HCS500-I Datasheet, PDF (6/34 Pages) Microchip Technology – KEELOQ Code Hopping Decoder

HCS500
3.3 Validation of Codes
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 Steps
Validation consists of the following steps:
1. Search EEPROM to find the Serial Number
Check Value Match
2. Decrypt the Hopping Code
3. Compare the 10 bits of the discrimination value
with the lower 10 bits of serial number
4. Check if the synchronization counter value falls
within the first synchronization window.
5. Check if the synchronization counter value falls
within the second synchronization window.
6. If a valid transmission is found, update the
synchronization counter, else use the next
transmitter block, and repeat the tests.
FIGURE 3-2: DECODER OPERATION
Start
No _Transmission
Received?
Yes
No
Does
Ser # Check Val
Match?
Yes
Decrypt Transmission
No
Is
decryption
valid?
Yes
Is
counter within
Yes
16?
No
No
Is
counter within
16K?
Execute
Command
and
Update
Counter
Yes
Save Counter
in Temp Location
DS40153C-page 6
© 2002 Microchip Technology Inc.