© 2002 Microchip Technology Inc.

Preliminary

DS41099C-page 9

HCS412

TABLE 3-1:

ENCODER MODE ACTIVATION

3.2

Transmitted Code Word

The HCS412 transmits a 69-bit code word in response

to a button or proximity activation (Figure 3-1). Each

code word contains a 50% duty cycle preamble,

header, 32 bits of encrypted data and 37 bits of fixed

code data followed by a guard period before another

code word can begin.

The 32 bits of Encrypted Data include 4 button bits, 2

counter overflow bits, 10 discrimination bits and the 16-

bit synchronization counter value (Figure 3-2).

The content of the 37 bits of Fixed Code Data varies

with the extended serial number (XSER) option

(Figure 3-2).

• If the extended serial number option is disabled

(XSER = 0), the 37 bits include 5 status bits, 4

button status bits and the 28-bit serial number.

• If the extended serial number option is enabled

(XSER = 1), the 37 bits include 5 status bits and

the 32-bit serial number.

FIGURE 3-1:

CODE WORD FORMAT

4-Bit Button Status

SEED

TMPSD

Resulting Transmission

LC0

(Note 1)

S2

S1

S0

X

0

0

1

X

X

Code hopping transmission

X

0

1

0

X

X

Code hopping transmission

X

0

1

1

0

0

Code hopping transmission

01

SEED transmissions temporarily enabled until the 7lsb’s of the synchro-

nization counter wrap 7Fh to 00h. Then only code hopping code words.

1

0

1

1

Code hopping transmission (2 key IFF enabled)

X

1

0

1

X

X

Code hopping transmission

X

1

0

0

X

X

Code hopping transmission

X

1

1

0

X

X

Code hopping transmission

X

1

1

1

0

0

Code hopping transmission

01

Limited SEED transmissions - temporarily enabled until the 7lsb’s of the

synchronization counter wrap 7Fh to 00h.

1

0

SEED transmission

1

1

Code hopping transmission (2 key IFF enabled)

1

0

0

0

X

X

Proximity activated code hopping transmission.

Note 1: The transmitted button status will reflect the state of the LC0 input when the button inputs are sampled.

Preamble

Header

Encrypted Portion

of Transmission

Fixed Portion of

Transmission

Guard

Time

50% Duty Cycle