Electrical characteristics of CLK under normal operation conditions

Symbol Conditions Minimum Maximum Unit
Vih Either (1) IIih max = +/- 200uA 2.4 VCC (2) V
Iih max = +/- 20uA 0.7*VCC VCC (2) V
Iih max = +/- 10uA VCC-0.7 VCC (2) V
Vil Iil max = +/-200 uA 0 (2) 0.5 V
Voh Either (2) Iol max = +/- 100uA 2.4 VCC V
Iol max = +/- 20uA 3.8 VCC V
tr, tf Cin = 30pF    9% of the period
with a max:0.5us
(1) For the interface device, take into account three conditions.
(2) The voltage on CLK shall remain between 0.3V and Vcc+0.3V.

 

 

Electrical characteristics of RST under normal operation conditions

Symbol Conditions Minimum Maximum Unit
Vih Either (1) IIih max = +/- 200uA 4 VCC (2) V
Iih max = +/- 20uA VCC-0.7 VCC (2) V
Vil Iil max = +/-200 uA 0 (2) 0.6 V
(1) For the interface device, take into account both conditions.
(2) The voltage on RST shall remain between 0.3V and VCC+0.3V.

 

 

Electrical characteristics of VCC under normal operation conditions.

Symbol Minimum Maximum Unit
Vcc 4.75 5.25 V
Icc    200 mA

 

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Supply current
The card’s microcontroller obtains its supply voltage, and thus its supply current, via contact C1. According to theGSM11.11 specification, this current may not exceed 10 mA. The current must lie below a certain value to allow the hardware in the terminal to be designed to supply a corresponding maximum current. The first version of the ISO/IEC 7816-3 standard in 1989 specified a maximum current of 200 mA with a 5-V supply voltage and 5-MHz clock, but even then that was too much. Since that time, the values have been significantly reduced and made dependent on the various supply voltage classes.
 
The most important factor is that the current consumption of a microcontroller is directly proportional to both the applied clock frequency and the supply voltage. It is also somewhat dependent on the temperature of the microcontroller. The current version of ISO/IEC 7816-3 specifies a maximum current of 60mAfor voltage class A (5 V) at a maximum clock frequency of 5 MHz and a maximum ambient temperature of 50 ◦C. With regard to smart cards for financial transactions, in the EMV 2000 specification the ISO/IEC 7816-3 value for the maximum current is reduced from 60 mA to 50 mA, but there are no other significant supplementary restrictions. In the telecommunications sector, current consumption has been a critical factor since the very beginning. Consequently, in this sector are there is a complicated set of rules specifying the maximum current as a function of the clock rate and operating state of the smart card. A detailed presentation of the maximum current for the various voltage classes is given in Table 3.2.
 
A technically interesting innovation has been introduced for smart cards that conform to the USIM specification. Here two different operating states are specified with regard to current consumption. The first state encompasses the time from the reset until selection of the application, while the second state comprises the subsequent application-specific session. The maximum allowable current consumption in the first state is significantly lower than in the second state. Furthermore, the mobile telephone can use an application-specific data object to determine the current demand of the currently selected application. This is because the current consumption of a smart card is considerably higher when its numeric coprocessor or internal frequency multiplier is enabled in order to achieve higher performance. With this mechanism, itwould at least be theoretically possible to have a mobile telephone use only those ‘smart applications’ whose current consumption it can adequately support. Unfortunately, the corresponding USIM specification does not include any procedure to allow a mobile telephone and a smart card to negotiate the maximum available current, as with PPS. With the current version of the specification, the only option available to a mobile telephone if its smart card demands too much current is to deactivate the smart card. Modern microcontrollers for smart cards have current consumptions on the order of 350 μA per megahertz of clock frequency. Using this value, we can write the following formula for the current consumption of a microcontroller as a function of the applied clock frequency or the clock frequency generated inside the chip:
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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 operating 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 consumption 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.
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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 terminal manufacturers who choose to ignore it. All current microcontrollers employ CMOS technology. Under certain conditions, large short-circuit current can occur briefly during transistor 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.