Single Interface Chip Controls Two Smart Cards – Design Note 289 Steven Martin Introduction There are considerable challenges to smart card interfacing, including various voltage levels (both input and output) and stringent fault handling requirements. To produce a robust card reading system, designers must comply with extensive and often difficult software as well as hardware standards. Furthermore, there are other complications like in-circuit ESD and pin-to-pin shorts to contend with. The LTC ®1955 dual smart card interface provides all of the required power management, control, ESD and fault detection circuitry for two smart cards. Employing a voltage doubling charge pump and two low dropout linear regulators, this device generates two independent levels of either 5V, 3V or 1.8V from a 2.7V to 5.5V input. Both channels have the required pins to support the EMV (Europay, MasterCard, Visa) and the ISO7816 smart card standards. One channel has extra control pins (smart card contact pad locations C4 and C8) to support existing memory cards. The entire chip is controlled by a microcontroller-friendly serial interface. Features The LTC1955 includes considerable security and functionality, yet remains easy to use. Two independent circuits detect the presence or absence of a smart card. Card insertion is debounced with a 40ms delay to ensure that the contacts are well seated before the L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 0.1μF 0.1μF 17 4 RXEN DREN VCC 16 21 45 47k 19 47k RESET 37 1 1k MOD B VDD VRH XIRQ 4 5 262k 180k FAULT 0.1μF 23 + Li-ION 12, 13 UNDERV DVCC 3 4.7μF VBATT VCC18 VCC3 VCCA RST LTC1348CG 36 1 MC68L11E9PB2 TD 2 7 3 8 DR1OUT DR1IN RX1IN RX1OUT 25 40 24 39 PD1 (TXD) IRQ PD0 (RXD) (MOSI) PD3 5 0.1μF FAULT LTC1955EUH 3 C8 4 C4 5 C7 6 RST A 7 CLK A 8 VCCA C2 C3 C1 5 C1+ C3 + 27 6 C1 – C3 – 26 2 C2 + 3 C2 C8A C4A 38 42 41 (MISO) PD2 43 (SCK) PD4 44 (SS) PD5 GND 0.1μF 24 2 DB9 RD RST LTC1728ES5-1.8 GND I/O A 27 DIN 28 DOUT 26 SCLK 25 LD 1μF CARD DETECT PB1 PB0 (IC3) PA0 PC0 24 8 9 1 28 31 I/O B ASYNC 32 SYNC 30 RIN 29 DATA RST B CLK B VCCB 1 0.1μF 28 15 VRL VSS MODA EXTAL 18 20 22 26 27 11 0.1μF C+ 14 CPO GND 15 NC/NO 9, 10 4.7μF 10M DN289 F01 8.000MHz 27pF 1μF 27pF Figure 1 Battery-Powered RS232 to Dual Smart Card Interface 07/02/289_conv C7 19 18 C2 C3 CARD B C1 17 0.1μF C5 C– XTAL 20 1μF PRES B GND 2 0.1μF (2MHz) E V– 0.1μF C5 PRES A – V+ CARD A 22 21 CARD DETECT card is activated. If a card is removed during a transaction, the LTC1955 automatically deactivates it before its pads leave the connector’s contact pins. Figure 3 shows the sequencing of the smart card pins during an automatic deactivation. current source is applied to the pin thereby accelerating its rise time. Once the pin reaches its local supply level the acceleration current is disabled. Figure 2 shows an example of the data waveforms on the smart card and microcontroller pins. Providing power to 5V cards from 3V, the charge pump operates in constant frequency mode when heavily loaded and has an autoburst feature for power savings under lightly loaded conditions. The constant frequency operation allows the use of tiny, low profile capacitors. The charge pump is powerful enough to supply both smart cards at rated current requirements. For the smart card clock pins, special clock divider and synchronization circuitry allows easy interfacing to the microcontroller. Separate clock input pins are available to support either asynchronous smart cards or synchronous memory cards. Internal low dropout linear regulators independently control the voltage of both smart cards. All three smart card classes (1.8V, 3V and 5V) are supported and the smart card signals are shifted to the appropriate level for the cards, independent of the microcontroller supply voltage (which can range from 1.7V to 5.5V). The data communication pins (I/OX and DATA) are bidirectional and full duplex. This feature allows true acknowledge data to be returned to the microcontroller interface. The bidirectional pins also have special accelerating pull-up sources* to ensure fast rise times (see Figure 2). These sources are faster than a resistor without dissipating excessive power when the pin is held low. They sense the edge rate on the pin and compare it to a preset limit. If the limit is exceeded, an additional Ease of Use Figure 1 shows an example of the LTC1955 used in a dual smart card to RS232 application powered by a single Li-Ion battery. A simple 4-wire command and status interface plus a 4-wire smart card communications interface are all that is required. The command/ status serial port can be easily daisy-chained and the smart card communications port can be paralleled to expand this application to four or more smart cards while maintaining the same number of wires to the microcontroller. Conclusion Requiring a minimum of external components and available in a small 5mm × 5mm × 0.75mm leadless package, the LTC1955 provides a compact, simple and cost effective solution to the difficult problems facing smart card system designers. RST 5V/DIV I/O A 2V/DIV CLK 5V/DIV I/O 5V/DIV DATA 2V/DIV VCC 5V/DIV 100ns/DIV DN289 F02 Figure 2. Bidirectional Pin Waveforms with Pull-Up Acceleration 10μs/DIV DN289 F03 Figure 3. Smart Card Deactivation Sequence *Patent pending Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn289f_conv LT/TP 0702 371.5K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2002