ã 1997 Microchip Technology Inc.

DS00665A-page 5

AN665

Proposing a Demo Keypad/Dongle

Implementation

The password generator fits between the keyboard and

the PC. A 5-pin plug connects to the PC, supplying

power to our device, and the keyboard plugs into the

5-pin socket (Figure 7). The clock and data lines are

passed between the PC and keyboard, allowing normal

keyboard operations. When S1 is activated, the

PIC12C508 receives the new message (16 hex digits)

PIC12C508 will then emulate the keyboard, sending

the appropriate sequence of key press and key release

messages to the PC. To prevent the keyboard from

interpreting this transmission as a ‘request to send’

from the PC, it is necessary to isolate the keyboard

from the clock line and data line during the transmis-

sion.

gle or can be removable, like a key, in order to allow dif-

ferent encoders with different encryption keys or serial

numbers to be easily exchanged.

Power consumption has to be the lowest possible in

order not to excessively load the line. Size and compo-

nent count should also be kept to the possible minimum

in order to allow for a very small package. Ideally, the

whole circuit should fit into a small gap between the two

connectors.

In the implementation we are proposing, an HCS300

PIC12C508 microcontroller.For simplicity, a standard

CMOS quadruple switch (4066) is used to alternatively

connect the dongle or the keyboard to the PC line. The

HCS300 and the PIC12C508 (both available in 8-pin

DIP or SOIC packages), draw extremely low currents

as well as internally produce the clock required to oper-

ate the dongle. Beside a couple of pull-up resistors

required for the clock line and data line, no other com-

ponents are required to obtain a fully functional hop-

ping password dongle (Figure 7).

FIGURE 7: KEYPAD/DONGLE SCHEMATIC

S0

S1

S2

S3

LED

PWM

LED

HCSIN

NU

CLK

DATA

SWITCH

HCS300

5V

1

2

3

4

8

7

6

5

1

2

3

4

8

7

6

5

100 nF

1K

5

1

2

3

To PC keyboard socket

24

13

GND

5V

5,13

5K6

5K6

GND

5V

PIC12C508

4066

From keyboard

5

1

2

3