HCS200
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.
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.
FIGURE 1-2: BASIC OPERATION OF TRANSMITTER (ENCODER)
Transmitted Information
KeeLoq
Encryption
Algorithm
Button Press
Information
32 Bits of
Encrypted Data
Serial Number
EEPROM Array
Encryption Key
Sync Counter
Serial Number
FIGURE 1-3: BASIC OPERATION OF RECEIVER (DECODER)
Check for
Match
EEPROM Array
Decrypted
KEELOQ
Decryption
Algorithm
Synchronization
Encryption Key
Sync Counter
Counter
Check for
Match
Serial Number
Manufacturer Code
32 Bits of
Encrypted Data
Button Press
Information
Serial Number
Received Information
1996 Microchip Technology Inc.
Preliminary
DS40138A-page 3
MICROCHIP [ MICROCHIP ]
