HCS500 Datasheet, PDF (17/24 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

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 66/67 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 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.
7.2 Code Word Organization
The HCS encoder transmits a 66/67-bit code word
when a button is pressed. The 66/67-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.
FIGURE 7-1: CODE WORD TRANSMISSION FORMAT
LOGIC ‘0’
Bit
Period
Preamble
TP
LOGIC ‘1’
Code Hopping Portion
Header
of Transmission
TH
THOP
Fixed Portion of
Transmission
TFIX
Guard
Time
TG
FIGURE 7-2: CODE WORD ORGANIZATION
Non-encrypted Data
CRC1*
Repeat
CRC0*
VLOW
(1 bit)
Button Status
S2S1S0S3
(4 bits)
28-bit
Serial
Number
Encrypted Data
Button Status
S2S1S0S3
(4 bits)
Discrimination
bits (12 bits)
16-bit
Sync.
Counter
Value
3/2 bits
*HCS360/361
+
Serial Number and
+ 32 bits of Encrypted Data 66/67 bits
Button Status (32 bits)
of Data
Transmitted
© 1997 Microchip Technology Inc.
Preliminary
DS40153B-page 17