HCS301

DS21143A-page 10

Preliminary

© 1996 Microchip Technology Inc.

5.4

Secure Learn

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 HCS301 which is stored in EEPROM

and can only be transmitted when all four 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.

TABLE 5-1:

PIN ACTIVATION TABLE

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 Auto-shutoff bit (Section 3.3.1).

Setting this bit high will enable the function (turn

Auto-shutoff function on) and setting the bit low will dis-

able the function. Time-out period is approximately 25

seconds.

S3

S2

S1

S0

Notes

10

0

0

1

1

20

0

1

0

1

30

0

1

1

1

40

1

0

0

1

50

1

0

1

1

60

1

1

0

1

70

1

1

1

1

81

0

0

0

1

91

0

0

1

1

10

1

0

1

0

1

11

1

0

1

1

1

12

1

1

0

0

1

13

1

1

0

1

1

14

1

1

1

0

1

15

1

1

1

1

2

Note 1: Transmit generated 32-bit code hopping

word.

2: Transmit 32-bit seed value.

5.6

VLOW: Voltage LOW indicator

The VLOW bit is transmitted with every transmission

(Figure 8-5) and will be transmitted as a one if the

operating voltage has dropped below the low voltage

trip point. The trip point is selectable between two

values, based on the battery voltage being used. See

Section 3.6.5 for a description of how the low voltage

select option is set. This VLOW signal is transmitted so

the receiver can alert the user that the transmitter bat-

tery is low.

5.7

RPT: Repeat indicator

This bit will be low for the first transmitted word. If a

button is held down for more than one transmitted code

word, this bit will be set to indicate a repeated code

word and remain set until the button is released.

5.8

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

5 Hz during the transmission (Section 3.6.5). The limit-

ing resistor is selectable between two nominal values

(Section 3.6.5).