HCS201 Datasheet, PDF (11/28 Pages) Microchip Technology – KEELOQ Code Hopping Encoder

HCS201
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 VLOW: Voltage LOW Indicator
The VLOW bit is transmitted with every transmission
(Figure 8-4) and will be transmitted as a one if the
operating voltage has dropped below the low voltage trip
point. The trip point is selectable based on the battery
voltage being used. See Section 3.7.2 for a description
of how the low voltage select option is set. This VLOW
signal is transmitted so the receiver can give an audible
signal to the user that the transmitter battery is low.
5.3 Auto-Shutoff
The auto-shutoff function automatically stops the device
from transmitting if a button inadvertently gets pressed
for a long period of time. This will prevent the device from
draining the battery if a button gets pressed while the
transmitter is in a pocket or purse. Time-out period is TTO.
5.4 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.
TABLE 5-1: PIN ACTIVATION TABLE
Standby
Hopping Code
Seed Code
Function S2 S1 S0
0
00 0
1
00 1
2
01 0
-
-
-
-
5
10 1
6
11 0
7
11 1
5.5 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.
FIGURE 5-1:
BLANK ALTERNATE CODE WORD (BACW)
Amplitude
BRS = 0
A
Code Word
Code Word
Code Word
Code Word
BRS = 1
2A
Time
© 2001 Microchip Technology Inc.
DS41098C-page 11