Electrical connections
Smart cards have either six or eight contacts on the front side, which form the electrical interface between the terminal and the microcontroller in the card. All electrical signals are passed via these contacts. However, according to ISO/IEC 7816-2, two of the eight contacts (C4 and C8) are reserved for the auxiliary contacts AUX1 and AUX2, which can be used in the future for interfaces such as USB. Presently, some smart card modules have only six contacts, since this slightly reduces manufacturing costs. However, they have the same functionality as modules with eight contacts. The contacts are numbered sequentially from top left to bottom right. Figure 3.34 shows the ISO designations and electrical assignments of the eight defined contacts.
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Figure 3.34 Electrical assignments and numbering of smart card contacts, per ISO 7816-2

Until the late 1980s, it was necessary to apply an external voltage to program and erase the EEPROM, since the microcontrollers then in use did not have charge pumps. Contact C6 was reserved for this purpose. However, since the early 1990s it has been standard practice to generate this voltage directly in the chip using a charge pump, so this contact is no longer used. Nevertheless, it cannot be employed for some other function, as this would conflict with the ISO standard. Thus, every smart card has a contact that has no real function, but which must still be present. Since the programming voltage contact lies between two others that are necessary for the operation of the card, it cannot simply be eliminated. This somewhat reduces the drawback of having a superfluous contact.

Table 3.3 Contact designations and functions according to ISO 7816-2

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Supply voltage
The supply voltage for smart cardswas originally 5 volts, with a maximum tolerance of±10 %. This voltage, which is the same as that used for conventional TTL circuits, was the standard value for all commercial smart cards and all applications. As with other semiconductor components, increasingly smaller structure widths of semiconductor components and the need for reduced current consumption has made it necessary to markedly reduce the operating voltage range. This has been given extra impetus by the mobile telephone sector. The market-driven demand for reducing the weight of mobile telephones required changing from 6-V batteries to 3-V types, and since all other components for mobile telephones were available in 3-V technology, for a while the smart card was the only component in a mobile telephone that still needed 5 V. Consequently, an expensive voltage converter
was needed to provide electrical power to the smart card, resulting in an avoidable extra cost.

Consequently, in the international standards the voltage range for smart cards was first extended to 3–5 V with a tolerance of ±10 %. This yields an effective range of 2.7 V to 5.5 V. However, it can already be foreseen that this extension will be insufficient. Consequently, the revised version of ISO/IEC 7816-3 and ISO/IEC 7816-3 Amd. 1 will again be revised in the relatively near future to permit smart cards using a supply voltage of 1.8 V with a tolerance of ±10 %. The extended voltage range does not pose a problem for the microprocessor or most types of memory, particularly since the core voltage for semiconductors built with 0.13-μm technology is usually only 1.8 V. However, EEPROMs are also integrated into smart card microcontrollers. These EEPROMs and their associated charge pumps form the greatest obstacle to low-voltage smart cards. Nevertheless, with a certain amount of technical ingenuity it is certainly possible to integrate EEPROMs and their charge pumps into microcontrollers that can work over a supply voltage range of 1.62 to 5.5 V.

The ISO/IEC 7816-3 standard and its amendment define three classes for characterizing the voltage ranges of smart cards. Class A covers the voltage range of 5 V ±10 %, Class B
covers the range of 3 V ±10% and Class C covers the range of 1.8 V ±10 %. All three classes can be used individually or in any desired combination. For instance, if a smart card meets the requirements for both Class A and Class B, it can be used with both 5-V and 3-V supply voltages. However, it must be borne in mind that the range between 3.3 V and 4.5 V lies outside the specified ranges, so the smart card need not necessarily be able to operate in this range. Nevertheless, smart cards can usually be used without any problems between the upper and lower limits of the specified voltage ranges. The ISO/IEC 7816-3 standard imposes yet another equally important requirement, which is that under no circumstances may the microcontroller of a smart card be damaged if the card is powered from voltage not supported by the microcontroller. This is an essential requirement
for ensuring the upward compatibility of newtypes of smart cards with older types of terminals. The objective is to eliminate the possibility that using a 3-V card in a 5-V terminal, for example, could destroy the IC in the card.

The three possible voltage ranges defined by ISO/IEC 7816-3 and ISO/IEC 7816-3 Amd. 1 have not yet been used in the area of smart cards for financial transactions. In this area, the original operating voltage of 5 V ± 10% still prevails, since stationary terminals can easily provide a voltage of 5 V without additional technical components. The situation for smart cards used in the telecommunications area is completely different. In this area, smart cards that can be used only with 5-V supplies (‘5-V-only cards’) have fully disappeared. Since the end of the 1990s, 3 V has become the standard operating voltage for GSM devices. In the area of the new UMTS mobile telephone network, it is already clear that there will not be any mobile telephones that support 5 V, with the 3-V supply voltage being supported only for reasons of compatibility. In the medium term, the future standard operating voltage will be 1.8 V. The ISO/IEC 7816-3 standard specifies a particular procedure for selecting the supply voltage, which essentially amounts to trying the voltages for each of the three classes in turn. As soon as the terminal can receive an ATR, it analyzes the ATR to see whether the smart card prefers a particular class. If so, the terminal initiates a new activation sequence using the desired class. If the ATR does not include any information about the voltage range, the smart card will be used with the voltage with which the first ATR could be received. The selection procedure is shown in the form of a flow chart in Figure 3.35. It is already certain that an additional voltage class will be introduced in the future to allow a supply voltage of 1.2 V. Particularly in the light of the steadily decreasing structure widths of semiconductor devices, in the medium term this could become a typical operating voltage for microcontrollers.

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Figure 3.35 Flow chart showing the actions taken by a terminal when selecting the operating voltage based on the classes specified in ISO/IEC 7816-3 and ISO/IEC 7816-3 Amd. 1. With mobile end-user equipment, the process is usually started using the lowest supply voltage