hcs500 Microchip Technology Inc., hcs500 Datasheet - Page 6

• Manufacturer Part Number: hcs500
• Description: Keeloq Code Hopping Decoder
• Manufacturer: Microchip Technology Inc.

HCS500
3.3
The decoder waits for a transmission and checks the
serial number to determine if it is a learned transmitter.
If it is, it takes the code hopping portion of the transmis-
sion and decrypts it, using the crypt key. It uses the dis-
crimination value to determine if the decryption was
valid. If everything up to this point is valid, the
synchronization counter value is evaluated.
3.4
Validation consists of the following steps:
1.
2.
3.
4.
5.
6.
DS40153C-page 6
Search EEPROM to find the Serial Number
Check Value Match
Decrypt the Hopping Code
Compare the 10 bits of the discrimination value
with the lower 10 bits of serial number
Check if the synchronization counter value falls
within the first synchronization window.
Check if the synchronization counter value falls
within the second synchronization window.
If a valid transmission is found, update the
synchronization counter, else use the next
transmitter block, and repeat the tests.
Validation of Codes
Validation Steps
FIGURE 3-2:
No
No
No
No
Decrypt Transmission
in Temp Location
Ser # Check Val
counter within
counter within
Save Counter
Transmission
decryption
Received?
Match?
valid?
16K?
Does
16?
Start
Is
Is
Is
DECODER OPERATION
Yes
Yes
Yes
Yes
No
2002 Microchip Technology Inc.
Yes
Command
Execute
Counter
Update
and