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The complete iButton datalogger starter pack includes one DS1921G iButton, a Blue Dot adapter, USB adapter and Windows-based software. By Clive Seager* The DS1921 iButton . . . a temperature datalogger in a 16mm stainless steel can! One of a range of datalogging iButtons, the DS1921 houses a battery and an embedded computer chip that integrates a temperature sensor, a clock/calendar and memory. T HE MEASUREMENT RANGE for the DS1921 iButton’s temperature sensor is -30°C to +85°C in 0.5° increments, while the clock measures seconds to years accurately to +1 minute per month from 0°C to 45°C. Incredibly, the iButton can log data for siliconchip.com.au more than 10 years, or up to 1 million temperature measurements. As well as the datalogging memory, these devices also incorporate a further 512 bytes of user memory. This space could be used to store a product description, shipping manifest or even a student’s name, for example. All iButtons include a unique 16-digit silicon serial number for identification. As described in our previous iButton article, this serial number can be read out using a simple PICAXE08M circuit and BASIC program. Unfortunately, PICAXE BASIC does not include any commands that enable us to easily access the DS1921’s other resources over the 1-Wire bus. However, iButton manufacturer Dallas Semiconductor and Revolution Education have produced a generalpurpose Windows-based application October 2005  91 A Commercial Application: Making Sure The Fish Stayed Cold The DS1921 iButton belongs to the Thermochron family of dataloggers, which include devices with high accuracy measurements, larger memories and even humidity measurement capabilities. These devices continue to be applied in a wide range of commercial applications, including food quality control, as highlighted in the following example. Around the coast of Scotland and Canada, aqua-culture farms rear salmon for sale to restaurants around the world. Many salmon are destined for Japan and therefore must be transported by air, packed in crates of ice. It is naturally very important that the fish are maintained at the correct temperature during the journey, particularly if they are subjected to delays at the airport terminals. For this reason, it is desirable to constantly monitor the temperature of the fish over the duration of the journey. The DS1921 iButton temperature that provides full access to the iButton’s resources. The software is well suited for use in a classroom or laboratory situation, as it provides all that’s necessary to initiate datalogging and retrieve mission results. To use the software, the iButton’s 1-Wire bus must be hooked up to the PC’s serial or USB ports. Let’s see how it all works. datalogger is ideal for this purpose. This small, low-cost iButton is easily clipped onto a piece of plastic and placed beside the fish in each crate. The iButton is programmed to take readings every 30 minutes and its memory is programmed with the shipping details, such as lot number and batch quantity. As the iButton contains an internal battery, it operates completely autonomously, requiring no external connection to perform its pre-programmed datalogging tasks. When the crate arrives at its destination in Japan, the merchant can retrieve the iButton, connect it to a laptop or Palm computer and upload the data. The temperature readings can be displayed as a graph and the merchant can check that the salmon have been stored between the correct temperature thresholds for the entire journey. If the temperature limits have been exceeded the fish are rejected and destroyed. This ensures that only the finest quality fish are sold on to the restaurants. The iButtons are collected and then mailed back to the salmon farms so that they can be reused. As the iButton has a life span of up to 10 years, it can be reused many times, making it a very cost-effective datalogging device. Before the computer chip in an iButton will begin logging temperature and time information, the user must first set a number of key parameters. These are as follows: • Set the internal clock to the current date and time. • Enter mission details (eg, student name and purpose of experiment). • Set the sample rate (1 minute to 255 minutes). • Set the upper and lower temperature alarm threshold values. • Set the mission start delay (time before the mission starts – up to 45 days). To perform the configuration, the iButton is clipped into a “Blue Dot” adapter, which is in turn connected to a computer via its serial or USB port. The Windows software can then be used to set all of the parameters and initiate the mission. Once configuration is complete, the iButton is removed from the Blue Dot adapter and placed in the logging position. The iButton’s stainless steel case withstands dirt, moisture and rough treatment and its small size means that it can be attached unob- Fig.1: software supplied with the starter pack makes it very easy to configure the iButton for a mission. Fig.2: once a mission has been launched, its status can be viewed at any time. Fig.3: current mission data is automatically downloaded from the iButton when the software is launched. Configuring the iButton 92  Silicon Chip siliconchip.com.au trusively to most container surfaces or walls. This could include bottles, boxes, crates, pallets, air cargo containers, refrigerators, semi-trailers, etc. Retrieving mission data When the mission is complete, the iButton is retrieved and clipped into the Blue Dot adapter again. The software is then used to extract mission data. Reports and graphs can be generated from the data and it can be exported in CSV format for use in other applications (eg, Excel). Data storage in detail The iButton stores data in three different ways that serve different application needs, as follows: (1). TEMPERATURE LOGGING: the iButton can take 2048 time and date stamped temperature readings at equal intervals, spaced between 1 and 255 minutes. At a 1-minute interval, this equates to about 1.4 days, whereas at a 255-minute interval, this equates to almost a year of continuous operation. The battery lasts for 10 years, so almost a million readings are possible in this time. To fully utilise the recording mission, the user chooses the time to begin temperature taking, sets a sampling rate, sets high and low alarm thresholds, and determines whether to “rollover” when 2048 time and temperature readings are completed (or to simply stop logging at that point). (2). HISTOGRAM BINS: the iButton also simultaneously stores each temperature sample in a histogram. The histogram memory consists of 56 bins in 2° increments; each bin can hold up to 65,500 temperature readings. The histogram method of data storage serves applications that require long-term monitoring or the ability to instantly assess whether a minimum or maximum threshold has been compromised. For example, when storing blood or other biomedical products, it is critical to know if certain user-defined temperature thresholds have been exceeded. The histogram instantly reveals this information, together with the length of time the thresholds have been exceeded. The histogram keeps recording after the standard log is full. (3). THRESHOLD ALARMS: both upper and lower threshold alarms siliconchip.com.au The “Blue Dot” adapter provides a connection point for one or two iButtons during configuration and data retrieval. Hook-up to the PC is made via an RS232 serial or USB adapter. Fig.4: graphs and charts of the results can be instantly displayed and printed. They can also be exported in CSV format from the main toolbar. can be set. If the iButton temperature exceeds these thresholds at any point an alarm time/date stamp will be recorded. This reveals exactly when, and for how long, an alarm threshold was exceeded. Summary Dallas Semiconductors’ iButtons open up a completely new world of datalogging possibilities. Check out their website at www.iButton.com for all the details. As well as their many commercial applications (see panel), Revolution Education believe that the low-cost DS1921 temperature dataloggers will find use in the lab and classroom. To this end, they are making a complete “starter pack” available. The iButton datalogger starter pack (Part No. LOG001) contains a DS1921 iButton, Blue Dot adapter, USB adapter and PC software on CD and is avail- able from www.microzed.com.au or phone MicroZed on (02) 4351 0886. Individual iButtons are also available SC from MicroZed. * About the author: Clive Seager is the Technical Director of Revolution Education Ltd, the developers of the PICAXE system. Select your microcontroller kit and get started... From $295* RCM3400 Fax a copy of this ad and receive a 5% discount on your order! Feature rich, compiler, editor & debugger with royalty free TCP/IP stack • Prices exclude GST and delivery charges. Tel: + 61 2 9906 6988 Fax: + 61 2 9906 7145 www.dominion.net.au 4007 October 2005  93