HCS301 datasheet(7/38 Pages) MICROCHIP

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DS21143C-page 7

HCS301

3.2

SYNC (Synchronization Counter)

This is the 16-bit synchronization value that is used to

create the hopping code for transmission. This value

will increment after every transmission.

3.3

Reserved

Must be initialized to 0000H.

3.4

SER_0, SER_1

(Encoder Serial Number)

SER_0 and SER_1 are the lower and upper words of

the device serial number, respectively. Although there

are 32 bits allocated for the serial number, only the

lower order 28 bits are transmitted. The serial number

is meant to be unique for every transmitter.

3.4.1

AUTO-SHUTOFF TIMER ENABLE

The Most Significant bit of the serial number (Bit 31) is

used to turn the Auto-shutoff timer on or off. This timer

prevents the transmitter from draining the battery

should a button get stuck in the on position for a long

period of time. The time period is approximately

25 seconds, after which the device will go to the Time-

out mode. When in the Time-out mode, the device will

stop transmitting, although since some circuits within

the device are still active, the current draw within the

Shutoff mode will be higher than Standby mode. If the

Most Significant bit in the serial number is a one, then

the Auto-shutoff timer is enabled, and a zero in the

Most Significant bit will disable the timer. The length of

the timer is not selectable.

3.5

SEED_0, SEED_1 (Seed Word)

The 2-word (32-bit) seed code will be transmitted when

all three buttons are pressed at the same time (see

Figure 4-2). This allows the system designer to imple-

ment the secure learn feature or use this fixed code

word as part of a different key generation/tracking pro-

cess.

3.6

CONFIG (Configuration Word)

The Configuration Word is a 16-bit word stored in

EEPROM array that is used by the device to store

information used during the encryption process, as well

as the status of option configurations. The following

sections further explain these bits.

TABLE 3-2:

CONFIGURATION WORD

3.6.1

DISCRIMINATION VALUE

(DISC0 TO DISC9)

The discrimination value aids the post-decryption

check on the decoder end. It may be any value, but in

a typical system it will be programmed as the 10 Least

Significant bits of the serial number. Values other than

this must be separately stored by the receiver when a

transmitter is learned. The discrimination bits are part

of the information that form the encrypted portion of the

transmission (Figure 4-2). After the receiver has

decrypted a transmission, the discrimination bits are

checked against the receiver’s stored value to verify

that the decryption process was valid. If the discrimina-

tion value was programmed as the 10 LSb’s of the

serial number then it may merely be compared to the

respective bits of the received serial number; saving

EEPROM space.

3.6.2

OVERFLOW BITS

(OVR0, OVR1)

The overflow bits are used to extend the number of

possible synchronization values. The synchronization

counter is 16 bits in length, yielding 65,536 values

before the cycle repeats. Under typical use of

10 operations a day, this will provide nearly 18 years of

use before a repeated value will be used. Should the

system designer conclude that is not adequate, then

the overflow bits can be utilized to extend the number

Bit Number

Bit Description

Discrimination Bit 0

Discrimination Bit 1

Discrimination Bit 2

Discrimination Bit 3

Discrimination Bit 4

Discrimination Bit 5

Discrimination Bit 6

Discrimination Bit 7

Discrimination Bit 8

Discrimination Bit 9

Overflow Bit 0 (OVR0)

Overflow Bit 1 (OVR1)

Low Voltage Trip Point Select

(VLOW SEL)

Baud rate Select Bit 0 (BSL0)

Baud rate Select Bit 1 (BSL1)

Reserved, set to 0

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