AN665

DS00665A-page 6

ã 1997 Microchip Technology Inc.

Software Implementation

The software is composed of three short code

segments:

• Keyboard Emulation Routines

• Main loop routine.

Receive Routine for the HCS300 Encoder

(RECEIVE)

The RECEIVE routine gathers the first 64 bits transmit-

ted by the HCS300, ignoring the last two bits (repeat

and battery status) as they carry no useful information

for this application, and packs them into a 8-bytes

buffer (Buffer0…Buffer7).

Keyboard Emulation Routines

(Sendbit, SendKey)

These implement the transmission of the key scan

codes according to the IBM-PC/AT keyboard protocol.

Main Loop

While the CMOS switch connects the PC to the key-

board clock and data lines, the LED output line is con-

tinuously polled to detect the activation of the HCS300

Note:

Any combination of its four input lines after

debouncing activates the encoder.

When the LED line goes low, the CMOS switch is acti-

vated to isolate the clock and data lines from the key-

board. The RECEIVE routine is called.

Upon successfully receiving a transmission, a loop is

entered where 16 hex digits from the receive buffer are

transmitted as a sequence of key press and key

release messages, separated by appropriate delays

repetitively calling the SENDKEY subroutine.

The software has been developed in the simplest pos-

sible form and, therefore, is open to a number of opti-

mizations. For example:

• The PIC12C508 could be put to “sleep” to further

reduce power consumption.

• The encoder could be removable and its pres-

ence/activation should be properly detected.

• For simplicity, the presented RECEIVE routine

requires a 400

ms transmission speed being con-

figured in the encoder, while a more flexible multi-

baud rate routine can be used as presented in

various other application notes.

• A second code word could be compared with the

first code word received to recognize transmission

errors (although highly improbable when the

encoder is wired to the PIC12508), since there is

no decryption, there is no other means to tell that

the transmission has not been corrupted.

Decoding Options

A code hopping password can be used to validate

access to a wide variety of electronic services provid-

ing the recipient application (typically, it will be some

software running on a server) is capable of following

some simple decryption and verification steps. The

fixed unencrypted part of the code (last 8 digits) can be

used to identify the user (7 digits) and the function acti-

vated on the encoder (1 out of 15, corresponding to the

last digit).

The hopping part has to be decrypted using the appro-

priate 64-bit decryption key. Depending on the desired

level of security, many different key generation and

management techniques can be adopted. For exam-

ple, the key could be deduced by the User ID and a

Manufacturer’s Key, by the “seed” code of the encoder

or could simply be a fixed 64-bit constant.

Learning techniques can also be applied so that the

application

actually

autonomously

acquires

the

required keys and builds a database of users, ID codes,

and decryption keys. For a further analysis, consult the

following literature:

.

AN645

PIC16C57 Based Code Hopping Secu-

rity System (DS00645)

AN662

Secure Learn (DS00662)

AN663

(DS00663)

TB001

An Introduction to KeeLoq Code Hopping

(DS91000A)

TB003

(DS91002A).