements and thermostat operation was performed using a programmable digital thermometer DS1621 from Maxim An open source hard ware circuit was built to implement 2 wire data communication using computer s printer port The IC interface circuit supports digital read write operation necessary to facilitate temperature measurements and data logging operation The LabVIEW software was writ ten to emulates the behaviour of a microprocessor hardware for serial communication with Ds1621 A set of VIs were created to build a graphical user interface for two channel TMC system Finally the device operation was validated in a laboratory experiment involving a temperature controlled box meant to enclose a narrow band optical filter for solar studies ACKNOWLEDGMENTS We thank Dr Shibu K Mathew and T Yogeshnanda Singh for their help and support during the initial stages of the work 1 B Ravindra and Ravinder K Banyal A Dual Fabry Perot based imaging Spectrometer for the
22. rief outline and explanation of essential programming steps in Main vi e User enters various parameters such as printer port address temperature set point Tout polarity time delay and I2C device address directly from the front panel e In Main vi we used several stacked sequences to streamline the programme flow The outermost sequence contains five frames designated as 0 4 as shown in Figure 0 and 10 to accomplish different tasks For example The DS1621 sensors are configured in frame 0 the high temperature Ty and low temperature Tr thresholds are set in frame 1 and frame 2 the process of temperature conversion begins at frame 3 and 16 IOOCOOOOOOO0OCOOO0OUUUO0N 0 3 P oReReR ReeReheheleReReRen heheheheh en hene gt FIG 10 LabVIEw code a to read the two byte temperature register b to read the counter register finally a continuous temperature readout is performed in frame 4 Each frame of the outermost sequence in turn has 2 frame stacked sequence designated as 0 1 meant to send identical instructions to control two sensors e The bit 0 and bit 1 of the configuration register determine the mode of operation and polarity of the thermostat Tout pin respectively The LabVIEW code shown in Figure 9 a configures DS1621 sensors for continuous temperature conversion mode bit 0 0 with user selectable polarity of the Tou pin from the front panel The access configuration register command ACh is issued b
23. rintPort vi This VI has three output terminals carrying the address of the Data Base Status Base 1 and Control Base 2 register Input terminal is wired to the printer port address Base 0xBD00 of the current PC 2ByteRead vi This VI reads the 2 byte temperature data from DS 1621 The DO bit is set high to release the SDA line in Oth frame of outer sequence structure shown in LabVIEW code Figure 8 Each data byte is read by separate For loop The For loop inside the 1st 14 frame See Figure 8 of the outer sequence is programmed to read the 1st byte of the temperature data The 3 frames of a sequence structure inside this For loop are required to read each data bit In Oth frame the clock SCL line is pulled HIGH The In Port vi reads the Status register in 1st frame as shown in the LabVIEW code in Figure 8 S 7 bit of the Status register is extracted using Index Array function The SCL line is then pulled low in 2nd frame so that DS1621 can load the next bit of data onto the SDA line An 8 bit data array collected outside the For loop is inverted using Reverse 1D Array LabVIEW function to get the correct bit order After reading the 1st byte of data the master sends an acknowledgment AKN signal to slave indicating that first byte of data has been successfully received and another byte of data may be sent This is achieved in 2nd frame of the outer sequence by pulling the SDA D0 bit line LOW The line should be stable LOW
24. rom Maxim for our application The approach outlined in this paper is amenable to similar devices from other manufacturers Configuration Status Register Temperature Sensor Analog to Digital Convertor ADC Address Lines TH TL Register Register 4 4 Logic Comparator COUNT COUNT REMAIN PER C FIG 1 Functional diagram of DS1621 temperature sensor A DS1621 Temperature Sensor The DS 1621 is an 8 pin programmable IC from Maxim with onboard thermal diode that is capable of measuring ambient temperature from 55 C to 125 C The functional compo nents of the DS1621 hardware are depicted in the block diagram shown in Figure 1 Only a brief description of relevant features is provided here Details related to device specifications electronic ratings pin description and operating conditions of the DS1621 can be found in the product data sheet 7 Some of the notable features pertaining to on chip functionality of the DS1621 are e Configuration Status Register The DS1621 can be operated either in continuous temperature conversion mode or one shot temperature conversions modes In former case DS1621 continuously converts the temperature and stores the results in 2 byte temperature register Reading the temperature register does not affect the conversion process In one shot conversion mode DS1621 performs a single temperature mea surement stores the result in temperature register and then goes to power shutdown mode
25. sed with In Port and Out Port VIs to implement IC protocols A collection of IC specific LabVIEW programmes shown in Figure 4 were written to create a quick and easy graphical interface for DS1621 These Sub VIs are used in the Main vi programme to perform temperature measurements thermostat and data logging operations A detailed description of each VIs is provided in the following 4 r SDA a La SDA TAE Eey L Aj S START STOP a Condition Condition Base 0x378 Delay out fa Ki FIG 5 Timing diagram for START condition a visual depiction and b the LabVIEW code Start vi A START condition occurs when master pulls the SDA line low while SCL is high This is illustrated in timing diagram of SDA and SCL line shown in Figure 5 a Start vi uses three frames of a stacked sequence structures of LabVIEW In the Oth frame D0 bit SDA line of the Data register Base address 0x378 is set high by writing a value 1 to the input terminal of the Out Port vi In the 1st frame the SCL line which is C 1 bit of the Control register Base 2 is set high by writing a value 2 to the input terminal of the Out Port vi In the 2nd frame shown in Figure 5 b DO bit is pulled low to initiate the START condition Address vi Following the START condition master sends a control byte address byte comprising 7 bit device address and a
26. uctor material in thermometry and second the IC fabrication technology that has significantly matured in last few decades It is now feasible to build a thermal sensor with all the device functionality integrated on a single Silicon chip thus making them extremely compact and cost effective solution for a variety of TMC applications These devices operate on the principle of temperature dependent behavior of voltage current relationship at semiconductor junctions forming diode or a transistor For example the difference of forward base emitter voltage Vgg between two identical transistors operating at a constant collector current ratio is proportional to the absolute temperature The temperature is calculated using an in built bandgap circuit that measures the difference in bandgap voltage at two different current densities 5 6 The bandgap circuit produces a voltage that varies linearly with temperature By integrating on chip electronic sub systems the device functionality of an IC based temperature sensors have been further enhanced to suit the specific requirements in TMC applications Temperature ICs are now commercially available from several reputed companies such as NXP Semiconductors Maxim National Semiconductors Microchip and Analog Devices Typical examples include LM35 LM335 and LM95233 LM95235 series from National Semiconductor and AD590 and other variants from Analog Devices As a readily available test samples we chose DS1621 IC f
27. unction before writing In 2nd frame SCL line is pulled HIGH to indicate the valid data condition during which the slave reads the data bit This completes writing of the first address bit to DS1621 The For loop then increments to write the remaining bits in the same manner CheckACK vi After transferring each bye of data the master has to release the SDA line so that slave can pull it LOW to acknowledge ACK the successful transfer of the data In the 1st frame of the code shown in Figure 7 the SDA line was released by setting DO bit high In the 2nd frame the clock was pulled HIGH and in the third frame shown in Figure 7 the Status port was read using the In Port vi The bit 7 S7 bit was extracted to check the ACK status I CWrite vi DS1621 has a pre defined hex command set to perform various tasks 7 Each command is written to DS1621 using Write vi This LabVIEW code is similar to Address vi The hex code should be converted to 8 bit array using Number To Boolean 13 POO 3 0 4 PP Checking the AKN signal from Slave Printer Port H Pl ELIE fF FIG 7 Reading the device acknowledgement with CheckACK vi Array function of LabVIEW and reversed to transmit the bits in correct order i e the MSB first Reverse 1D Array S Paine Dy OM 1 0 2 P oo rj FIG 8 Block diagram of 2ByteRead vi to implement 2 byte temperature readout from DS1621 P
28. utely critical a software based approach can be used to facilitate the device control and data flow operations In general any programming language which can access read write the computer ports can be used to transmitted serial data and control instructions to the IC device Likewise the serial data can be read by the software by sampling the digital I O lines of the device at periodic intervals A knowledge of basic electronics and some experience in high level programming are the only prerequisites to implement this approach The report is divided into VII Sections The principle of semiconductor thermometry and the functional details of the DS1621 IC temperature sensor are discussed in Section II An interface circuit that converts 2 wire device signal to parallel port voltage levels is presented in the Section III The necessary features of the Iz protocols for serial data transfer between PC and I device are explained in Section IV The details of the LabVIEW software developed to configure DS1621 sensor for TMC operations are given in Section V The temperature regulation of an interference filter build and tested in the laboratory is discussed in Section VI Finally conclusions are presented in Section VII II PRINCIPLE OF TEMPERATURE MEASUREMENTS Most commonly used temperature sensors include thermocouple resistance temperature detector RTD thermistors and silicon semiconductors In each case the analog output voltage current
29. via two wires with microcontroller or PC for supporting device read write operations Temperature Threshold Registers Two programmable registers to store e g the high Ty and low temperature T limits for thermal control Logic Comparator A logic comparator for generating digital interrupt signal to issue alarm control thermostat operation for temperature regulation or to shut down an equipment to prevent a catastrophic failure when the measured temperature exceeds or drops below a preset temperature limits This feature also relieves the system controller or PC from continuously reading the temperature Thermostat control Tour with programmable hysteresis is necessary to prevent the fan or heater from oscillating with the slow changing inputs Device Address Pins Programmable address lines AO Al and A2 to uniquely identify the device on IC bus e Counter and Slope Register A counter and slope registers to obtain higher tem perature resolution 0 125 C The high resolution is computed using the following expression Slope Count TEMPERATURE TEMP_READ 0 25 4 1 Slope where Temp_Read is the 1st byte temperature readout A Read Slope and a Read Counter command should be used to obtain these values The method to obtain high resolution temperature measurements is explained in reference 8 1t Byte 2 4 Byte Bit 7 Bit 0 Bit 7 Bit 0 Pele e ToT Efe Joo e e fo foc vo t Sign bit 0 ve amp 1 ve
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