HCS200-ISN MICROCHIP [Microchip Technology], HCS200-ISN Datasheet - Page 3

• Manufacturer Part Number: HCS200-ISN
• Description: KEELOQ CODE HOPPING ENCODER
• Manufacturer: MICROCHIP [Microchip Technology]

The 16-bit synchronization value is the basis for the
transmitted code changing for each transmission, and
is updated each time a button is pressed. Because of
the complexity of the code hopping algorithm, a change
in one bit of the synchronization value will result in a
large change in the actual transmitted code. There is a
relationship (Figure 1-2) between the key values in
EEPROM and how they are used in the encoder. Once
the encoder detects that a button has been pressed,
the encoder reads the button and updates the synchro-
nization counter. The synchronization value is then
combined with the encryption key in the encryption
algorithm and the output is 32 bits of encrypted infor-
mation. This data will change with every button press,
hence, it is referred to as the hopping portion of the
code word. The 32-bit hopping code 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 4.3.
FIGURE 1-2:
FIGURE 1-3:
1996 Microchip Technology Inc.
EEPROM Array
Encryption Key
Serial Number
Sync Counter
Manufacturer Code
BASIC OPERATION OF TRANSMITTER (ENCODER)
BASIC OPERATION OF RECEIVER (DECODER)
EEPROM Array
Encryption Key
Serial Number
Sync Counter
Button Press
Information
Encryption
Algorithm
KeeLoq
Received Information
Serial Number
Check for
Match
Preliminary
Encrypted Data
32 Bits of
Any type of controller may be used as a receiver, but it
is typically a microcontroller with compatible firmware
that allows the receiver to operate in conjunction with a
transmitter based on the HCS200. Section 7.0 provides
more detail on integrating the HCS200 into a total sys-
tem.
Before a transmitter can be used with a particular
receiver, the transmitter must be ‘learned’ by the
receiver. Upon learning a transmitter, information is
stored by the receiver so that it may track the
transmitter, including the serial number of the
transmitter, the current synchronization value for that
transmitter, and the same encryption key that is used
on the transmitter. If a receiver receives a message of
valid format, the serial number is checked, and, if it is
from a learned transmitter, the message is decrypted,
and the decrypted synchronization counter is checked
against what is stored. If the synchronization value is
verified, then the button status is checked to see what
operation is needed. Figure 1-3 shows the relationship
between some of the values stored by the receiver and
the values received from the transmitter.
Encrypted Data
K
Decryption
Algorithm
32 Bits of
EE
L
Transmitted Information
OQ
Serial Number
Synchronization
Check for
Decrypted
Match
Counter
Button Press
Information
HCS200
DS40138A-page 3