HCS512 Datasheet, PDF (13/20 Pages) Microchip Technology – KEELOQ CODE HOPPING DECODER

HCS512
8.0 KEELOQ ENCODERS
8.1 Transmission Format (PWM)
The KEELOQ encoder transmission is made up of sev-
eral parts (Figure 8-1). Each transmission begins with
a preamble and a header, followed by the encrypted
and then the fixed data. The actual data is 56/66/67 bits
which consists of 32 bits of encrypted data and 24/34/
35 bits of non-encrypted data. Each transmission is
followed by a guard period before another transmission
can begin. The encrypted portion provides up to four
billion changing code combinations and includes the
button status bits (based on which buttons were acti-
vated) along with the synchronization counter value
and some discrimination bits. The non-encrypted por-
tion is comprised of the status bits, the function bits,
and the 24/28-bit serial number. The encrypted and
non-encrypted combined sections increase the number
of combinations to 7.38 x 10¹⁹.
8.2 Code Word Organization
The HCSXXX encoder transmits a 66/67-bit code word
when a button is pressed. The 66/67-bit word is con-
structed from an encryption portion and a non-
encrypted code portion (Figure 8-2).
The Encrypted Data is generated from four button bits,
two overflow counter bits, ten discrimination bits, and
the 16-bit synchronization value.
The Non-encrypted Data is made up from 2 status
bits, 4 function bits, and the 28/32-bit serial number.
FIGURE 8-1: CODE WORD TRANSMISSION FORMAT
LOGIC ‘0’
Bit
Period
LOGIC ‘1’
Preamble
TP
Header
TH
Encrypted Portion
of Transmission
THOP
Fixed Portion of
Transmission
TFIX
Guard
Time
TG
FIGURE 8-2: CODE WORD ORGANIZATION
Non-encrypted Data
CRC1*
Repeat
CRC0*
Vlow Button Sta-
28-bit
(1 bit) tus (4 bits) Serial Number
Encrypted Data
Button
Status
(4 bits)
Discrimina-
tion bits
(12 bits)
16-bit
Sync Value
3/2 bits
*HCS360/361
+
Serial Number and But-
ton Status (32 bits)
66/67 bits
+ 32 bits of Encrypted Data of Data
Transmitted
© 1997 Microchip Technology Inc.
DS40151C-page 13