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  Microchip Technology Semiconductor Electronic Components Datasheet  

AN652 Datasheet

Code Hopping Decoder Using Secure Learn

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AN652 pdf
AN652
Code Hopping Decoder Using Secure Learn
Author: Steven Dawson
Standard Microcontroller and
ASSP Division
OVERVIEW
This application note fully describes the working of a
code hopping decoder implemented on a Microchip
PIC16C56 microcontroller. This decoder uses the
secure learn (seed-based) method of learning new
transmitters. This application note describes the vari-
ous KEELOQ code hopping encoders that can be used
with the decoder, the decoder hardware, and the vari-
ous software modules comprising the system. The soft-
ware can be used to implement a stand alone decoder
or integrate with full function security systems. The
decoder supports the Microchip HCS200, HCS300,
HCS301, HCS360, and HCS361 KEELOQ code hop-
ping encoders.
KEY FEATURES
• Stand alone decoder
• Compatible with Microchip HCS200, HCS300,
HCS301, HCS360, and HCS361 encoders
• Automatic bit rate detection
• Automatic encoder type detection
• Four function outputs
• Six learnable transmitters
• RC Oscillator
FIGURE 1: PIC16C56 KEELOQ DECODER
LEARN INIT
LEARN IND
Vcc
MCLR
GND
S0
S1
S2
S3
1
2
3
4
5
6
7
8
9
18 RFIN
17 NC
16 OSC OUT
15 OSC IN
14 Vcc
13 BUT LRNT
12 EE CS
11 EE CK
10 EE DIO
THE INFORMATION CONTAINED IN THIS DOCUMENT IS PROPRIETARY AND
CONFIDENTIAL INFORMATION OF MICROCHIP TECHNOLOGY INC. THEREFORE,
ALL PARTIES ARE REQUIRED TO ENTER INTO A NONDISCLOSURE AGREEMENT
BEFORE RECEIVING THIS DOCUMENT.
KEELOQ is a registered trademark of Microchip Technology Inc.
© 1996 Microchip Technology Inc.
This document was created with FrameMaker 4 0 4
DS00652B_C-page 1


  Microchip Technology Semiconductor Electronic Components Datasheet  

AN652 Datasheet

Code Hopping Decoder Using Secure Learn

No Preview Available !

AN652 pdf
AN652
INTRODUCTION TO KEELOQ
ENCODERS
All KEELOQ encoders use the KEELOQ code hopping
technology to make each transmission by an encoder
unique. The encoder transmissions have two parts.
The first part changes each time the encoder is acti-
vated and is called the hopping code part and is
encrypted. The second part is the unencrypted part of
the transmission, principally containing the encoder’s
serial number identifying it to a decoder. Refer to
DS91002, Introduction to KEELOQ.
Hopping Code
The hopping code contains function information, a dis-
crimination value, and a synchronization counter. This
information is encrypted by an encryption algorithm
before being transmitted. A 64-bit encryption key is
used by the encryption algorithm. If one bit in the data
that is encrypted changes, the result is that an average
of half the bits in the output will change. As a result, the
hopping code changes dramatically for each transmis-
sion and can not be predicted.
Function Information
The encoder transmits up to four bits of function infor-
mation. Up to 15 different functions are available.
Discrimination Value
Stored in the encoder EEPROM, this information can
be used to check integrity of decryption operation by a
decoder. If known information is inserted into the trans-
mitted string before encryption, the same information
can be used at the decoder to check whether the infor-
mation has been decrypted correctly. 12 bits (including
overflow bits) are available in the Microchip HCSXXX
encoders.
Synchronization Counters
The transmitted word contains a 16-bit synchronization
counter. The synchronization information is used at the
decoder to determine whether a transmission is valid or
is a repetition of a previous transmission. Previous
codes are rejected to safeguard against code grabbers.
The HSC300 and HCS301 encoders transmit two over-
flow bits which may be used to extend the range of the
synchronization counter from 65,536 to 196,608 button
operations. The HCS360 and HCS361 encoders trans-
mit one overflow bit which can be used to extend the
range of the synchronization counter from 65,536 to
131,071 button operations.
Unencrypted Code
Serial Number
The encoder’s serial number is transmitted every time
the button is pressed. The serial number is transmitted
unencrypted as part of the transmission and serves to
identify the encoder to the decoder.
Other Status and Function Information
The HCS200, HCS300, and HCS301 encoders include
provision for four bits of function information and two
status bits in the fixed code portion of its transmission.
The two status bits indicate whether a repeated trans-
mission is being sent, and whether the battery voltage
is low. The HCS200 does not send repeated transmis-
sion information, and the bit is permanently set to ‘0’.
The HCS360/361 encoders transmit two bits that are
used as a Cyclic Redundancy check. These bits can be
used to check the integrity of the reception. Addition-
ally, the HCS360 and HCS361 encoders can extend
the length of the serial number from 28 bits to 32 bits,
replacing the unencrypted function code.
Seed Transmissions
The Microchip HCSXXX encoders all have the ability to
transmit a fixed seed. The seed value is programmed
into the encoder when the encoder is first initialized
along with the counters, key, serial number, and other
information. The seed length differs from encoder to
encoder with the HCS200, HCS300, and HCS301 hav-
ing a 32-bit seed. The HCS360 and HCS361 encoders
have a 48-bit seed. The HCS200, HCS300, and
HCS301 encoders transmit the seed if all the inputs are
activated simultaneously (S0, S1, S2, and S3). The
HCS360 and HCS361 encoders transmit the seed
immediately if S0 and S3 are activated or delayed if S0
and S1 are activated for more than 3 seconds.
Table 1 summarizes seed transmissions in the Micro-
chip HCSXXX encoders
DS00652B_C-page 2
© 1996 Microchip Technology Inc.


Part Number AN652
Description Code Hopping Decoder Using Secure Learn
Maker Microchip
Total Page 30 Pages
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