Datasheet

LTC1955
9
1955fd
For more information www.linear.com/LTC1955
operaTion
Serial Port
The microcontroller compatible serial port provides all
of the command and control inputs for the LTC1955, as
well as the status of the two smart cards. Data on the D
IN
input is loaded on the rising edge of SCLK. D15 is loaded
first and D0 last. At the same time, the command bits are
being shifted into the D
IN
input, the status bits are being
shifted out of the D
OUT
output. The status bits are presented
to D
OUT
on the rising edge of SCLK. Once all bits have
been clocked into the shift register, the command data is
loaded into the command latch by bringing LD low. At this
time, the command latch is updated and the LTC1955 will
begin to act on the new command set. The status data
is latched into the shift register on the rising edge of LD.
SCLK should be low when LD is brought low and should
be high when LD is brought high. This requires a 9th clock
cycle per transaction. Figure 1 shows the recommended
operation of the serial port.
Multiple LTC1955s may be daisychained together by
connecting the D
OUT
pin of one LTC1955 to the D
IN
pin of
another. Figure 6 shows an example of multiple LTC1955s
daisychained together.
The maximum clock rate for the serial port is 10MHz.
The serial port controls the following parameters of each
smart card socket:
• Selection/deselection of a smart card
• V
CC
voltage level of each card (5V/3V/1.8V/0V)
• Clock mode of each card (synchronous or asynchronous)
• Operating mode of asynchronous cards (clock stop
high, low, ÷1, ÷2, ÷4 or ÷8)
• Selection of the I/O, C4 or C8 pins for card socket A
The serial port provides the following status data:
• It indicates the presence or absence of the smart
cards.
• It indicates the readiness of the smart card V
CC
supplies.
Communication with a smart card is disabled until its
power supply voltage has reached the final value.
• It indicates fault status. In the event of an electrical or
ATR fault, the fault is reported. For electrical faults, the
LTC1955 will automatically deactivate the smart card.
Table 1 illustrates the command inputs and status outputs
associated with each bit of the serial data word.
Three voltage options are available from the LTC1955: 5V,
3V and 1.8V. Bits D0, D1 (card B) and D8, D9 (card A)
determine which voltage is selected. Setting both control
bits of a channel to 0 deactivates that channel and sets
the smart card supply voltage to 0V. If both channels are
deactivated, the LTC1955 is in shutdown. Table 2 shows
the operation of the supply control bits.
The CLK A/CLK B pins to the smart cards can be pro
-
grammed for various modes. Both synchronous and asyn-
chronous cards are supported. There are several options
available with asynchronous cards. T
able 3 shows how
all clock options are obtained using bits D5–D7 (card B)
and D13–D15 (card A). The default state of the LTC1955
on power up is synchronous mode.
D
IN
SCLK
LD
XD0D15X D1
D
OUT
D15 FROM
INPUT
D0 D15
1955 F01
D14 D5D15 D1
t
LC
t
DH
t
DD
t
DS
t
H
t
L
t
CL
t
LFC
t
CL
READ/WRITE CYCLE
D14 D2
Figure 1. Serial Port Timing Diagram