HCS320_11 datasheet(12/38 Pages) MICROCHIP

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HCS320

DS41097D-page 12

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)

Code Word

BACW Disabled

(All words transmitted)

BACW Enabled

(1 out of 2 transmitted)

BACW Enabled

(1 out of 4 transmitted)

Amplitude

Time

Code Word

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