HCS410_11 Datasheet, PDF(10/46 Page) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder 2-bit CRC, 2-bit queue

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HCS410_11 Datasheet, PDF (10/46 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder 2-bit CRC, 2-bit queue

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HCS410

2.3 Code Hopping Mode Special Features

2.3.1 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. 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 CODE WORD BLANKING ENABLE

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

constraints on the average power that can be

transmitted by a device, and CWBE effectively

prevents continuous transmission by only allowing the

transmission 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 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 2-1: CRC CALCULATION

CRC[1]n + 1 = CRC[0]n â Din

and

CRC[0]n + 1 = (CRC[0]n â Din) â CRC[1]n

with

CRC[1, 0]0 = 0

and Din the nth transmission bit 0 Ã° n Ã° 64

FIGURE 2-12: CODE WORD BLANKING ENABLE

Amplitude

One Code Word

CWBE Disabled

(All words transmitted)

CWBE Enabled

(1 out of 2 transmitted)

CWBE Enabled

(1 out of 4 transmitted)

Time

â¢Patents have been applied for.

DS40158F-page 10

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