HCS361 datasheet(8/42 Pages) MICROCHIP

8 / 42 page

HCS361

DS40146F-page 8

3.5.6

BACW: BLANK ALTERNATE CODE

WORD

BACW = 1 selects the encoder to transmit every sec-

ond code word. This can be used to reduce the aver-

age power transmitted over a 100 ms window and

thereby

transmit

higher

peak

power

(see

Section 5.2).

3.5.7

XSER: EXTENDED SERIAL

NUMBER

If XSER = 0, the four Most Significant bits of the Serial

Number are substituted by S[3:0] and the code word

format is compatible with the HCS200/300/301.

If XSER = 1, the full 32-bit Serial Number [SER_1,

SER_0] is transmitted.

3.5.8

DISCRIMINATION VALUE

While in other KEELOQ encoders its value is user

selectable, the HCS361 uses directly the 8 Least Sig-

nificant bits of the Serial Number as part of the infor-

mation that form the encrypted portion of the

transmission (Figure 3-2).

The discrimination value aids the post-decryption

check on the decoder end. After the receiver has

decrypted a transmission, the discrimination bits are

checked against the encoder Serial Number to verify

that the decryption process was valid.

3.5.9

USRA,B: USER BITS

User bits form part of the discrimination value. The user

bits together with the IND bit can be used to identify the

counter that is used in Independent mode.

FIGURE 3-2:

CODE WORD ORGANIZATION

Note:

Since the button status S[3:0] is used to

detect a Seed transmission, Extended

Serial Number and Secure Learn are

mutually exclusive.

Discrimination Bits

(12 bits)

... S

... E

... R

...

Fixed Code Portion of Transmission

Encrypted Portion of Transmission

67 bits

of Data

Transmitted

MSB

LSB

CRC

(2-bit)

VLOW

(1-bit)

Button

Status

(4 bits)

28-bit

Serial Number

Button

Status

(4 bits)

Discrimination

bits

(12 bits)

16-bit

Sync Value

Button Status

(4 bits)

Fixed Code Portion of Transmission

Encrypted Portion of Transmission

MSB

LSB

CRC

(2-bit)

VLOW

(1-bit)

32-bit

Extended Serial Number

Button

Status

(4 bits)

Discrimination

bits

(12 bits)

16-bit

Sync Value

XSER=1

XSER=0

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