HCS201 Datasheet, PDF (9/28 Pages) Microchip Technology – KEELOQ Code Hopping Encoder

HCS201
4.0 TRANSMITTED WORD
4.1 Code Word Format
The HCS201 code word is made up of several parts
(Figure 4-1). Each code word contains a 50% duty
cycle preamble, a header, 32 bits of encrypted data and
34 bits of fixed data followed by a guard period before
another code word can begin. Refer to Table 8-4 for
code word timing.
FIGURE 4-1:
CODE WORD FORMAT
TE TE TE
LOGIC ‘0’
4.2 Code Word Organization
The HCS201 transmits a 66-bit code word when a
button is pressed. The 66-bit word is constructed from
a Fixed Code portion and an Encrypted Code portion
(Figure 4-2).
The 32 bits of Encrypted Data are generated from 4
button bits, 12 discrimination bits and the 16-bit sync
value. The encrypted portion alone provides up to four
billion changing code combinations.
The 34 bits of Fixed Code Data are made up of 2 sta-
tus bits, 4 button bits and the 28-bit serial number. The
fixed and encrypted sections combined increase the
number of code combinations to 7.38 x 10¹⁹.
Bit
Period
LOGIC ‘1’
50% Duty Cycle
Preamble
TP
Header
TH
Encrypted Portion
of Transmission
THOP
Fixed Portion of
Transmission
TFIX
Guard
Time
TG
FIGURE 4-2:
CODE WORD ORGANIZATION
34 bits of Fixed Portion
32 bits of Encrypted Portion
1
MSb
VLOW Button
(1 bit) Status
S2 S1 S0 S3
Serial Number
(28 bits)
Button
Status
S2 S1 S0 S3
DISC
(12 bits)
Sync Counter
(16 bits)
LSb
66 Data bits
Transmitted
LSb first.
VLOW Button
Serial Number
SEED
1 (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 Microchip Technology Inc.
DS41098C-page 9