HCS500_15 Datasheet, PDF (19/42 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder

HCS500
7.0 KEELOQ ENCODERS
7.1 Transmission Format (PWM)
The KEELOQ encoder transmission is made up of
several parts (Figure 7-1). Each transmission begins
with a preamble and a header, followed by the
encrypted and then the fixed data. The actual data is
66/69 bits which consists of 32 bits of encrypted data
and 34/35 bits of non-encrypted data. Each
transmission is followed by a guard period before
another transmission can begin. The code hopping
portion provides up to four billion changing code
combinations and includes the button Status bits
(based on which buttons were activated), along with
the synchronization counter value and some
discrimination bits. The non-code hopping portion is
comprised of the Status bits, the function bits, and the
28-bit serial number. The encrypted and non-encrypted
combined sections increase the number of
combinations to 7.38 x 10¹⁹.
FIGURE 7-1: TRANSMISSION FORMAT (PWM)
7.2 Code Word Organization
The HCS encoder transmits a 66/69-bit code word
when a button is pressed. The 66/69-bit word is
constructed from a code hopping portion and a non-
code hopping portion (Figure 7-2).
The Encrypted Data is generated from four button bits,
two overflow counter bits, ten discrimination bits, and
the 16-bit synchronization counter value.
The Non-encrypted Data is made up from two Status
bits, four function bits, and the 28/32-bit serial number.
LOGIC "0"
LOGIC "1"
TE TE TE
TBP
50% Preamble
10xTE
Header
Encrypted
Portion
Fixed Code
Portion
Guard
Time
FIGURE 7-2:
CODE WORD ORGANIZATION
34 bits of Fixed Portion
32 bits of Encrypted Portion
Repeat VLOW Button
(1-bit) (1-bit) Status
MSb
S2 S1 S0 S3
Serial Number
(28 bits)
Button
OVR DISC Sync Counter
Status (2 bits) (10 bits) (16 bits)
S2 S1 S0 S3
66 Data bits
LSb
Transmitted
LSb first.
Repeat VLOW Button
Serial Number
SEED
(1-bit) (1-bit) Status
(28 bits)
(32 bits)
MSb
1 111
LSb
SEED replaces Encrypted Portion when all button inputs are activated at the same time.
 2001-2015 Microchip Technology Inc.
DS40000153E-page 19