LTC1955 Linear Technology, LTC1955 Datasheet - Page 13

• Manufacturer Part Number: LTC1955
• Description: Dual Smart Card Interface
• Manufacturer: Linear Technology

OPERATIO
Electrical Fault Detection
Several types of faults are detected by the LTC1955. They
include V
CLK A/CLK B, RST A/RST B, C8A, C4A short circuit, card
removal during a transaction, failed answer to reset (ATR),
supply undervoltage or UNDERV and chip overtemperature.
To prevent false errors from plaguing the microcontroller,
the electrical faults are acted upon only after a 5 s (min)
timeout period. Card removal during transaction faults
initiate the deactivation sequence immediately.
V
paring the actual output voltage with the internal reference
voltage. If the output is more than ~5% below its set point
for the entire timeout period, the fault is reported and the
deactivation sequence is initiated.
V
the output current of the LDOs with an internal reference
level. If the current of an LDO is more than 100mA (typ) for
the entire timeout period, the fault is reported and the
deactivation sequence is initiated.
CLK A/CLK B and RST A/RST B faults are detected by
comparing the outputs of these pins with their expected
signals. If the signal on a pin is incorrect for the entire
timeout period, the fault is reported and the deactivation
sequence is initiated.
The clock channels are a special case. Since they can have
a free running clock, the error indication is accumulated
over a longer period of time without being cleared. Even
though the clock may be running, an error will still be
detected.
An overtemperature fault is detected by sensing the junc-
tion temperature of the IC. If the junction temperature
exceeds approximately 150 C for the entire timeout
period, the fault is reported by setting both fault bits (D4
and D12) and the deactivation sequence is initiated.
A card removal fault is determined as soon as the PRES A/
PRES B pin is high (for NC/NO = 0). Once this occurs the
fault is reported and the deactivation sequence is initiated.
If no card is present, and the application software attempts
to power up a card socket, an automatic fault will result on
that channel.
CCA
CCA
www.DataSheet4U.com
/V
/V
CCB
CCB
CCA
overcurrent faults are detected by comparing
under voltage faults are determined by com-
/V
CCB
U
undervoltage, V
CCA
/V
CCB
overcurrent,
Short circuits on the I/O A/I/O B lines will not be detected
by the fault detection hardware; however, a short circuit
from these lines to their respective V
compliant with the maximum current limits set by appli-
cable standards (<15mA).
Answer to Reset (ATR) Fault Detection
Answer to Reset faults are detected by an internal counter
that is started once the RST A/B line goes high. If the DATA
pin remains high for 40,000 clock cycles, the ATR fault bit
for a given channel is set in the serial port’s status register
(see Table 1) and the FAULT pin is brought low.
An ATR fault can not occur if the clock mode of a channel
is set to synchronous. ATR faults will only occur for
asynchronous smart cards.
ATR faults are cleared by bringing the RST A/B pin low for
the faulted channel. This will also clear the FAULT pin to
the Hi-Z state (assuming no other errors are causing
FAULT to be low).
An ATR fault will not automatically deactivate a card
channel. It is the application programmer’s responsibility
to check the status register for ATR faults and deactivate
the smart card channel in accordance with smart card
standards. Generally the application has 50ms (EMV
2.1.3.1, 2.1.3.2) from the 40,000th clock pulse to deacti-
vate the card. Once the LTC1955 receives the deactivation
command, it will shut down a card channel in less than
250 s.
Using the FAULT Pin
The FAULT pin can be used as an interrupt to a
microcontroller. It is an open-drain output and generally
requires a pull-up resistor. The FAULT pin will go low when
either an electrical fault or an answer to reset fault occurs
on either channel. Thus there are four possible faults that
can cause it to indicate a problem. The serial port’s status
register must be polled to find out what type of fault
occured and on which channel. The FAULT pin is logically
equivalent to D4+D5+D12+D13 (see Table 1).
CCA
LTC1955
/V
CCB
pins will be
sn1955 1955fs
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