HCS500-IP Microchip Technology, HCS500-IP Datasheet - Page 3

• Manufacturer Part Number: HCS500-IP
• Description: KEELOQ CODE HOPPING ENCODER
• Manufacturer: Microchip Technology

The 16-bit synchronization counter 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 K
change in one bit of the synchronization counter value
will result in a large change in the actual transmitted
code. There is a relationship (Figure 1-2) between the
in the encoder. Once the encoder detects that a button
has been pressed, the encoder reads the button and
updates the synchronization counter. The synchroniza-
tion value is then combined with the encoder key in the
K
of encrypted information. 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 hop-
ping portion is combined with the button information
and the serial number to form the code word transmit-
ted to the receiver.
FIGURE 1-2:
FIGURE 1-3:
encoder key values in EEPROM and how they are used
EE
1997 Microchip Technology Inc.
Sync. Counter Value
L
EEPROM Array
OQ
Serial Number
Encoder Key
encryption algorithm, and the output is 32 bits
Sync. Counter Value
Manufacturer Code
EEPROM Array
BASIC OPERATION OF A CODE HOPPING TRANSMITTER (ENCODER)
BASIC OPERATION OF A CODE HOPPING RECEIVER (DECODER)
Serial Number
Encoder Key
EE
L
OQ
Button Press
Information
encryption algorithm, a
Encryption
Algorithm
K
EE
L
Received Information
OQ
Serial Number
Check for
Match
Preliminary
Encrypted Data
32 Bits of
1.3
Before a transmitter and receiver can work together, the
receiver must first ‘learn’ and store certain information
from the transmitter. This information includes a ‘check
value’ of the serial number, the encoder key, and cur-
rent synchronization counter value.
When a validly formatted message is detected, the
receiver first compares the serial number. If the serial
number check value is from a learned transmitter, the
message is decrypted. Next, the receiver checks the
decrypted synchronization counter value against what
is stored in memory. If the synchronization counter
value is verified, then a valid transmission message is
sent. Figure 1-3 shows the relationship between some
of the values stored by the receiver and the values
received from the transmitter.
Encrypted Data
Decryption
Algorithm
32 Bits of
K
EE
Transmitted Information
L
HCS Decoder Overview
OQ
Serial Number
Synchronization
Check for
Decrypted
Match
Counter
Button Press
Information
HCS500
DS40153B-page 3