HCS200

DS40138A-page 8

Preliminary

© 1996 Microchip Technology Inc.

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 HCS200 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

is

activated

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

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

determined in conjunction with the baud rate selection

bit BSL0. Using the BACW allows the user to transmit a

higher amplitude transmission if the transmission

length is shorter. The FCC puts constraints on the aver-

age power that can be transmitted by a device, and

BACW 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

Seed Transmission

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 learn function. This can be done by utilizing the

seed value on the HCS200 which is stored in EEPROM

and can only be transmitted when all three button inputs

are pressed at the same time (Table 5-1). Instead of the

normal key generation method being used to create the

encryption key, this seed value is used.

5.4

(Figure 4-2 and Figure 8-5) and will be transmitted as a

zero if the operating voltage is above the voltage trip

can give an indication to the user that the transmitter

battery is low.

FIGURE 5-1:

BLANK ALTERNATE CODE WORD (BACW)

TABLE 5-1:

PIN ACTIVATION TABLE

External

Internal

S2

S1

S0

S3

S2

S1

S0

Standby after CC

0

0

00000

Hop

0

0

10001

0

1

00010

0

1

10011

1

0

01100

1

0

11110

1

1

01110

Fixed

1

1

11111

One Code Word

BACW Disabled

(All words transmitted)

BACW Enabled

(1 out of 2 transmitted)

A

2A

100ms

100ms

100ms

100ms

Amplitude