NCN6000
http://onsemi.com
29
Bidirectional Level Shifter
The NCN6000 carries out the voltage difference between
the MPU and the Smart Card I/O signals. When the start
sequence is completed, and if no failures have been detected,
the device becomes essentially transparent for the data
transferred on the I/O line. To fulfill the ISO7816−3
specification, both sides of the I/O line have built in pulsed
circuitry to accelerate the signal rise transient. The I/O line is
connected on both side of the interface by a NMOS switch
which provide the level shifter and, due to its relative high
internal impedance, protects the Smart Card in the event of
data collision. Such a situation could occurs if either the MPU
of the smart card forces a signal in the opposite logic level
direction.
When the CS signal goes High, or if the MPU is running
any of the programming functions, the built in register holds
the previous state presents on the input I/O pin. This
mechanism is useful to force the CRD_IO card pin in either
a High or a Low pre−defined logic state. It is the responsibility
of the programmer to set up the I/O line according to the
system’s activity
Device Q4 provides a low impedance to ground when the
CRD_IO line is deactivated. This mechanism avoids noise
presence on this line during any of the power operation.
When either side of this level shifter is forced to Low, the
externally connected device will be forward biased by the DC
current flowing through the pull up resistors as depicted in
Figure 30. Since these two resistors will carry 350A max
each under the worst case conditions, care must be observed
to make sure the external device will be capable to handle this
level of current. Note: the typical series impedance of the
internal MOS device (Q3, Figure 30) is 400 .
The oscillograms in Figure 31 give the worst case operation
when the stray capacitance is 15 pF.
Q1 Q2
Q3
GND
V
bat
I/O
200 ns
20 k 20 k
CRD_IO
Q4
CRD_VCC
LOGIC
CARD ENABLE
Seq 1
Figure 30. Basic Internal I/O Level Shifter
200 ns
Figure 31. Typical CRD_IO Rise Time
I/O
CRD_IO
CRD_VCC = 5.0 V
Note: The I/O data depends solely upon the smart card ATR
content, the NCN6000 being not involved in these data.
Figure 32. Typical I/O and RST Signals During an ATR Sequence.
CRD_VCC
I/O Card
Answer
Request Sends on
CRD_RST Line
NCN6000
Chip Select
CRD_VCC = 3.0 V
