HCS201 Datasheet, PDF (7/28 Pages) Microchip Technology – KEELOQ Code Hopping Encoder

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.
TABLE 3-2: DISCRIMINATION WORD
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
Not Used
13
Not Used
14
Not Used
15
Not Used
3.6 DISC
(Discrimination Word)
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
HCS201
serial number then it may merely be compared to the
respective bits of the received serial number; saving
EEPROM space.
3.7 CONFIG
(Configuration Word)
The Configuration Word is a 16-bit word stored in
EEPROM array that is used by the device to store infor-
mation used during the encryption process, as well as
the status of option configurations. Further explana-
tions of each of the bits are described in the following
sections.
TABLE 3-3: CONFIGURATION WORD
Bit Number
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
OSC0
OSC1
OSC2
OSC3
VLOWS
BRS
MTX4
TXEN
S3SET
XSER
Not Used
Not Used
Not Used
Not Used
Not Used
Not Used
Bit Name
3.7.1
OSCILLATOR TUNING BITS
(OSC0 AND OSC3)
These bits are used to tune the frequency of the
HCS201 internal clock oscillator to within ±10% of its
nominal value over temperature and voltage.
3.7.2
LOW VOLTAGE TRIP POINT
SELECT (VLOWS)
The low voltage trip point select bit (VLOWS) and the S3
setting bit (S3SET) are used to determine when to send
the VLOW signal to the receiver.
TABLE 3-4: TRIP POINT SELECT
VLOWS
0
0
1
1
S3SET*
0
1
0
1
Trip Point
4.4
4.4
9
6.75
* See also Section 3.7.6
© 2001 Microchip Technology Inc.
DS41098C-page 7