HCS360 Datasheet, PDF (12/24 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

HCS360
5.0 SPECIAL FEATURES
5.1 Code Word Completion
Code word completion is an automatic feature that
ensures that the entire code word is transmitted, even
if the button is released before the transmission is com-
plete and that a minimum of two words are completed.
The HCS360 encoder powers itself up when a button is
pushed and powers itself down after two complete
words are transmitted if the user has already released
the button. If the button is held down beyond the time
for one transmission, then multiple transmissions will
result. If another button is activated during a
transmission, the active transmission will be aborted
and the new code will be generated using the new
button information.
5.2 Long Guard Time
Federal Communications Commission (FCC) part 15
rules specify the limits on fundamental power and
harmonics that can be transmitted. Power is calculated
on the worst case average power transmitted in a
100ms window. It is therefore advantageous to
minimize the duty cycle of the transmitted word. This
can be achieved by minimizing the duty cycle of the
individual bits and by extending the guard time between
transmissions. long guard time (LNGRD) is used for
reducing the average power of a transmission. This is a
selectable feature. Using the LNGRD allows the user to
transmit a higher amplitude transmission if the
transmission time per 100 ms is shorter. The FCC puts
constraints on the average power that can be
transmitted by a device, and LNGRD effectively
prevents continuous transmission by only allowing the
transmission of every second word. This reduces the
average power transmitted and hence, assists in FCC
approval of a transmitter device.
5.3 CRC (Cycle Redundancy Check) Bits
The CRC bits are calculated on the 65 previously trans-
mitted bits. The CRC bits can be used by the receiver
to check the data integrity before processing starts. The
CRC can detect all single bit and 66% of double bit
errors. The CRC is computed as follows:
EQUATION 5-1: CRC CALCULATION
CRC[1]_{n + 1} = CRC[0]_n ∧ Di_n
and
CRC[0]_{n + 1} = (CRC[0]_n ∧ Di_n) ∧ CRC[1]_n
with
CRC[1, 0]₀ = 0
and
Di_n the nth transmission bit 0 ≤ n ≤ 64
5.4 Secure Learning
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 learning function. This can be done by utilizing
the seed value on the HCS360 which is stored in
EEPROM. Instead of the normal key generation method
being used to create the encryption key, this seed value
is used and there should not be any mathematical rela-
tionship between serial numbers and seeds for the best
security.
5.5 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. This func-
tion can be enabled or disabled and is selected by set-
ting or clearing the time-out bit (Section 3.5.5). Setting
this bit will enable the function (turn Auto-shutoff func-
tion on) and clearing the bit will disable the function.
Time-out period is approximately 25 seconds.
5.6 VLOW: Voltage LOW Indicator
The VLOW bit is transmitted with every transmission
(Figure 4-2) and will be transmitted as a one if the
operating voltage has dropped below the low voltage
trip point, approximately 3.8V at 25°C. This VLOW signal
is transmitted so the receiver can give an indication to
the user that the transmitter battery is low.
5.7 LED Output Operation
During normal transmission the LED output is LOW. If
the supply voltage drops below the low voltage trip
point, the LED output will be toggled at approximately
1Hz during the transmission.
DS40152C-page 12
Preliminary
© 1996 Microchip Technology Inc.