HCS410 datasheet(24/36 Pages) MICROCHIP

24 / 36 page

HCS410

DS40158E-page 24

Preliminary

 2001 Microchip Technology Inc.

4.3

CH Mode Decoder Operation

In a typical decoder operation (Figure 4-4), the key

generation on the decoder side is done by taking the

serial number from a transmission and combining that

with the manufacturer’s code to create the same

encoder key that is stored in the HCS410. Once the

encoder key is obtained, the rest of the transmission

can be decrypted. The decoder waits for a transmission

and immediately checks the serial number to determine

if it is a learned transmitter. If it is, the code hopping por-

tion of the transmission is decrypted using the stored

key. It uses the discrimination bits to determine if the

decryption was valid. If everything up to this point is

valid, the synchronization counter value is evaluated.

FIGURE 4-4:

TYPICAL CH MODE

DECODER OPERATION

Transmission

Received

Does

Serial Number

Match

Decrypt Transmission

Decryption

Valid

Counter

Within 16

Counter

Within 32K

Update

Counter

Execute

Command

Save Counter

in Temp Location

Start

Yes

and

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