LTC1955 Linear Technology, LTC1955 Datasheet - Page 12

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

LTC1955
OPERATIO
Smart Card Detection Circuits
The PRES A/PRES B pins are used to detect the presence
of a smart card. An automatic debounce circuit waits until
a smart card has been present for a continuous period of
typically 35ms. Once a valid card indication exists, the
status bit for that channel is updated and may be polled by
cycling data through the serial port. The D
lent to D15) of the serial port can be used to indicate the
presence of a card on channel A in real time if LD is held
low.
The PRES A/PRES B pins have built-in pull-up current
sources so no external components are required for
switch detection. The pull-up current sources are de-
signed to have a small current when the pin voltage is
below approximately 1V but somewhat higher current
when the pin voltage reaches 1V. This helps maintain low
power dissipation when a card is present and yet fast
response time to a card removal.
The PRES A/PRES B pins can be configured to respond to
either normally open or normally closed switches via the
NC/NO pin.
Activation/Deactivation
For maximum flexibility, the activation sequencing of the
smart card is left to the application programmer. Upon
activation, to comply with relevant smart card standards,
none of the smart card signal pins will be allowed to go
high before the smart card supply voltage (V
reached its final value. Deactivation can be achieved either
manually or automatically. An electrical fault condition will
trigger the automatic deactivation.
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U
OUT
CCA
pin (equiva-
/V
CCB
) has
Manual deactivation may be performed under software
control by setting the smart card pins to 0V in the desired
sequence via the control pins (SYNC, ASYNC, R
and the serial port). For most applications this will be
cumbersome and the built-in deactivation will be used
instead.
Automatic Deactivation
The built-in deactivation sequence can be executed via the
serial port simply by setting the appropriate control bits
(D0 and D1 or D8 and D9) to 0. The deactivation sequence
is outlined below.
1. The RST A/RST B pin for that channel is immediately
2. The deactivation of the CLK A/CLK B pins depends upon
3. The I/O A/I/O B, C4A and C8A pins for that channel are
4. The V
If an error occurs on one smart card, operation of the other
card is unaffected.
brought low.
which type of card is used:
If the smart card was set to asynchronous mode then
the CLK A/CLK B pin will be latched low on its next falling
edge. If no falling edges occur within 5 s (min) then the
CLK A/CLK B line is forced low.
If the smart card was set to synchronous mode then the
CLK A/CLK B pin is immediately latched at its current
value (either high or low) and then forced low after a
duration of 5 s (min). During the 5 s timeout period
changes on SYNC will be ignored.
brought low.
CCA
/V
CCB
pin is brought low.
IN
sn1955 1955fs
, DATA