HCS360_01 Datasheet, PDF (13/28 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder

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.
HCS360
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
Note:
The CRC may be wrong when the battery
voltage is around either of the VLOW trip
points. This may happen because VLOW is
sampled twice each transmission, once for
the CRC calculation (PWM is low) and once
when VLOW is transmitted (PWM is high).
VDD tends to move slightly during a transmis-
sion which could lead to a different value for
VLOW being used for the CRC calculation
and the transmission
.
Work around: If the CRC calculation is incor-
rect, recalculate for the opposite value of
VLOW.
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.
 2001 Microchip Technology Inc.
DS40152D-page 13