HCS320 Datasheet, PDF (12/32 Pages) Microchip Technology – KEELOQ Code Hopping Encoder

HCS320
5.6 Blank Alternate Code Word
Federal Communications Commission (FCC) part 15
rules specify the limits on fundamental power and
harmonics that can be transmitted. Power is calcu-
lated 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 that is deter-
mined in conjunction with the baud rate selection bits
BSL0 and BSL1. Using the BACW allows the user to
transmit a higher amplitude transmission if the trans-
mission 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 or every fourth code word. This reduces
the average power transmitted and hence, assists in
FCC approval of a transmitter device.
FIGURE 5-2:
BLANK ALTERNATE CODE WORD (BACW)
Amplitude
BACW Disabled
A
Code Word
(All words transmitted)
BACW Enabled
2A
(1 out of 2 transmitted)
Code Word
Code Word
Code Word
4A
BACW Enabled
(1 out of 4 transmitted)
Time
DS41097C-page 12
© 2001 Microchip Technology Inc.