Testing the "System on a Chip"
Much has been written about the concept of a "system on a chip," the ever-increasing integration of logic and analog functions on one silicon die or chip. This paradigm is about to change. The results of work by universities, national labs, and companies such as Motorola, Inc., are paving the way for a true system on a chip, or SOC. These new SOCs will not only analyze data, but will measure, analyze, and react to their environment.
The integration of power and analog elements with a CMOS microcontroller unit (MCU) has been possible for several years. Products have been introduced such as an integrated 68HC05 motor controller with integral power devices in an H-bridge configuration (1990). In 1993, a product called a System Chip MCU was introduced that provided a Society of Automotive Engineers J1850 interface, including the physical layer. This chip could withstand 40 V, based on the combination of power and analog capability with the MCU. However, the system input was not included in previous monolithic designs.
What is the most recent development that promises to truly enable a system on a chip? It is the ability to combine CMOS and MEMS (microelectromechanical systems) structures into one process flow. Photo 1 illustrates a 68HC05 microcontroller with a 100 kPa pressure sensor integrated onto a single silicon die. A likely application is a side air bag sensor.
A pressure sensor, inside the door panel of a car, could detect the change in pressure when the panel crumples under an impact. The ability to program the onchip microcontroller will enable the auto manufacturer to embed the control algorithm inside the chip. To complete an entire system, only a mechanism for actuating the air bag need be added. This actuation capability could be yet another step in the continuous integration of silicon and electronics/electromechanical systems. This platform provides a first step in the integration of electronics with electromechanical structures and at the same time raises several issues that must be resolved before a low-cost, high-quality product can be mass produced. One of these issues is that of testability.
Typical logic circuits have many years of accumulated test data that can be used as a foundation for building the next generation of product. With sensors, however, very little previous technology can be reused. The reasons are the relative infancy of sensor technology and the uniqueness of each type of sensor. For example, the technology used to measure pressure (a thin diaphragm with integral strain gauge) is much different from that used for measuring acceleration (a proof mass forming a moving capacitor). The testing technology is different as well. Pressure measurements require a pressure source to be connected to the sensor; acceleration or shock detection requires shaking the device at some known frequency and force.
System Configuration
To develop a proof-of-concept vehicle (see Figure 1), a 100 kPa pressure sensor was integrated onto Motorola's standard 8-bit 68HC05 microcontroller core along with the associated analog circuitry [1]. To this basic core was added analog circuitry for signal conditioning, a voltage and current regulator, and 10-bit A/D and 8-bit D/A converters. A temperature sensor was also incorporated into the design for compensation purposes.
The pressure transducer is temperature dependent and has an inherent nonlinearity. To increase the accuracy of the system, a calibration or conditioning algorithm must be programmed into the microcontroller.
The pressure transducer's output is conditioned by a variable gain and input offset amplifier that is controlled by the program stored in the MCU. The A/D converter is used to read the temperature sensor's and the pressure transducer's outputs. The band gap voltage regulator supplies a constant voltage for the pressure sensor, amplifier, and A/D converter. The band gap current regulator provides a constant current source for the temperature sensor.
Calibration Method
The MCU calibrates and compensates the pressure sensor's nonlinearity and temperature drift. To provide the maximum accuracy, an A/D input resolution of 10 bits was chosen and the calculation resolution was set at 16 bits, fixed point. To calibrate span and offset and compensate the nonlinearity of the sensor output, calibration software performs a second-order polynomial correction of sensor output described as:
Vout = c0 + c1Vp + c2Vp2 (1)
Cp = (c0, c1, c2 ) (2)
where:
Vout = calibrated output
Vp = uncompensated pressure sensor output
To compensate the temperature dependency of Cp, calibration software is used to calculate Cp using a second-order polynomial fitting equation to temperature:
c0 = c00 + c01Vt + c02 Vt2 (3)
c1 = c10 + c11Vt + c12 Vt2 (4)
c2 = c20 + c21Vt + c22Vt2 (5)
(6)
where:
Vt = temperature sensor output
The Cts are read during the calibration procedure and stored in EPROM. The MCU calculates Cp from the temperature sensor output, Vt, and Ct. Cp is then used to calculate the calibrated pressure sensor output using the pressure transducer's output, Vp.
Calibration Procedure
The calibration system first adjusts the gain and offset of the amplifier to use the full A/D range. Then the characteristics of the uncompensated pressure sensor output are examined over several temperature points. At each temperature, a second-order polynomial described in Equation 1 is obtained by least square fitting and the coefficient set, Cp, is determined. After completing the calculation of Cp over all temperature points, Ct is determined by the least square fitting of Equations 3, 4, and 5 to determine Cp over the temperature points. At present, at least three separate temperature sampling points are required to complete the fitting calculation.
Figure 2. The uncompensated output of the sensor-based system on a chip is plotted at four different temperatures.
Characteristics
Figure 2 shows the uncompensated sensor output characteristics over various temperatures after adjusting gain and offset. Based on these data, the coefficients for calibration were calculated and written into the onchip EPROM by the calibration system. The compensation value was rounded off to 8 bits. Figure 3 shows the calibrated and compensated output of the integrated MCU. Figure 4 shows the error from expected values. Since 1 bit is 0.4% error, the result indicates the error is within 0.4% of full-scale output.
Figure 3. Compensated output of the system on a chip is improved through testing and calibration at three temperatures.
Test Issues
Several issues are raised by this initial work, including the different types of testing required, unique test equipment, and the need for multipass testing. To make a low-cost integrated solution possible, these concerns must be addressed.
The integration of a physical measurement function onto the already complex mixed-mode analog-digital chip raises the need for an additional type of testing. The physical medium being tested must be applied to the device and the response must be measured. Measuring the response to a physical stimulus is not a
Figure 4. Bit error in the compensated output is within 1 bit at both 30°C and 85°C
standard test for the semiconductor industry, especially under multiple temperatures. Standard equipment can test the digital and analog portions of the chip, but the application of a physical stimulus and the procedure of heating and cooling the device under test rapidly and accurately drive the need for a modified and unique tester. These testers are one of a kind and are not available as a standard. The tester therefore represents a large part of the final unit's cost.
Not only are the testers expensive, but the throughput is limited. This raises the cost of each part because of the increased depreciation costs allocated to each device. The cost is further increased by the need for multipass testing. Remember that each part is first tested, using at least three different temperatures, to determine the transducer's output characteristics over temperature. Then these values are used to derive the compensation algorithm, which is loaded into the onchip EPROM. To complete the cycle, the device is once again tested over temperature to prove accuracy. Hence, not only is a special tester required, but it becomes a bottleneck since it must be used twice to complete each device—once to measure the characteristics and a second time to verify the result.
Future Directions
Finding ways to reduce the cost of testing is one of the keys to making a low-cost integrated sensor and MCU a reality. Ideas that could prove promising include:
Thoroughly characterizing the design
Limiting the operating temperature
Limiting the accuracy
Programming the MCU to take data during testing
Loading the test and compensation algorithm into the MCU before testing
Since this is a first proof-of-concept device, further characterization could provide a way to limit the number of temperatures required for compensations. Limiting the operating temperature range could also reduce the number of temperatures required for compensation testing. Data shown in Figure 3 indicate a 5% accuracy from 5°C to 25°C. Another potential cost reduction step would be to use the MCU's programmability for data logging during test. By storing the compensation program in the onchip EPROM prior to test, and then logging the uncompensated output into the EPROM during test, it might be possible to develop an algorithm for a one-pass test over temperature.
Without a breakthrough in lowering the cost of testing this new integrated sensor and MCU, the system designer may be limited to the continued use of the present day solution—separate MCU and sensor.
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All the DS18B20 sensors, used for the multipoint test temperature, are connected with MCU on one of IO bus, and temperature data are collected by turns. If the system has a large amount of sensors, the time of MCU used in processing the temperature data is obviously prolonged, so the cycle of alternate test gets longer. In this paper, a new method that DS18B20 are rationally grouped is presented, and some measures are taken in software; as a result, the speed of alternate test advances distinctly.
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现在，生物技术的发展更是突飞猛进，这必将促成生物检测方法的不断补充和完善。 下面是我为大家整理的食品生物技术论文，希望你们喜欢。
食品检测中的生物技术分析
摘要：近年来，食品安全问题得到了全社会的关注，食品检测技术得到了更多的重视，生物技术等新兴的食品检测技术也因此而得到了广泛的应用。文章在简要介绍生物技术的基础上，详细阐述了生物技术在食品检测中的应用，以期为生物技术的发展与应用提供新的思路。
关键词：食品检测 生物技术 应用
食品安全问题是由于食品中含有毒、有害物质，对人体健康产生危害而造成的公共卫生问题。近年来，食品安全问题已成为人们普遍关注的社会热点问题，引起了政府和公众的广泛重视。目前，国内的食品安全问题的产生既有政府监管不严、制度体系不完全的原因，也有食品检测技术不够科学先进的原因。随着食品工业的快速发展，对食品检测技术提出了更高的要求，传统分析方法难以满足当前食品检测的需要，灵敏度高、特异性强、简便快捷的生物技术逐渐在食品检测领域大放异彩，文章将对此进行详细论述。
一、生物技术概述
生物技术是利用生物有机体及其组成部分，或是利用其组织、细胞、酶来合成、转化、降解，从而实现生产产品等目的的技术。生物技术在食品领域的应用已经有几百年的历史，从最初的面包、酱油生产，如今已延伸到食品领域的各个方面，得到了长足的发展和不断的完善。现代生物技术是建立在细胞生物学等学科基础之上的高科技技术，包括细胞工程、酶工程、基因工程、发酵工程等诸多类技术。细胞工程是以动物、植物细胞及细胞融合技术为基础的一类生物技术，主要用于食品生产;酶工程是通过特定细胞酶来控制食品生产过程中的物质转化;基因工程是通过重组基因来改造食品生物特性，起到生产特殊产品的作用;食品发酵技术如今已发展为发酵工程学，用于预定食品及成分的生产。
二、生物技术在食品检测中的应用
生物技术在食品检测中的应用，表现在食品中微生物、转基因成分等对人体有毒有害物质的检测。例如借助细菌学、血清学方法可以检测食品中是否含有致病菌，但是这些传统生物技术方法操作繁琐，耗时较长，目前应用更多的是操作简便、快捷且精准的生物芯片、胶体金免疫层技术、PCR技术、酶联免疫吸附法、基因探针等生物技术。
1.生物芯片的应用
生物芯片技术是建立在现代生物化学、物理化学、计算机科学等诸多学科交叉的基础上的，检测原理是利用生物分子间的抗原、抗体等亲和反应或碱基对互补杂交，检测、分析样品中的成分。由于生物芯片技术可在小面积内对多种生物分子进行并行检测分析，分析量很大，因而检测效率较高，检测结果具有很好的可比性。
生物芯片包括基因芯片和蛋白质芯片。基因芯片是将基因探针固化在检测工具表面，利用软件分析检测工具与样品间发生的基因杂交信息，从而检测出遗传信息。基因芯片可同时进行定性定量检测，能够快速检测分析大量序列的杂交信息。蛋白质芯片的原理则是利用生物分子间的特异性结合来测定样品成分，具体操作与基因芯片技术类似。基于基因芯片和蛋白质芯片的原理及特点，生物芯片技术通常用于转基因食品、原料、病原微生物的检测。
2.胶体金免疫层技术的应用
一直以来，胶体金免疫层技术在医学领域得到了广泛的应用，近年来逐渐应用于食品检测领域。胶体金免疫层技术具有操作简单、耗时较短等优势，一般需要定性分析或半定量分析，主要用于有害微生物、药物残留、违禁药物的检测。该技术用于有害微生物的检测较多，例如检测食品中是否含有大肠杆菌、沙门氏菌、霍乱弧菌等致病菌，较为常见的检测方法是双抗体夹心法;用于药物残留的检测是通过制得的抗体抗原与药物残留反应来分析食品中是否含有黄曲霉毒素、磺胺类药物、氯霉素等残留;用于违禁药物的检测一般是利用竞争免疫层析法来分析食品中是否含有罂粟碱、吗啡等物质。目前国内应用胶体金免疫层技术仍处于初级阶段，尚未投入广泛的应用，还有待新型免疫层析产品的开发、研制。
3.PCR技术的应用
PCR技术是上世纪八十年代产生的一种技术，借助体外扩增DNA来实现转基因食品以及病原微生物的检测。传统PCR技术早于1992年便用于病原菌的检测，但直到近年来才得到广泛应用，目前可用于检测沙门氏菌、肠出血性大肠杆菌、金黄色葡萄球菌等。但在实际应用中，传统技术存在一些缺陷，无法定量检测，而且存在死细菌的环境下检测结果不准确，难以检测微生物毒素，因此在传统技术的基础上经过一系列改进和技术融合产生了多种改进的PCR技术，包括实时定量的PCR技术、PCR―DGGE技术、巢式及半巢式PCR技术等。定时定量的PCR技术是在传统技术中加荧光基团来实现实时检测，能够做定量分析，主要应用于检测外源基因污染、病原微生物、掺假量等，例如检测葡萄中的曲霉菌、肉骨粉中的牛羊源成分。PCR―DGGE技术在传统PCR技术基础上结合变性梯度凝胶电泳技术，不仅特异性强，而且敏感度高。巢式及半巢式PCR技术通过设计两对或1对半引物来降低假阳性结果的产生，使检测下限大幅度下降，检测结果通常无需其他方法再验证。
4.酶联免疫吸附法的应用
酶联免疫吸附法是利用免疫或酶促反应来进行食品检测，具有操作简便、特异性强、耗时短、灵活、可批量检测的优势。酶联免疫吸附法用于有毒有害物质的检测比常规培养法耗时少三至四天，而且无需特殊设备支持，结果易于观察辨别，样品易于保存，例如有研究用该法检测牛奶中的沙门氏菌敏感性100%、特异性99.7%，检测时间不超过3天，因而广泛应用在黄曲霉毒素等毒素检测、残留药物检测、过敏原检测、生理活性物质检测、转基因食品检测等领域。
5.DNA探针技术的应用
DNA探针技术利用碱基对结合原理制成DNA探针，能够检测样品中的碱基序列，从而判定样品基因序列。由于该技术操作简便，而且检测结果精确度高，应用十分广泛，通常用于大肠杆菌、金黄色葡萄球菌等检测。DNA探针技术主要有异相杂交和同相杂交两种技术，其关键在于针对检测目标构建相应的DNA探针，只有DNA探针的基因序列具有针对性和特异性，方能取得理想的检测结果。
三、结语
近年来，随着生物技术的发展和进步，其在食品安全领域发挥了越来越重要的作用，在食品检测的各个方面得到了广泛的应用。然而虽然生物技术普遍具有成本低、操作简便、效率高、特异性高等优势，但是在实际应用中各种生物检测技术均存在自身的局限性，需要结合实际需要灵活选择、搭配。为了更好的提高食品检测水平，解决食品安全问题，还需要开发新的生物检测技术和方法，对现有技术方法不断进行优化，这需要相关领域的专家学者持续不懈的努力
参考文献
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[5]吴彤.现代生物技术在食品检测领域中的应用[J].大众标准化，201l(S1).
[6]张奇志.DNA探针和 PCR技术在食品检测中的应用[J].广东农工商职业技术学院学报.2007(23).
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