HCS361 datasheet(12/24 Pages) MICROCHIP

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HCS361

DS40146C-page 12

Preliminary

5.0

SPECIAL FEATURES

5.1

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 and that a minimum of two words are completed.

The HCS361 encoder powers itself up when a button is

pushed and powers itself down after the current trans-

mission is ﬁnished, if the user has already released the

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

two transmissions, then multiple transmissions will

result. The HCS361 transmits at least two transmis-

sions before powering down. If another button is acti-

vated during a 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. Using the

BACW 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 BACW effectively prevents

continuous transmission by only allowing the transmis-

sion of every second word. This reduces the average

power transmitted and hence, assists in FCC approval

of a transmitter device.

5.3

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 0-1:

CRC CALCULATION

and

with

and

Din the nth transmission bit 0 ≤ n ≤ 64

5.4

Secure Learning

In order to increase the level of security in a system, it

is possible for the receiver to implement what is known

as a secure learning function. This can be done by uti-

lizing the seed value on the HCS361 which is stored in

EEPROM. Instead of the normal key generation

method being used to create the encryption key, this

seed value is used and there should not be any mathe-

matical relationship between serial numbers and seeds

for the best security.

5.5

Auto-shutoff

The Auto-shutoff function automatically stops the

device from transmitting if a button inadvertently gets

pressed for a long period of time. This will prevent the

device from draining the battery if a button gets pressed

while the transmitter is in a pocket or purse. This func-

tion can be enabled or disabled and is selected by set-

ting or clearing the time-out bit (Section 3.5.7). Setting

this bit will enable the function (turn Auto-shutoff func-

tion on) and clearing the bit will disable the function.

Time-out period is approximately 25 seconds.

5.6

VLOW: Voltage LOW Indicator

The VLOW bit is transmitted with every transmission

(Figure 4-2) and will be transmitted as a one if the

operating voltage has dropped below the low voltage

trip point, typically 3.8V at 25

°C. This VLOW signal is

transmitted so the receiver can give an indication to the

user that the transmitter battery is low.

5.7

LED Output Operation

During normal transmission the LED output is LOW. If

the supply voltage drops below the low voltage trip

point, the LED output will be toggled at approximately

1Hz during the transmission.

FIGURE 5-1:

BLANK ALTERNATE CODE WORD

CRC 1

[ ]

CRC 0

[ ]

∧

CRC 0

[ ]

CRC 0

[ ]

∧

(

) CRC 1

[ ]

∧

CRC 1 0

[

]

One Code Word

BACW Disabled

(All words transmitted)

BACW Enabled

(1 out of 2 transmitted)

100ms

Amplitude

Time

Min Tx Length

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