HCS300 Datasheet, PDF (6/28 Pages) Microchip Technology – KEE LOQ Code Hopping Encoder

HCS300
3.0 EEPROM MEMORY
ORGANIZATION
The HCS300 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 64-bit encryption key
(word 0) LSb’s
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) MSb’s
4
SYNC 16-bit synchronization
value
5
RESERVED Set to 0000H
6
SER_0 Device Serial Number
(word 0) LSb’s
7
SER_1 Device Serial Number
(Note) (word 1) MSb’s
8
SEED_0 Seed Value (word 0)
9
SEED_1 Seed Value (word 1)
10
RESERVED Set to 0000H
11
CONFIG Config 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 Crypt Key)
The 64-bit crypt key is used to create the encrypted
message transmitted to the receiver. This key is calcu-
lated and programmed during production using a key
generation algorithm. The key generation algorithm
may be different from the KEELOQ algorithm. Inputs to
the key generation algorithm are typically the transmit-
ter’s serial number 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 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.
3.5 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.
3.5.1 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.
DS21137F-page 6
© 2001 Microchip Technology Inc.