HCS301 Datasheet, PDF (10/20 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

HCS301
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
S3 S2 S1
S0
Notes
1
0
0
0
1
1
2
0
0
1
0
1
3
0
0
1
1
1
4
0
1
0
0
1
5
0
1
0
1
1
6
0
1
1
0
1
7
0
1
1
1
1
8
1
0
0
0
1
9
1
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.
5.5 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.
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).
DS21143A-page 10
Preliminary
© 1996 Microchip Technology Inc.