HCS360 datasheet(12/24 Pages) MICROCHIP

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HCS360

DS40152C-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 HCS360 encoder powers itself up when a button is

pushed and powers itself down after two complete

words are transmitted 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

Long Guard Time

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 extending the guard time between

transmissions. long guard time (LNGRD) is used for

reducing the average power of a transmission. This is a

selectable feature. Using the LNGRD allows the user to

transmit a higher amplitude transmission if the

transmission time per 100 ms is shorter. The FCC puts

constraints on the average power that can be

transmitted by a device, and LNGRD effectively

prevents continuous transmission by only allowing the

transmission 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 5-1:

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 utilizing

the seed value on the HCS360 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 mathematical rela-

tionship 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 3.5.5). 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, approximately 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.

CRC 1

[ ]

CRC 0

[ ]

∧

CRC 0

[ ]

CRC 0

[ ]

∧

(

) CRC 1

[ ]

∧

CRC 1 0

[

]

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