HCS200SN Datasheet, PDF (9/34 Pages) Microchip Technology – KEELOQ Code Hopping Encoder

HCS200
5.0 SPECIAL FEATURES
5.1 Code Word Completion
The code word completion feature ensures that entire
code words are transmitted, even if the button is
released before the code word is complete. If the but-
ton is held down beyond the time for one code word,
multiple code words will result. If another button is acti-
vated during a transmission, the active transmission
will be aborted and a new transmission will begin using
the new button information.
5.2 Blank Alternate Code Word
Federal Communications Commission (FCC) part 15
rules specify the limits on worst case average funda-
mental power and harmonics that can be transmitted in
a 100 ms window. For FCC approval purposes, it may
therefore be advantageous to minimize the transmis-
sion duty cycle. This can be achieved by minimizing the
duty cycle of the individual bits as well as by blanking
out consecutive code words. Blank Alternate Code
Word (BACW) may be used to reduce the average
power of a transmission by transmitting only every sec-
ond code word (Figure 5-1). This is a selectable feature
that is determined in conjunction with the baud rate
selection bit BSL0.
Enabling the BACW option may likewise allow the user
to transmit a higher amplitude transmission as the time
averaged power is reduced. BACW effectively halves
the RF on time for a given transmission so the RF out-
put power could theoretically be doubled while main-
taining the same time averaged output power.
5.3 Seed Transmission
In order to increase the level of security in a system, it
is possible for the receiver to implement what is known
as a secure learn function. This can be done by utilizing
the seed value stored in EEPROM, transmitted only
when all three button inputs are pressed at the same
time (Table 5-1). Instead of the normal key generation
inputs being used to create the crypt key, this seed
value is used.
5.4 VLOW: Voltage LOW Indicator
The VLOW signal is transmitted so the receiver can give
an indication to the user that the transmitter battery is
low. The VLOW bit is included in every transmission
(Figure 4-2 and Figure 9-5) and will be transmitted as a
zero if the operating voltage is above the low voltage
trip point. Refer to Figure 4-2. The trip point is select-
able based on the battery voltage being used. See Sec-
tion 3.6.3 for a description of how the low voltage trip
point is configured.
FIGURE 5-1:
BLANK ALTERNATE CODE WORD (BACW)
Transmission Amplitude = A
BACW Disabled
A
Code Word
(All words transmitted)
Code Word
Code Word
Code Word
BACW Enabled
(1 out of 2 transmitted)
2A
Code
Word
Code
Word
Code
Word
Code
Word
TABLE 5-1: PIN ACTIVATION TABLE
External
Internal
S2 S1 S0 S3 S2 S1 S0
Standby
0
0
0
0
0
0
0
0
0
1
0
0
0
1
Hopping Code
0
1
0
0
0
1
0
Transmission
0
1
1
0
0
1
1
1
0
0
1
1
0
0
1
0
1
1
1
1
0
1
1
0
1
1
1
0
Seed Transmission 1
1
1
1
1
1
1
© 2011 Microchip Technology Inc.
DS40138D-page 9