HCS410-IP Microchip Technology, HCS410-IP Datasheet - Page 11

• Manufacturer Part Number: HCS410-IP
• Description: KEELOQ CODE HOPPING ENCODER AND TRANSPONDER
• Manufacturer: Microchip Technology

2.3
2.3.1
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. The HCS410 encoder powers itself up when a
button is pushed and powers itself down after the com-
mand is finished (Figure 2-7). If MTX3 is set in the con-
figuration word, a minimum of three transmissions will
be transmitted when the HCS410 is activated, even if
the buttons are released.
If less than seven words have been transmitted when
the buttons are released, the HCS410 will complete the
current word. If more than seven words have been
transmitted, and the button is released, the PWM out-
put is immediately switched off.
2.3.2
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
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 blanking out consecutive words.
Code Word Blanking Enable (CWBE) is used for
reducing the average power of a transmission
(Figure 2-12). Using the CWBE allows the user to
transmit a higher amplitude transmission if the
transmission length is shorter. The FCC puts
FIGURE 2-12: CODE WORD BLANKING ENABLE
1997 Microchip Technology Inc.
(1 out of 2 transmitted)
(1 out of 4 transmitted)
(All words transmitted)
CWBE Disabled
CWBE Enabled
CWBE Enabled
Code Hopping Mode Special Features
CODE WORD COMPLETION
CODE WORD BLANKING ENABLE
4A
2A
Amplitude
A
Preliminary
constraints on the average power that can be
transmitted by a device, and CWBE effectively prevents
continuous transmission by only allowing the transmis-
sion of every second or fourth word. This reduces the
average power transmitted and hence, assists in FCC
approval of a transmitter device.
The HCS410 will either transmit all code words, 1 in 2
or 1 in 4 code words, depending on the baud rate
selected and the code word blanking option. See
Section 3.7 for additional details.
2.3.3
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 2-1:
and
with
and Di
Time
CRC 0
n
One Code Word
the nth transmission bit 0
CRC (CYCLE REDUNDANCY CHECK) BITS
CRC 1
n
+
1
=
CRC 1 0
n
+
CRC 0
1
CRC CALCULATION
=
CRC 0
n
0
=
Di
HCS410
0
n
n
n
DS40158C-page 11
Di
64
CRC 1
n
n