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.

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-k pull-up resistor, as shown in Figure 3.37. In combination with the agreed convention of never sending an active 5-V level, this avoids any problems that might occur if the two parties attempted to drive the data line to two different levels as the result of a communications error. Whenever the I/O line has to be set to a +5-V level during communications, the party in question simply switches its output to a highimpedance state (tri-state level), and the line is raised to the +5-V level by the pull-up resistor alone.

SLE5542 Printed Cards

Figure 3.37 The circuit of the I/O channel between the terminal and the smart card

Activation and deactivation sequences
All smart card microcontrollers are protected against electrostatic charges and potentials on the contacts. In order to avoid undefined states, precisely specified activation and deactivation sequences are prescribed, and they must be strictly adhered to. This is also reflected in the relevant part of ISO/IEC 7816-3. These sequences define the electrical aspects of activating and deactivating the card and have nothing to do with the sequence of establishing mechanical contact with the card, which is anyhow not specified. Nevertheless, mechanical contact is first made with the ground contact of the card as an intelligent precaution in order to ensure well-defined electrical connection and disconnection.

As shown in Figure 3.38, the electrical ground connection must be made first, followed by the supply voltage connection. After this comes the clock connection. If an attempt were made to connect the clock before the supply voltage, for example, the microcontroller would try to draw its entire supply current via the clock line. This could irreversibly damage the chip, causing complete functional failure. A faulty deactivation sequence could also have similar effects on the microcontroller. When the microcontroller is operating, it can be reset via the reset line. This requires a low level to be first applied to this line, with the actual reset being initiated by the subsequent rising edge. Such a reset during operation is called a warm reset, as in other computer systems. By contrast, a cold reset is one that occurs when all the supply lines are switched as specified in the ISO standard.

ATMEL AT88SC153 Cards,AT88SC153,Card Module Contact,

Figure 3.38 Smart card activation and deactivation sequences according to ISO/IEC 7816-3. The intervals t1 and t2 lie in the ranges 400/ f ≤ t1 ≤ 40, 000/ f and t2 ≤ 200/ f