HCS301
5.6
Seed Transmission
5.0
5.1
SPECIAL FEATURES
Code Word Completion
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 stored in EEPROM, transmitted only
when all three button inputs are pressed at the same
time (Table 5-1). Instead of the normal key generation
inputs being used to create the crypt key, this seed
value is used.
The code word completion feature ensures that entire
code words are transmitted, even if the button is
released before the code word is complete. If the but-
ton is held down beyond the time for one code word,
multiple code words will result. If another button is acti-
vated during a transmission, the active transmission
will be aborted and a new transmission will begin using
the new button information.
TABLE 5-1:
PIN ACTIVATION TABLE
Function S3 S2 S1
S0
0
1
0
-
5.2
LED Output Operation
Standby
0
1
0
0
0
-
0
0
0
-
0
0
1
-
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
5Hz during the transmission (Section 3.6.4).
2
Hopping Code
Seed Code
-
5.3
RPT: Repeat Indicator
13
14
15
1
1
1
1
1
1
0
1
1
1
0
1
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.
5.4
VLOW: Voltage LOW Indicator
The VLOW signal is transmitted so the receiver can give
an indication to the user that the transmitter battery is
low. The VLOW bit is included in every transmission
(Figure 4-2 and Figure 8-5) and will be transmitted as a
zero if the operating voltage is above the low voltage
trip point. Refer to Figure 4-2. The trip point is select-
able based on the battery voltage being used. See Sec-
tion 3.6.3 for a description of how the low voltage trip
point is configured.
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 function can be enabled or disabled and is
selected by setting or clearing the Auto-shutoff bit (see
Section 3.4.1). Setting this bit high will enable the func-
tion (turn Auto-shutoff function on) and setting the bit
low will disable the function. Time-out period is approx-
imately 25 seconds.
2001 Microchip Technology Inc.
DS21143B-page 11
MICROCHIP [ MICROCHIP ]
