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AN663

Using KEELOQ® to Generate Hopping Passwords

Microchip Technology 

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)

produced by the KEELOQ HCS300 Encoder. The

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.

The KEELOQ HCS300 Encoder can be part of the don-

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.

FIGURE 7: KEYPAD/DONGLE SCHEMATIC

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

KEELOQ code hopping encoder is used together with a

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).

To PC keyboard socket

5123

5V

1 S0

2 S1

3 S2

4 S3

VDD 8

LED 7

PWM 6

VSS 5

HCS300

5V

5K6

GND

100 nF

1 VDD

VSS 8

2 LED

CLK 7

3 HCSIN DATA 6

4 NU SWITCH 5

PIC12C508

5K6

1K

24

4066

1

5V

5,13

3

GND

5123

From keyboard

ã 1997 Microchip Technology Inc.

DS00665A-page 5

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