hcs361 Microchip Technology Inc., hcs361 Datasheet - Page 14

• Manufacturer Part Number: hcs361
• Description: Keeloq Code Hopping Encoder
• Manufacturer: Microchip Technology Inc.

HCS361
5.0
5.1
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 HCS361 encoder powers itself up when a button is
pushed and powers itself down after the current trans-
mission is finished, if the user has already released the
button. If the button is held down beyond the time for
two transmissions, then multiple transmissions will
result. The HCS361 transmits at least two transmis-
sions before powering down. If another button is acti-
vated during a transmission, the active transmission
will be aborted and the new code will be generated
using the new button information.
5.2
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 100
ms 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 blanking out consecutive words. Blank
Alternate Code Word (BACW) is used for reducing the
average power of a transmission (Figure 5-1). This is a
selectable feature. Using the BACW allows the user to
transmit a higher amplitude transmission if the
transmission length is shorter. The FCC puts
constraints on the average power that can be
transmitted by a device, and BACW 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.
FIGURE 5-1:
DS40146E-page 14
BRS = 0
BRS = 1
SPECIAL FEATURES
Code Word Completion
Blank Alternate Code Word
BLANK ALTERNATE CODE WORD (BACW)
Amplitude
2A
A
Code Word
Code Word
5.3
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:
and
with
and
Di
.
Note: The CRC may be wrong when the battery
n
the nth transmission bit 0 ð n ð 64
CRC 0
Time
Work around: If the CRC calculation is incor-
rect, recalculate for the opposite value of
V
CRC (Cycle Redundancy Check)
Bits
voltage is around either of the V
points. This may happen because V
sampled twice each transmission, once for
the CRC calculation (PWM is low) and once
when V
V
sion which could lead to a different value for
V
and the transmission
LOW
CRC 1
DD
LOW
Code Word
n
tends to move slightly during a transmis-
.
+
being used for the CRC calculation
1
LOW
=
n
CRC 1 0
+
1
CRC 0
CRC Calculation
is transmitted (PWM is high).
=
CRC 0
2002 Microchip Technology Inc.
0
n
Code Word
=
Di
n
0
n
Di
n
CRC 1
LOW
LOW
n
trip
is