HCS301 datasheet(9/20 Pages) MICROCHIP

9 / 20 page

Preliminary

DS21143A-page 9

HCS301

5.0

SPECIAL FEATURES

5.1

Code Word Completion

Code word completion is an automatic feature that

makes sure that the entire code word is transmitted,

even if the button is released before the transmission is

complete. The HCS301 encoder powers itself up when

a button is pushed and powers itself down after the

command is ﬁnished, if the user has already released

the button. If the button is held down beyond the time

for one transmission, then multiple transmissions will

result. If

another

button

activated

during

transmission, the active transmission will be aborted

and the new code will be generated using the new

button information.

5.2

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 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.

Blank Alternate Code Word (BACW) is used for

reducing

the

average

power

transmission

(Figure 5-1). This is a selectable feature that is

determined in conjunction with the baudrate selection

bits BSL0 and BSL1. Using the BACW allows the user

to transmit a higher amplitude transmission if the

transmission

length

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 transmis-

sion of every second or every fourth code word. This

reduces the average power transmitted and hence,

assists in FCC approval of a transmitter device.

5.3

Envelope Encryption Option

Envelope Encryption is a user selectable option which

is meant to offer a higher level of security for a code

hopping system. During a normal transmission with the

envelope encryption turned off, the 28-bit serial number

and function code are transmitted in the clear (unen-

crypted). If envelope encryption is selected, then the

serial number and function code are also encrypted

before transmission. The encryption for the serial num-

ber is done using a different algorithm than the trans-

mission algorithm. The envelope encryption scheme is

not nearly as complex as the KEELOQ algorithm and,

hence, not as secure. When the envelope encryption is

used, the serial number must be decrypted using the

envelope key and envelope decryption. After the serial

number is obtained, the normal decryption method can

be used to decrypt the hopping code.

FIGURE 5-1:

BLANK ALTERNATE CODE WORD (BACW)

One Code Word

BACW Disabled

(All words transmitted)

BACW Enabled

(1 out of 2 transmitted)

BACW Enabled

(1 out of 4 transmitted)

100ms

Amplitude

Time

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