DN289 - Single Interface Chip Controls Two Smart Cards

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
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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
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