Electrical Properties 59 This formula is useful for making initial estimates, but it must be remembered that the current

consumption depends not only on the clock frequency, but also on the supply voltage, the temperature and of course the type of chip.

With a supply voltage of 5 V and an assumed current consumption of 60 mA, a smart card has a power consumption of 300 mW. This value is so low that there is no need to be concerned about overheating of the chip while it is operating, even though this amount of power is dissipated over an area of approximately 20 mm2_.

All smart card microcontrollers have one or more special power-saving modes. The operat- ing principle of such modes is based on disabling all of the functional components of the chip that are not being used. In principle, only the interrupt logic of the I/O interface, the processor registers and the RAM need to remain energized in order to save the current operating state. In practice, the processor often remains energized as well, but the ROM and EEPROM are switched off. When the microcontroller is in this sleep mode, or idle state, its current con- sumption drops dramatically, since most parts of the chip are isolated from the supply voltage. In addition to this sleep mode, many smart card microcontrollers support another mode in which the applied clock can be switched off, called the ‘clock stop mode’. The main purpose of this mode is to allow the hardware components in the terminal that generate the clock to be switched off, which makes this mode particularly attractive for battery-operated terminal devices. According to ISO/IEC 7816-3, the maximum allowable current in the sleep mode with the clock stopped is 500 µA for all three classes. Even this value is too high for the mobile telecommunications area. For instance, GSM 11.11 specifies an upper limit of 200 µA for 5-V smart cards at a clock frequency of 1 MHz.

current consumption clock rate 0 mA 2 mA 4 mA 6 mA 8 mA 10 mA 0 MHz 1 MHz 5 MHz 10 MHz 15 MHz 20 MHz

Figure 3.36 Microcontroller current consumption versus clock frequency in the normal operating mode (not the sleep mode). The current consumption in the sleep mode with the clock applied is also linearly dependent on the clock frequency and is approximately 50 µA at 5 MHz, depending on the microcontroller type

Another important detail regarding the supply current causes severe headaches for ter- minal manufacturers who choose to ignore it. All current microcontrollers employ CMOS

technology. Under certain conditions, large short-circuit current can occur briefly during tran- sistor switching processes. These produce current spikes that are many times greater than the nominal operating current, with durations in the nanosecond range. These spikes can also occur when the EEPROM charge pump switches on. If the terminal cannot supply such large currents during these short intervals, the supply voltage will drop below the permitted value. This can produce a write error in the EEPROM or trigger the undervoltage detector in the chip.

For this reason, references to such spikes can now be found in practically every relevant standard and specification. For instance, ISO/IEC 7816-3 requires power sources for class-A (5-V) cards to be able to handle spikes with a maximum duration of 400 ns and a maximum amplitude of 100 mA. Assuming a triangular spike, this amounts to a charge of 20 nA–s that must be supplied. This requirement can be met in a simple manner by connecting a 100-nF ceramic capacitor between circuit ground and the supply voltage line very close to the contacts for the card.

3.3.4 External clock

Smart card processors usually do not have internal clock generators. An externally supplied clock is therefore necessary. This clock also provides the reference for data transmission rates. According to ISO/IEC 7816-3 and most other standards and specifications, the duty factor of the clock must be 50 %. The usual tolerance is a duty factor range of 40 to 60 %.

The clock signal applied to the contact is not necessarily the same as the internal clock provided to the processor. Some microcontrollers have a clock multiplier or divider that may optionally be inserted between the external and internal clocks. The clock divider frequently has a division factor of 2, so the internal clock rate is only half of the external clock rate. This is partly due to the characteristics of the chip hardware and partly because it allow oscillators already present in terminals to be used as the source of the clock signal for the chip.

Most smart card microcontrollers allow the clock signal to be switched off when the CPU is in the sleep mode. In this case, switching off the clock means holding the clock line at a defined level. Depending on the preference of the chip manufacturer, the ‘off’ level may be either high or low.

Since smart cards draw only a few microamperes from the clock line, switching off the clock may at first glance appear somewhat curious. Nevertheless, the amount of power saved within the terminal is substantial, so it can be worthwhile in certain applications.

3.3.5 Data transmission

If an error occurs during data transmission, it may happen that the terminal and the card attempt to send data at the same time. This results in a data collision on the connecting I/O line. Quite apart from the problems this causes at the application level, at the physical level it could produce currents in the I/O line that might be large enough to destroy the interface components. To prevent damage to the semiconductors in such an event, the I/O line in the terminal is tied to the+5-V level via a 20-kpull-up resistor, as shown in Figure 3.37. In

3.3 Electrical Properties 61