Microchip Technology HCS300 - Page 8

HCS300
DS21137G-page 8 © 2011 Microchip Technology Inc.
4.0 TRANSMITTED WORD
4.1 Code Word Format
The HCS300 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 9-4 for
code word timing.
4.2 Code Word Organization
The HCS300 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 10
19
.
FIGURE 4-1: CODE WORD FORMAT
FIGURE 4-2: CODE WORD ORGANIZATION
LOGIC ‘0’
LOGIC ‘1’
Bit
Period
Preamble
Header
Encrypted Portion
of Transmission
Fixed Portion of
Transmission
Guard
Time
TP
TH
THOP
TFIX
TG
TE
TE
TE
50% Duty Cycle
Repeat
(1 bit)
VLOW
(1 bit)
Button
Status
S2 S1 S0 S3
Serial Number
(28 bits)
Button
Status
S2 S1 S0 S3
OVR
(2 bits)
DISC
(10 bits)
Sync Counter
(16 bits)
Repeat
(1 bit)
VLOW
(1 bit)
Button
Status
1 1 1 1
Serial Number
(28 bits)
SEED
(32 bits)
34 bits of Fixed Portion 32 bits of Encrypted Portion
66 Data bits
Transmitted
LSb first.
LSb
MSb
MSb
LSb
Note: SEED replaces Encrypted Portion when all button inputs are activated at the same time.