HCS500-I Datasheet, PDF (18/34 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 sev-
eral 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 fol-
lowed 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 hop-
ping portion is comprised of the status bits, the function
FIGURE 7-1: TRANSMISSION FORMAT (PWM)
bits, and the 28-bit serial number. The encrypted and
non-encrypted combined sections increase the number
of combinations to 7.38 x 10¹⁹.
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 con-
structed 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 2 status
bits, 4 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
LSb
66 Data bits
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.
DS40153C-page 18
© 2002 Microchip Technology Inc.