Smartcard providing an improved standby mode

The invention relates to a smartcard (10) comprising: —a microcontroller (30); —a contact communication interface (11); —a supply line (Vcc) connecting the contact communication interface to the microcontroller, a clock line (Clk) connecting the contact communication interface to the microcontroller. The smartcard further comprises a power management circuit (20) including a switch (23) adapted to selectively open or close the supply line connection between the contact communication interface and the microcontroller, the power management circuit being adapted to detect the loss of a clock signal on the clock line (Clk), adapted to keep said switch closed during a predetermined period after a loss of clock signal is detected and adapted to open said switch after said predetermined period.

BACKGROUND

The present invention generally relates to low power devices and more particularly to low power devices performing transactions with smartcards.

Smart cards are plastic cards having an embedded smartcard chip. A smartcard chip is an integrated microcontroller generally comprising a central processing unit, a random access memory, a ROM memory and an EEPROM memory. The smartcard is provided with a contact communication interface. Such a smartcard can carry out a transaction through its contact interface when it is hosted in a smartcard reader. Smart cards are widely used to store sensitive information such as cryptographic keys or software routines that implement valuable algorithms or know-how. Smartcards are notably used for authentication purposes in relation with communication modems.

An increasing number of communication modems are used in environments where their power supply is limited. The use of such modems is notably known for a communication between an energy provider and the consumption counter located at the home of a final user. Such modems can notably be linked to a sensor measuring the electric power consumption, a gas flow or a water flow. On a regular basis, the modem sends the amount measured by the sensor to the energy provider.

Such a modem can be led to work during a significant time or permanently without any external power supply. The modem may notably be left months or years without any external intervention. The operation of such a modem then relies on a battery power supply. If the battery is not charged frequently enough or if it is not timely replaced, the modem may have to power itself off. The modem is then unable to communicate with remote premises. To delay the necessity to proceed to a power off, modems and smartcards need to be provided with efficient low consuming modes.

As the remote communications of the modem are timely spaced (for instance several minutes, hours or days), the modem and the smartcard remain idle most of the time. To benefit from the idle periods, such a modem can be provided with a low power mode. Its static power consumption is thereby reduced during the idle periods in order to increase its autonomy.

When the modem enters in low power mode, it performs a standby process. During the standby process, the modem stops sending APDUs to the smartcard. The smartcard then enters in standby mode. The modem then stops the clock sent to the smartcard. The smartcard then switches from the standby mode to a clock stop mode.

The clock stop mode is the mode where the smartcard has the lowest consumption. Most of the power consumption in clock stop mode is due to leakage currents. Unfortunately, security requirements induce a significant leakage current in conventional smartcards. The smartcards providing the lowest clock stop mode consumption still face a leakage current superior to 50 Microamperes. Thus, even if very efficient modems are used (for instance providing a leakage current as low as several hundreds of nanoamperes), the modem will have to power the smartcard which has a significantly higher power consumption. The smartcard in clock stop mode thus seriously reduces the modem autonomy. The modem and smartcard can thus reveal inappropriate for many applications due to their low autonomy.

An alternative could be to suppress the smartcard power supply when entering into low power mode. In this case when the modem switches back to active mode, the smartcard is powered up. During its start, the smartcard has to load applications and has to communicate with the modem, for instance to send an ATR (Answer to Reset) to modem or to define a PPS (Protocol and Parameter Select) value for the future communications. This solution has two drawbacks:This protocol is not compliant with ETSI standard, and requires software modifications of the modem.Starting the smartcard is a quite lengthy process. Each modem remote communication would be delayed by this starting process.

SUMMARY

Thus, there is a need for a smartcard solving one or more of these drawbacks. The invention proposes a smartcard comprising:a microcontroller;a contact communication interface;a supply line connecting the contact communication interface to the microcontroller, a clock line connecting the contact communication interface to the microcontroller.

The smartcard further comprises a power management circuit including a switch adapted to selectively open or close the supply line connection between the contact communication interface and the microcontroller, the power management circuit being adapted to detect the loss of a clock signal on the clock line, adapted to keep said switch closed during a predetermined period after a loss of clock signal is detected and adapted to open said switch after said predetermined period.

According to an embodiment, said power management circuit is adapted to detect the clock signal restoration and is adapted to close said switch after a clock signal restoration is detected.

According to another embodiment, said microcontroller comprises a non volatile memory, said microcontroller being adapted to detect the loss of a clock signal on the clock line and to adapt to save its execution context in the non volatile memory when a loss of clock signal is detected.

According to a further embodiment, said contact interface complies with ISO 7816 standards requirements.

According to another embodiment, the saved execution context includes the PPS value of the transaction performed when the loss of clock signal is detected.

According to a further embodiment, said microcontroller is adapted to check that no reset signal is received when it is powered up, to restore the execution context when no reset signal is detected and to resume its execution based on the execution context.

According to an embodiment, the ATR transmission is managed by software executed by the microcontroller, said microcontroller is adapted to check that no reset signal is received when it is powered up and adapted to block the ATR transmission when no reset signal is detected.

According to a further embodiment, wherein the microcontroller and the power management circuit are formed on distinct semiconductor substrates, the power management circuit providing a leakage current at least 20 times lower than the microcontroller.

Alternatively, the microcontroller and the power management circuit can be formed on a single semiconductor chip.

The invention further relates to a method for switching a smartcard into standby mode, comprising the steps of:detecting the loss of a clock signal on a clock line connecting a contact interface and a microcontroller of the smartcard;maintaining a connection between said contact interface and said microcontroller through a power supply line during a predetermined period after the loss of clock signal has been detected;opening the power supply line connection after said predetermined period.

According to an embodiment, the microcontroller comprises a non volatile memory, the microcontroller detecting the loss of the clock signal and saving its execution context in the non volatile memory during said predetermined period.

DETAILED DESCRIPTION

FIG. 1is a schematic view of a mobile support9, such as a transport container. A modem8is fastened to the support9and is designed to communicate remotely. Such a support9is intended to be transported using either ships or trucks and can be stored during several days on warehouse spaces. During its use, the support9may be located at places where no external power supply is available for its modem8. The modem8may comprise a Global Positioning System in order to regularly send its location to a remote server.

The modem8comprises a microcontroller82, a contactless communication interface81, a contact communication interface83and a battery84. The microcontroller82manages the contactless communication interface81and the contact communication interface83. The contact communication interface83is located in a smartcard reception slot, for instance at a SIM format. A smartcard10is inserted in the reception slot and comprises a contact communication interface connected to the contact communication interface83of the modem8. The microcontroller82can perform a transaction with smartcard10to have data ciphered or to proceed to an authentication to obtain an access to a contactless communication network, such as a mobile phone communication network reachable through the contactless communication interface81.

As known by someone skilled in the art, the modem8may have three smartcard management modes: an active mode where it performs a transaction with the smartcard, an inactive mode where it does not perform a transaction with the smartcard but provides the smartcard with a clock signal, and a low power mode where it stops providing the clock signal to the smartcard.

The modem8also comprises an interface for receiving an external power supply (not illustrated). Depending on the availability of an external power supply, the modem8is either powered by the external power supply or by the battery84. Each time the modem8is idle and intends to enter low power mode, it triggers a standby process to reduce the discharge of the battery84.

The invention proposes a smartcard structure for lowering its power consumption and for speeding up its availability for resuming the previous settings when the modem8has switched to low power mode.

FIG. 2is a schematic view of the structure of a smartcard10according to an embodiment of the invention. The smartcard10includes a plastic card12embedding a smartcard chip30. The smartcard chip30is an integrated microcontroller comprising a central processing unit31, a random access memory32, a ROM memory33and a non volatile memory34. The ROM memory33may notably store an operating system that is loaded by the central processing unit31at startup. The non volatile memory34, such as an EEPROM memory, may store various working parameters of the microcontroller30. The smartcard10is adapted to store sensitive information such as cryptographic keys or software routines, in order to perform ciphering or authentication processes. The microcontroller30may have a standby mode as known from the prior art. When it detects that no transaction has been performed with the modem8during a predetermined period, it switches into standby mode where its power consumption is reduced. In this standby mode, the microcontroller30remains powered.

The smartcard10is also provided with a contact interface11. The contact interface11is for instance compliant with the ISO 7816 standard requirements. The contact communication interface11provides surface connectors for receiving the following signals from the contact communication interface83. Corresponding connection lines connect the interface11to the microcontroller30:a Vcc line receives a power supply signal from interface83;a Gnd line is connected to the ground of the modem8;a Clk line receives a clock signal from interface83;an I/O line receives an input signal from interface83and sends an Output signal to interface83;an Rst line receives a reset signal from interface83.

The smartcard10further comprises a power management circuit20. This power management circuit20is interconnected between the interface11and the microcontroller30. The power management circuit20comprises a switch23adapted to selectively open or close the supply line connection between the interface11and the microcontroller30. The switch23can thereby selectively interrupt the power supply provided to the microcontroller30. The power management circuit20further comprises a clock signal detector22. The power management circuit20further comprises a control circuit21. The control circuit21is connected to the supply line upstream of the switch23, in order to keep a power supply even when the switch23is open. The control circuit21is also connected to the detector22. The detector22is adapted to detect a loss of clock signal on the Clk line. The detector22informs the control circuit21in case a loss of clock signal is detected on the clock signal line. If the microcontroller30is in standby mode at the time the loss of the clock signal is detected, the microcontroller30is activated to manage its switching into a clock-stop mode. The power management circuit20is used to lower the smartcard power consumption in comparison to the standby mode, when the modem8enters into low power mode.

The smartcard10can be automatically switched into clock stop mode by the modem8. When modem8switches into low power mode, it stops sending the clock signal to the communication interface83, while maintaining the power supply on this communication interface83.

The loss of clock signal is detected by the power management circuit20. The power management circuit20keeps the switch23closed during a predetermined period after a loss of clock signal has been detected. The power management circuit20may include a timer that triggers the opening of switch23at its expiry. The timer may be set to maintain the switch23closed during several hundreds of clock cycles of the smartcard10.

In the clock stop mode of the smartcard10, the switch23is open. The microcontroller30has thus no power consumption in this clock stop mode. The clock stop mode is thus an improved standby mode. Since the overall power consumption is reduced by the use of such a clock stop mode, there are no requirements to change the microcontroller30design technology to reduce its power consumption. Already available microcontroller technologies can be used in a smartcard10according to the invention, while providing satisfactory levels of consumption reduction in clock stop mode.

FIG. 3is a logical diagram illustrating the switching of the smartcard10into a clock stop mode in case the modem8has no transaction to perform with the smartcard10.

At step100, the modem8determines that a low power mode switching condition is met, for instance because it shall remain idled for a long period and because no immediate transaction has to be carried out with the smartcard10.

At step102, the modem8stops sending the clock signal to the smartcard10.

At step104, the power management circuit20detects the loss of the clock signal. The power management circuit20triggers a standby timer. The microcontroller30also detects the loss of the clock signal. The microcontroller30thus determines that a switch into clock-stop mode is currently performed. The microcontroller30uses its own clock signal up to the opening of the switch23.

At step106, the microcontroller30saves its current execution context into its non volatile memory34.

At step108, the microcontroller30ends the saving of its execution context before the end of the standby timer of the power management circuit20.

At step110, the standby timer expires. The power management circuit20opens the switch23. The microcontroller30is not powered anymore. The smartcard10is thus maintained into a very low power consumption mode. The modem8still applies a power supply on the communication interface11, thus providing a power supply to the power management circuit20.

If the smartcard10is kept in clock stop mode further to the modem8not requiring its use, its power consumption is limited to the power supply of the power management circuit20. Such a management circuit20can have a very simple design. It is currently possible to design such a management circuit20using 0.18 μm etching technologies, which would lead to a power consumption of several tens of nA. The microcontroller30and the power management circuit20are advantageously formed on distinct semiconductor substrates, the power management circuit20providing a leakage current significantly lower than the microcontroller30.

FIG. 4is a logical diagram illustrating the switching of the smartcard10exiting the clock stop mode by switching into its active mode.

At step300, the modem8determines that an active mode switching condition is met, for instance because a transaction has to be carried out with the smartcard10. The modem8thus switches from the low power mode to the active mode. During this mode switching, no reset signal is sent to the smartcard since the smartcard processing is fully transparent for the modem8: the modem8. does not know the new state introduced by power management circuit.

At step302, the modem8restores the emission of the clock signal to the smartcard10. The modem8triggers a power up timer. Such a power up timer is already defined in ISO 7816 standards for a smartcard reset and amounts 400 clock cycles.

At step304, the power management circuit20is still provided with a power supply and detects the restoration of the clock signal. The power management circuit20closes the switch23further to this clock signal detection. The microcontroller30is thus powered up.

At step306, the microcontroller30determines that it is powered up further to a clock stop mode, since the modem8has not sent a reset signal. The microcontroller30loads the execution context from the non volatile memory34to resume its execution. Further to a standby detection, the microcontroller30also blocks the emission of an ATR (Answer To Reset) to the modem8. The ATR message is supposed to be sent to a smartcard reader at each reset, according to requirements of the ISO 7816 standards. The modem8thereby determines that the microcontroller was successfully switched into clock stop mode and that its execution is about to be resumed with the former execution context. Since the microcontroller30determines that it is resuming its execution, the microcontroller30advantageously blocks the negotiation of the PPS value in order to have the next transaction between the smartcard10and the modem8carried out with the previous PPS parameters. The microcontroller30starts the execution of its applications.

At step308, the power up timer of the modem8expires. The modem8then sends its first transaction request to the smartcard10.

At step310, the microcontroller handles the first transaction request and resumes the execution of its applications based on the loaded execution context.

The saved execution context may comprise the content of various memory registers of the RAM32, of various RAM areas used by the applications run by the microcontroller30, the content of transitory values stored in the RAM, the identification of the ciphering keys in use, the identification of the applications whose execution is called . . .

The microcontroller30uses a software ATR management. With a software ATR management, the microcontroller30may easily block the emission of an ATR message when it is powered up if no reset is detected. Software ATR management is often performed by smartcards managing multiple contact interfaces. For instance, smartcard chips may provide a SWP (for Single Wire Protocol) or a USB interface. Such a smartcard may switch between an ISO communication mode and a non-ISO communication mode (USB protocol for instance).

The invention can advantageously be embodied without any impact on the ISO 7816 compliance. For instance, a standard power up timer can be used by the smartcard reader before it sends its first request to the smartcard10. The only signal triggering the switch into the clock stop mode/from the clock stop mode is the disappearance/restoration of the clock signal on the contact interface11, which is commonly managed by smartcard readers providing a low power mode. The device (the modem8in the example) receiving the smartcard10may thus be kept unchanged, its standard switching into low power mode being sufficient to trigger the clock stop mode of the smartcard10.