HCS301 Datasheet, PDF (2/20 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

HCS301
The HCS301 operates over a wide voltage range of
3.5 volts to 13.0 volts and has four button inputs in an
8-pin configuration. This allows the system designer the
freedom to utilize up to 15 functions. The only
components required for device operation are the but-
tons and RF circuitry, allowing a very low system cost.
1.0 SYSTEM OVERVIEW
Key Terms
• Manufacturer’s code – a 64-bit word, unique to
each manufacturer, used to produce a unique
encryption key in each transmitter (encoder).
• Encryption Key – a unique 64-bit key generated
and programmed into the encoder during the
manufacturing process. The encryption key
controls the encryption algorithm and is stored in
EEPROM on the encoder device.
1.1 Learn
The HCS product family facilitates several learn strate-
gies to be implemented on the decoder. The following
are examples of what can be done. It must be pointed
out that there exists some third-party patents on learn-
ing strategies and implementation.
The HCS301 is a code hopping encoder device that is
designed specifically for keyless entry systems,
primarily for vehicles and home garage door openers. It
is meant to be a cost-effective, yet secure solution to
such systems. The encoder portion of a keyless entry
system is meant to be held by the user and operated to
gain access to a vehicle or restricted area. The
HCS301 requires very few external components
(Figure 2-1).
1.1.1 NORMAL LEARN
The receiver uses the same information that is transmit-
ted during normal operation to derive the transmitter’s
secret key, decrypt the discrimination value and the
synchronization counter.
1.1.2 SECURE LEARN*
The transmitter is activated through a special button
combination to transmit a stored 48-bit value (random
seed) that can be used for key generation or be part of
the key. Transmission of the random seed can be dis-
abled after learning is completed.
Most low-end keyless entry systems transmit the same
code from a transmitter every time a button is pushed.
The relative number of code combinations for a low end
system is also a relatively small number. These
shortcomings provide the means for a sophisticated
thief to create a device that ‘grabs’ a transmission and
re-transmits it later, or a device that scans all possible
combinations until the correct one is found.
The HCS301 employs the KEELOQ code hopping
encryption algorithm to achieve a high level of security.
Code hopping is a method by which the code
transmitted from the transmitter to the receiver is
different every time a button is pushed. This method,
coupled with a transmission length of 66 bits, virtually
eliminates the use of code ‘grabbing’ or code
‘scanning’.
As indicated in the block diagram on page one, the
HCS301 has a small EEPROM array which must be
loaded with several parameters before use. The most
important of these values are:
• A 28-bit serial number which is meant to be
unique for every encoder
• An encryption key that is generated at the time of
production
• A 16-bit synchronization value
The serial number for each transmitter is programmed
by the manufacturer at the time of production. The
generation of the encryption key is done using a key
generation algorithm (Figure 1-1). Typically, inputs to
the key generation algorithm are the serial number of
the transmitter and a 64-bit manufacturer’s code. The
manufacturer’s code is chosen by the system
manufacturer and must be carefully controlled. The
manufacturer’s code is a pivotal part of the overall
system security.
FIGURE 1-1: CREATION AND STORAGE OF ENCRYPTION KEY DURING PRODUCTION
Transmitter
Serial Number
Manufacturer’s
Code
Key
Generation
Algorithm
Encryption
Key
HCS301 EEPROM Array
Serial Number
Encryption Key
Sync Counter
.
.
.
*KEELOQ learning patents pending.
DS21143A-page 2
Preliminary
© 1996 Microchip Technology Inc.