HCS301 Datasheet, PDF (6/20 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

HCS301
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
Bit Number
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
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
Baudrate Select Bit 0 (BSL0)
Baudrate Select Bit 1 (BSL1)
Envelope Encryption Select (EENC)
3.6.1 DISCRIMINATION VALUE
(DISC0 TO DISC9)
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 10 least significant 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 is to form the encrypted portion of the
transmission. After the receiver has decrypted a trans-
mission, the discrimination bits can be checked against
the stored value to verify that the decryption process
was valid.
3.6.2 OVERFLOW BITS (OVR0 AND OVR1)
The overflow bits are used to extend the number of pos-
sible 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 of
unique values. This can be done by programming
OVR0 and OVR1 to 1s at the time of production. The
encoder will automatically clear OVR0 the first time that
the synchronization value wraps from 0xFFFF to
0x0000 and clear OVR1 the second time the counter
wraps. Once cleared, OVR0 and OVR1 cannot be set
again, thereby creating a permanent record of the
counter overflow. This prevents fast cycling of 64K
counter. If the decoder system is programmed to track
the overflow bits, then the effective number of unique
synchronization values can be extended to 196,608.
3.6.3 ENVELOPE ENCRYPTION (EENC)
If the EENC bit is set to a 1, the serial number and func-
tion code will also be encrypted so that it will appear to
be random. The 16-bit envelope key and envelope algo-
rithm will be used for encryption.
3.6.4 BAUDRATE SELECT BITS (BSL0, BSL1)
BSL0 and BSL1 select the speed of transmission and
the code word blanking. Table 3-3 shows how the bits
are used to select the different baud rates and
Section 5.2 provides detailed explanation in code word
blanking.
TABLE 3-3: BAUDRATE SELECT
BSL1 BSL0
0
0
0
1
1
0
1
1
Basic Pulse
Element
400µs
200µs
100µs
100µs
Code Words
Transmitted
All
1 out of 2
1 out of 2
1 out of 4
DS21143A-page 6
Preliminary
© 1996 Microchip Technology Inc.