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KEELOQ

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 2001 Microchip Technology Inc. Preliminary DS00744A-page 1

AN744

OVERVIEW

This application note describes a KEELOQ code hop-

ping decoder implemented on a Microchip Mid-range

Enhanced FLASH MCU (PIC16F872). The software

has been designed as a group of independent modules

(standard C source files "C" ).

For clarity and ease of maintenance, each module cov-

ers a single function. Each module can be modified to

accommodate a different behavior, support a different

MCU, and/or a different set of peripherals (memories,

timers, etc.).

KEY FEATURES

The set of modules presented in this application note

implement the following features:

• Source compatible with HITECH and CCS C

compilers

• Pin out compatible with PICDEM-2 board

• Normal Learn mode

• Learn up to 8 transmitters, using the internal

EEPROM memory of PIC16F872

• Interrupt driven Radio Receive (PWM) routine

• Compatible with all existing K

EELOQ hopping code

encoders with PWM transmission format

selected, operating in "slow mode" (T

E = 400 µs)

• Automatic synchronization during receive, using a

4 MHz RC oscillator

FIGURE 1: DECODER PIN OUT

TABLE 1: FUNCTIONAL INPUTS AND

OUTPUTS

Notice:

This is a non-restricted version of Application Note AN745 which is available under the K

EELOQ License

Agreement. The license agreement can be ordered from the Microchip Literature Center as DS40149.

Author: Lucio Di Jasio

Microchip Technology Inc.

Name

Number

Input/

Output

Function

RFIN 3 I Demodulated

PWM signal from

RF receiver

LEARN 6 I Input to enter learn

mode

LEARN-

OUT

25 O Output to show the

status of the learn

process

OUT0..3 21,22,2

3, 24

O Function outputs,

correspond to

encoder input pin

LOW 26 O Low Battery indica-

tor, as transmitted

by the encoder

VDD 20 PWR 5V power supply

VSS 19, 8 GND Common ground

Note: All NU pins are tristate

MCLR

LRNOUT

LEARN

OSCIN

LOW

LEARNOUT

OUT3

OUT2

OUT1

OUT0

OSCOUT

Modular Mid-Range PICmicro

KEELOQ

Decoder in C

 2001 Microchip Technology Inc. Preliminary DS00744A-page 3

AN744

RECEIVER MODULE

The receiver module has been developed around a fast

and independent Interrupt Service Routine (ISR). The

whole receiving routine is implemented as a simple

state machine that operates on a fixed time base. This

can be used to produce a number of virtual timers. The

operation of this routine is completely transparent to

the main program and similar to a UART. In fact, the

interrupt routine consumes only 30% of the computa-

tional power of the MCU working in the background .

After a complete code-word of 66 bits has been prop-

erly received and stored in a 9 bytes buffer, a status flag

(

RF_FULL) is set and the receiver becomes idle.

It is the responsibility of the main program to make use

of the data in the buffer and to clear the flag to enable

the receiving of a new code-word.

In order to be compatible with all K

EELOQ encoders,

with or without oscillator tuning capabilities, the

receiver routine constantly attempts to resynchronize

with the first rising edge of every bit in the incoming

code-word. This allows the decoder to operate from an

inexpensive (uncalibrated) RC clock. In doing so, the

last rising edge/bit of every code-word is lost (resulting

in an effective receive buffer capacity of 65-bit).

For HCS20X and HCS30X encoders this implies that

the REPEAT bit (being the 66th) cannot be captured.

While for Advanced Encoders like the HCS36X or

HCS4XX, the reader can easily modify the definition of

the constant

BIT_NUM to 68 to receive all bits trans-

mitted with exception of the last queue bit Q1 (being the

69th), again rarely used.

The only resource/peripheral used by this routine is

Timer0 and the associated Overflow Interrupt. This is

available on every mid-range PICmicro microcontroller.

Timer0 is reloaded on overflow, creating a time base (of

about 1/3 T

E = 138 µs). The same interrupt service rou-

tine also provides a virtual 16-bit timer, derived from the

same base period, called

XTMR.

FIGURE 3: CODE-WORD TRANSMISSION FORMAT

FIGURE 4: CODE-WORD ORGANIZATION

LOGIC ‘0’

LOGIC ‘1’

Bit

Period

Preamble

Header

Encrypted Portion

of Transmission

Fixed Portion of

Transmission

Guard

Time

THOP

TFIX

16-bit

Discrimination

bits

(10 bits)

Overflow

bits

(2 bits)

Button

Status

(4 bits)

Transmission Direction

32 bits of Encrypted Data

Encrypted using

LOCK CIPHER

Algorithm

Encrypted Code Data

Sync Value

28-bit Serial Number

Button

Status

(4 bits)

VLOW and

Repeat Status

(2 bits)

Serial Number and Button

2 bits

of Status

Fixed Code Data

Status (32 bits)

AN744

DS00744A-page 4 Preliminary  2001 Microchip Technology Inc.

Since the radio input is polled (for 1 µs) on multiples of

the base period (138 µs), the chance of a glitch (short

noise pulse) disturbing the receiver is reduced.

Further, since the time base produced is constant, the

same interrupt service routine could easily be extended

to implement a second UART as a separate state

machine for full duplex asynchronous communication

up to 1,200 baud at 4 MHz.

Note: This would also require the main oscillator

to be crystal based.

Other implementations of the same receiver module

can be obtained using other peripherals and detection

techniques. These include:

• Using the INT pin and selectable edge interrupt

source

• Using the Timer1 and CCP module in capture

mode

• Using comparator inputs interrupt

All of these techniques pose different constraints on the

pin out, or the PICmicro MCU, that can be used. This

would lead to different performances in terms of

achievable immunity from noise and or CPU overhead,

etc.

FAST DECRYPTION MODULE

This module contains an implementation of the KEELOQ

decryption algorithm that has been optimized for speed

on a mid-range PICmicro microcontroller. It allows fast

decryption times for maximum responsiveness of the

system even at 4 MHz clock.

The decryption function is also used in all learning

schemes and represents the fundamental building

block of all K

EELOQ decoders.

KEY GENERATION MODULE

This module shows a simple and linear implementation

of the Normal Learn Key Generation .

This module uses the KEELOQ Decrypt routine from the

Fast Decryption module to generate the key at every

received code-word instead of generating it during the

learn phase and storing it in memory. The advantage is

a smaller Transmitter Record of 8 bytes instead of 16

bytes (see Table 2). This translates in a double number

of transmitters that can be learned using the 64 byte

internal EEPROM available inside the PIC16F872.

This space reduction comes at the expense of more

computational power required to process every code-

word. When a new code-word is received, the key gen-

eration algorithm is applied (Normal Learn) and the

resulting Description key is placed in the array

DKEY[

0..7]. During a continous transmission (the

user is holding the button on the transmitter), the key

generation is not repeated, to save time, the last com-

puted Decryption Key value is used safely instead (the

serial number being the same).

Due to double buffering of the receiver and the

PICmicro MCU execution speed and efficiency (even

running at 4 MHz only), it is possible to receive and

decrypt, at the same time, each and every incoming

code-word.

For an overview of some of the different security levels

that can be obtained through the use of different key

generation/management schemes, refer to the "Secure

Data Products Handbook" [DS40168] (Section 1,

EELOQ Comparison Chart, Security Level Summary).

A detailed description of the Normal Learn key gener-

ation scheme can be found in Technical Brief TB003

"An Introduction To K

EELOQ Code Hopping"

[DS91002].

More advanced Key Generation Schemes can be

implemented replacing this module with the techniques

described in Technical Brief TB001 "Secure Learning

RKE Systems Using K

EELOQ Encoders" [DS91000].

TABLE MODULE

One of the major tasks of a decoder is to properly main-

tain a database that contains all the unique ID’s (serial

numbers) of the learned transmitters. In most cases,

the database can be as simple as a single table, which

associates those serial numbers to the synchronization

counters (that are at the heart of the hopping code

technology).

This module implements the easiest of all methods, a

simple "linear list" of records.

Each transmitter learned is assigned a record of 8

bytes (shown in Table 2), where all the relevant infor-

mation is stored and regularly updated.

TABLE 2: TRANSMITTER RECORD

Offset Data Description

FCODE

Function code (4 bits) and

upper 4 Serial Number bits

[24..28]

IDLo

Serial Number bits [0..7]

IDHi

Serial Number bits [8..15]

IDMi

Serial Number bits [16..23]

SYNCH

Sync Counter 8 MSB

SYNCL

Sync Counter 8 LSB

SYNCH2

Second copy of SYNCH

SYNCL2

Second copy of SYNCL

 2001 Microchip Technology Inc. Preliminary DS00744A-page 5

AN744

The 16-bit synchronization counter value is stored in

memory twice because it is the most valuable piece of

information in this record. It is continuously updated at

every button press on the remote. When reading the

two stored synchronous values, the decoder should

verify that the two copies match. If not, it can adopt any

safe resync or disable technique required depending

on the desired system security level .

The current implementation limits the maximum num-

ber of transmitters that can be learned to eight. This is

due to the size of the internal EEPROM of the

PIC16F872.

This number can be changed to accommodate different

PICmicro models and memory sizes by modifying the

value of the constant

MAX_USER.

The simple "linear list" method employed can be scaled

up to some tens of users. But due to its simplicity, the

time required to recognize a learned transmitter grows

linearly with the length of the table.

It is possible to reach table sizes of thousands of trans-

mitters by replacing this module with another module

that implements a more sophisticated data structure

like a “Hash Table” or other indexing algorithms.

Again due to the simplicity of the current solution, it is

not possible to selectively delete a transmitter from

memory. The only delete function available is a Bulk

Erase (complete erase of all the memory contents) that

happens when the user presses the Learn button for up

to 10 seconds. (The LED will switch off. At the release

of the button, it will flash once to acknowledge the

delete command). To allow for selective transmitter

removal from memory, more sophisticated techniques

will be analyzed in future application notes, by simply

replacing/updating this module.

MEM-87X MODULE

This module is optimized to drive the internal EEPROM

of the PIC16F87X device.

The module make the memory generically accessible

by means of two routines RDword and WRword that

respectively read and write a 16-bit value out of an

even address specified in parameter IND.

Replacing this module with the appropriate drivers,

(and adapting the pin out) make possible the use of any

kind of nonvolatile memory. This includes internal and

external serial EEPROMs (Microwire

, SPI

™

or I

™

bus) of any size up to 64 Kbytes.

THE MAIN PROGRAM

The main program is reduced to a few pages of code.

The behavior is designed to mimic the basic behavior

of the HCS512 integrated decoder, although just the

parallel output is provided (no serial interface).

Most of the time, the main loop goes idle waiting for the

receiver to complete reception a full code-word.

Double buffering of the receiver is done in RAM, in

order to immediately re-enable the reception of new

codes and increase responsiveness and perceived

range.

CONCLUSION

The C language source increases the readability of the

program structure and eases the maintenance. This

benefit has come at the cost of the program size. That

in terms of memory words, has considerably increased

over the equivalent code written in assembly (more

than 30% larger).

Selecting a FLASH PICmicro microcontroller from the

mid-range family as the target MCU allows us to make

the code simpler and cleaner. It also provides larger

RAM memory space and a deeper hardware stack.

Interrupts have been used to "virtualize" the receiving

routine as a software peripheral and to free the design

of the hard real time constraint that it usually poses.

Still, many of the resources available on the

PIC16F872 are left unused and available to the

designer. These include:

• Timer1, a 16-bit timer

• Timer1 oscillator, a low power oscillator for real

time clock

• CCP module, capable of capture, compare and

PWM generation

• Timer2, an 8-bit timer, with auto reload

• 10-bit A/D converter with a 5 channel input

multiplexer

We resisted introducing extra features and optimiza-

tions in favor of clarity. For example:

• Speed optimizations and code compacting

• More complex key generation schemes

• Multiple manufacturer codes

• Co-processor functionality

• Advanced user entry and deletion commands

• Large memory tables (up to 8,000 users)

• Serial interface to PDAs and/or terminals for

memory management and logging

These are left as exercises to the advanced reader/

designer or as suggestions for further application

notes.

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