HCS301
4.2
Code Word Organization
4.0
4.1
TRANSMITTED WORD
Code Word Format
The HCS301 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 HCS301 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.
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
.
FIGURE 4-1:
CODE WORD FORMAT
TE TE
TE
LOGIC ‘0’
LOGIC ‘1’
Bit
Period
50% Duty Cycle
Preamble
TP
Encrypted Portion
of Transmission
Fixed Portion of
Transmission
TFIX
Guard
Time
TG
Header
TH
THOP
FIGURE 4-2:
CODE WORD ORGANIZATION
34 bits of Fixed Portion
32 bits of Encrypted Portion
Repeat VLOW
(1 bit) (1 bit)
Button
Status
Serial Number
(28 bits)
Button
Status
OVR
(2 bits) (10 bits)
DISC
Sync Counter
(16 bits)
S2 S1 S0 S3
S2 S1 S0 S3
MSb
MSb
LSb
LSb
66 Data bits
Transmitted
LSb first.
Repeat VLOW
(1 bit) (1 bit)
Button
Status
1 1 1 1
Serial Number
(28 bits)
SEED
(32 bits)
Note: SEED replaces Encrypted Portion when all button inputs are activated at the same time.
2001 Microchip Technology Inc.
DS21143B-page 9
MICROCHIP [ MICROCHIP ]
