HCS301 Datasheet, PDF (5/20 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

3.0 EEPROM MEMORY
ORGANIZATION
The HCS301 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
WORD
ADDRESS
MNEMONIC
DESCRIPTION
0
KEY_0
1
KEY_1
64-bit encryption key
(word 0)
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(Note) Device Serial Number
(word 1)
8
SEED_0
Seed Value (word 0)
9
SEED_1
Seed Value (word 1)
10
EN_KEY
16-bit Envelope Key
11
CONFIG
Configuration Word
Note: The MSB of the serial number contains a bit
used to select the auto shutoff timer.
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 is different from the
KEELOQ algorithm, although it too is a proprietary
encryption method. Inputs to the key generation
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.
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 be changed after every transmission.
3.3 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. The most
significant bit of the serial number (Bit 31) is used to
turn the auto shutoff timer on or off.
3.3.1 AUTO-SHUTOFF TIMER SELECT
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 more 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.
3.4 SEED_0, SEED_1 (Seed Word)
This is the two-word (32 bits) seed code that will be
transmitted when all four 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.
3.5 EN_Key (Envelope Encryption Key)
Envelope encryption is a selectable option that
encrypts the portion of the transmission that contains
the transmitter serial number and function code. Select-
ing this option is done by setting the appropriate bit in
the configuration word (Table 3-2). Normally, the serial
number and function code are transmitted in the clear
(unencrypted), but for an added level of security, the
system designer may elect to implement this option.
The envelope encryption key is used to encrypt the
serial number and function code portion of the trans-
mission, if the envelope encryption option has been
selected. The envelope encryption algorithm is a differ-
ent algorithm than the key generation or transmit
encryption algorithm. The EN_key is typically a random
number and the same for all transmitters in a system.
© 1996 Microchip Technology Inc.
Preliminary
DS21143A-page 5