© 1996 Microchip Technology Inc.

Preliminary

DS40138A-page 5

HCS200

3.0

EEPROM MEMORY

ORGANIZATION

The HCS200 contains 192 bits (12 x 16-bit words) of

EEPROM memory (Table 3-1). This EEPROM array is

used

to

store

the

encryption

key

information,

synchronization value, etc. Further descriptions of the

memory array is given in the following sections.

TABLE 3-1:

EEPROM MEMORY MAP

3.1

Key_0 - Key_3 (64-Bit Encryption Key)

The 64-bit encryption key is used by the transmitter to

create the encrypted message transmitted to the

receiver. This key is created and programmed at the

time of production using a key generation algorithm.

The key generation algorithm may be different from the

algorithm are the serial number for the particular

transmitter being used and the 64-bit manufacturer’s

code. While the key generation algorithm supplied from

Microchip is the typical method used, a user may elect

to create their own method of key generation. This may

be done providing that the decoder is programmed with

the

same

means

of

creating

the

key

for

decryption purposes.

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 be changed after every transmission.

3.3

Reserved

Must be initialized to 0000H.

WORD

ADDRESS

MNEMONIC

DESCRIPTION

0

KEY_0

64-bit

encryption

key

(word 0)

1

KEY_1

64-bit

encryption

key

(word 1)

2

KEY_2

64-bit

encryption

key

(word 2)

3

KEY_3

64-bit

encryption

key

(word 3)

4

SYNC

16-bit

synchronization

value

5

Reserved

Set to 0000H

6

SER_0

Device Serial Number

(word 0)

7

SER_1

Device Serial Number

(word 1)

8

SEED_0

Seed Value (word 0)

9

SEED_1

Seed Value (word 1)

10

Reserved

Set to 0000H

11

CONFIG

Config Word

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.

3.5

SEED_0, SEED_1 (Seed Word)

This is the 2-word (32-bit) seed code that will be

transmitted when all three buttons are pressed at the

same time. This allows the system designer to implement

the secure learn feature or use this fixed code word as

part of a different key generation/tracking process.

3.6

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. Further

explanations of each of the bits are described in the

following sections.

TABLE 3-2:

CONFIGURATION WORD

3.6.1

DISCRIMINATION VALUE

(DISC0 TO DISC11)

Bits 14 and 15 should be set to zero. The discrimination

value can be programmed with any value to serve as a

post decryption check on the decoder end. In a typical

system, this will be programmed with the 12 least sig-

nificant bits of the serial number or a constant value,

which will also be stored by the receiver system after a

transmitter has been learned. The discrimination bits

are part of the information that form the encrypted por-

tion of the transmission. After the receiver has

decrypted a transmission, the discrimination bits can

be checked against the stored value to verify that the

decryption process was valid.

Bit Number

Bit Description

0

Discrimination Bit 0

1

Discrimination Bit 1

2

Discrimination Bit 2

3

Discrimination Bit 3

4

Discrimination Bit 4

5

Discrimination Bit 5

6

Discrimination Bit 6

7

Discrimination Bit 7

8

Discrimination Bit 8

9

Discrimination Bit 9

10

Discrimination Bit 10

11

Discrimination Bit 11

12

13

Baudrate Select Bit 0 (BSL0)

14

Reserved

15

Reserved