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Part Number AN652
Manufacturers Microchip
Logo Microchip
Description Code Hopping Decoder Using Secure Learn
Datasheet AN652 DatasheetAN652 Datasheet (PDF)

AN652 Code Hopping Decoder Using Secure Learn Author: Steven Dawson Standard Microcontroller and ASSP Division 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 OVERVIEW This application note fully describes the working of a code hopping decoder implemented on a Microchip PIC16C56 microcontroller..

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AN652 Code Hopping Decoder Using Secure Learn Author: Steven Dawson Standard Microcontroller and ASSP Division 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 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 various KEELOQ code hopping encoders that can be used with the decoder, the decoder hardware, and the various software modules comprising the system. The software 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 hopping encoders. FIGURE 1: LEARN INIT LEARN IND Vcc MCLR GND S0 S1 S2 S3 PIC16C56 KEELOQ DECODER 1 2 3 PIC16C56 Secure Learn Decoder 4 5 6 7 8 9 18 17 16 15 14 13 12 11 10 RFIN NC OSC OUT OSC IN Vcc BUT LRNT EE CS EE CK 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. DS00652B_C-page 1 This document was created with FrameMaker 4 0 4 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 activated 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. 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 transmission is being sent, and whether the battery voltage is low. The HCS200 does not send repeated transmission 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. Additionally, the HCS360 and HCS361 encoders can extend the length of the serial number from 28 bits to 32 bits, replacing the unencrypted function code. Hopping Code The hopping code contains function information, a discrimination 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 transmission and can not be predicted. 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 having 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 Microchip HCSXXX encoders Function Information The encoder transmits up to four bits of function information. 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 transmitted string before encryption, the same information can be used at the decoder to check whether the information 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.


FLM3135-8F AN652 NLV32T-056J-PF


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