From an informatics perspective, the central component of a smart card is the microcontroller embedded under the contacts. It controls, initiates and monitors all of the card’s activities. The microcontrollers that have been specially designed and developed for this purpose are complete computers in their own right. This means that they contain processors, memory and interfaces to the outside world.

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Figure 3.39 Possible arrangement of the essential functional components on the die of a simple smart card microcontroller

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Figure 3.40 Photograph of a PC 83C852 smart card microcontroller with the following functional components (from top left to bottom right): ROM, EEPROM, processor with coprocessor and RAM. This chip has an area of 22.3 mm2 and contains 183,000 transistors. Although this chip is no longer produced, it clearly shows the arrangement of the functional components on the die

The most important functional components of a typical smart card microcontroller are the processor, the address and data buses and the three types of memory (RAM, ROM and EEPROM). The chip also has an interface unit that provides serial communication with the outside world. This interface should not be imagined to be a complex functional unit that can independently transmit and receive data. In the simplest case, the serial interface is just a location that can be addressed by the CPU and is connected to the I/O contact. In addition, some manufacturers provide special processors on the chip that act as a sort of mathematical coprocessor, although the functions provided by these components are limited to exponential and modulus operations on integers. Both of these operations are fundamental and necessary elements of public-key encryption procedures, such as the RSA algorithm. The semiconductor technologies that are presently commonly used to produce smart card microcontrollers work with structure widths of around 0.25 μm, 0.18 μm and 0.13 μm, which definitely lie in the range of the smallest currently achievable structure widths. Microcontrollers used in smart cards are not standard, widely available components. Instead, they have been specifically developed for this purpose, and they are not used in other applications. There are several important reasons for this, which are described below.

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Figure 3.41 Relative sizes of functionally identical smart card microcontrollers before and after chiparea reduction (‘shrink processing’). At the far left is an SLE 44C80 in 1-μm technology, with a surface area of 21.7 mm2. To its right is an SLE 44C80S in 0.8-μm technology, with a surface area of 10 mm2. The three gray rectangles show the relative sizes this smart card microcontroller would have if fabricated in 0.5-μm, 0.35-μm and 0.13-μm technologies (Photos: Infineon Technologies)

Manufacturing costs
The surface area of the microcontroller on the silicon wafer is one of the decisive factors with regard to manufacturing costs. A large chip area leads to more complicated packaging in the module, and thus increased costs. The chip area is thus kept as small as possible. Furthermore, many commercially available standard devices include functions that are not needed in smart cards. Since these functions take up extra space on thewafer, they can be deleted from chips designed for smart cards. Although only a small reduction in the manufacturing cost per chip is achieved by such efforts to minimize the chip size, these small savings add up to a significant amount when a large number of chips are produced. This justifies the modifications to the chip design.

Due to the need to integrate all the functional components of a computer into a single silicon chip, the available number of suitable semiconductor devices is extremely limited. Given the requirements of a minimum chip area, 5-V or 3-V supply voltage and a serial interface on the chip, all standard devices are effectively ruled out. In addition, the chip must contain a memory that can be written and erased but that does not require a permanent power supply for data retention (EEPROM or Flash EEPROM).

Since smart cards are primarily used in security-related areas that require both passive and active security features in the chip, developing chips specially designed for this purpose is an unavoidable necessity.

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Figure 3.42 An ST16623 smart card microcontroller with the following functional components (from left to right): ROM, EEPROM, CPU and RAM. Although this chip is no longer manufactured, it clearly shows the arrangement of the individual functional components on the die (Source: ST Microelectronics)