LTC1756 查看數據表（PDF） - Linear Technology

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LTC1756

Smart Card Interface

Linear Technology 

LTC1755/LTC1756

APPLICATIO S I FOR ATIO

released by comparing its slew rate with an internal

reference value. If a valid transition is detected, a large

pull-up current enhances the edge rate on the node. The

higher slew rate corroborates the decision to charge the

node thereby effecting a dynamic form of hysteresis. Once

the node has reached the power supply voltage the internal

comparator requires several hundred nanoseconds to

reset. Pulling down on the pin before the reset delay

expires will result in a momentary contention and a higher

current flow. Therefore, the comparator delay sets the

upper limit on the maximum data rate of the bidirectional

channels to about 500kHz.

The dynamic pull-up current sources are designed to

trigger with as much as 50pF of capacitive load on the

bidirectional pins. At approximately 90pF (or greater), the

edge rate on the node will be insufficient to trigger the edge

rate detector and the node will only ramp up at a rate given

by the ISTART current source and the load capacitance. In

these instances the edge rate of the bidirectional pin may

not meet the requirements of existing smart card stan-

dards. Therefore, it is recommended that the sum of both

explicit and parasitic capacitances on the bidirectional

pins be kept below 50pF.

If excessive capacitance (either explicit or parasitic) is

present on the bidirectional pins, the starting pull-up

current must also be increased. This can be accomplished

with a pull-up resistor to the respective supply. For the

smart card side (I/O, AUX1 and AUX2), the pull-up resis-

tor should be connected to VCC. For the microcontroller

side (DATA, AUX1IN and AUX2IN), the pull-up resistor

should be connected to DVCC on the LTC1755 (VIN on the

LTC1756). To maintain an edge rate of approximately

5V/µs, the following expression for RPULL-UP should be

applied:

( )( ) RPULL−UP =

VSUPPLY – 1V

CPAR – 50pF 5 • 106

where CPAR is the extra capacitance on the bidirectional

pin and VSUPPLY is the minimum local supply for the

bidirectional pin. For example, on the smart card side, 3V

should be assumed for VSUPPLY.

Note that the addition of a pull-up resistor will give a higher

output voltage when the bidirectional pin pulls down. Care

should be taken so that the VIL or VOL specifications are not

compromised with this technique.

Bidirectional Channels

As described in Pin Functions (Pins 10/6), the bidirec-

tional channels allow transmission in only one direction at

a time. Figure 2 shows a simplified block diagram of one

of the three bidirectional channels. The three channels

operate in an identical fashion.

Figure 3 shows an example of normal transmit and receive

operations as well as the two possible collision scenarios.

If a channel is activated from one direction and an L is

imposed in the other direction before both sides return H

a collision results. The result of the collision is that the

receiving side (Slave Side) will remain low until it is

released, but the transmitting side (first side to go low or

Master Side), will be allowed to return high if released. The

colliding L externally imposed on the slave side will not be

transmitted back through the channel.

12

CS

DVCC

CHARGE

PUMP

READY

VCC

DATA

TO

MICROCONTROLLER

BIDIRECTIONAL

LATCH

I/O

TO

SMART CARD

3.5mA

17756 F02

Figure 2. Bidirectional Channel Simplified Block Diagram

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