HCS515_11 Datasheet, PDF (6/38 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder stand-alone decoder

HCS515
3.2 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 encoder decryption key.
It uses the discrimination value to determine if the
decryption was valid. If everything up to this point is
valid, the synchronization counter value is evaluated.
3.3 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?
Yes
Save Counter
in Temp Location
Execute
Command
and
Update
Counter
DS40183E-page 6
© 2011 Microchip Technology Inc.