HCS410 Datasheet, PDF (4/36 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER AND TRANSPONDER

HCS410
1.2 KEELOQ Code Hopping Encoders
When the HCS410 is used as a code hopping encoder
device, it is ideally suited to 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 carried by the user and operated
to gain access to a vehicle or restricted area.
Most 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
retransmits it later or a device that scans all possible
combinations until the correct one is found.
The HCS410 employs the KEELOQ code hopping tech-
nology and an 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 69 bits,
virtually eliminates the use of code ‘grabbing’ or code
‘scanning’.
The HCS410 has a small EEPROM array which must
be loaded with several parameters before use. The
most important of these values are:
• A 28/32-bit serial number which is meant to be
unique for every encoder
• 64-bit seed value
• A 64-bit encoder key that is generated at the time
of production
• A 16-bit synchronization counter value.
• Configuration options
The 16-bit synchronization counter value is the basis
for the transmitted code changing for each transmis-
sion, and is updated each time a button is pressed.
Because of the complexity of the code hopping encryp-
tion algorithm, a change in one bit of the synchroniza-
tion counter value will result in a large change in the
actual transmitted code.
Once the encoder detects that a button has been
pressed, the encoder reads the button and updates the
synchronization counter. The synchronization counter
value, the function bits, and the discrimination value are
then combined with the encoder key in the encryption
algorithm, and the output is 32 bits of encrypted infor-
mation (Figure 1-1). The code hopping portion pro-
vides up to four billion changing code combinations.
This data will change with every button press, hence, it
is referred to as the code hopping portion of the code
word.
The 32-bit code hopping portion is combined with the
button information and the serial number to form the
code word transmitted to the receiver. The code word
format is explained in detail in Section 2.2.
FIGURE 1-1: BASIC OPERATION OF A CODE HOPPING TRANSMITTER (ENCODER)
Transmitted Information
EEPROM Array
Encoder Key
Sync Counter
Serial Number
KEELOQ
Encryption
Algorithm
32 Bits of
Encrypted Data
Serial Number
Button Press
Information
DS40158C-page 4
Preliminary
© 1997 Microchip Technology Inc.