HCS300 Datasheet, PDF (7/28 Pages) Microchip Technology – KEE LOQ Code Hopping Encoder

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
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 (VLOW
SEL)
Baud rate Select Bit 0 (BSL0)
Baud rate Select Bit 1 (BSL1)
Reserved, set to 0
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 12 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 discrimi-
nation value was programmed as the 12 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
© 2001 Microchip Technology Inc.
HCS300
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
BAUD RATE 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 5.7 provides detailed explanation in code word
blanking.
TABLE 3-3: BAUD RATE SELECT
BSL1 BSL0
0
0
0
1
1
0
1
1
Basic Pulse
Element
400 µs
200 µs
100 µs
100 µs
Code Words
Transmitted
All
1 out of 2
1 out of 2
1 out of 4
3.6.4
LOW VOLTAGE TRIP POINT
SELECT (VLOW SEL)
The low voltage trip point select bit is used to tell the
HCS300 what VDD level is being used. This information
will be used by the device to determine when to send
the voltage low signal to the receiver. When this bit is
set to a one, the VDD level is assumed to be operating
from a 5V or 6V VDD level. If the bit is set low, then the
VDD level is assumed to be 3.0 volts.
FIGURE 3-1:
Volts (V)
4.2
4.0
3.8
3.6
VLOW CHARACTERISTICS
VLOW
VLOW sel = 1
2.6
2.4
2.2
2.0
1.8
1.6
1.4
-40
VLOW sel = 0
Temp (C)
0
50 85
DS21137F-page 7