HCS410_11 Datasheet, PDF (11/46 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder 2-bit CRC, 2-bit queue

HCS410
2.3.4 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 learning function. This can be done by uti-
lizing the seed value on the HCS410 which is stored in
EEPROM. Instead of the normal key generation
method being used to create the encoder key, this seed
value is used and there should not be any mathemati-
cal relationship between serial numbers and seeds for
the best security. See Section 3.7.3 for additional
details.
2.3.5 PASSIVE PROXIMITY ACTIVATION
If the HCS410 is brought into a magnetic field it enters
IFF mode. In this mode it sends out ACK pulses on the
LC lines. If the HCS410 doesn't receive any response
to the first set of ack pulses within 50 ms the HCS410
will transmit a normal code hopping transmission for 2
seconds if XPRF is set in the configuration word. The
function code during this transmission is S2:S0 = 000.
2.3.6 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.
Time-out period is approximately 20 seconds.
2.3.7 VLOW: VOLTAGE LOW INDICATOR
The VLOW bit is transmitted with every transmission
(Figure 2-8). VLOW is set when the operating voltage
has dropped below the low voltage trip point, approxi-
mately 2.2V or 4.4V selectable at 25°C. This VLOW sig-
nal is transmitted so the receiver can give an indication
to the user that the transmitter battery is low.
2.3.8 QUE0:QUE1: QUEUING INFORMATION
If a button is pressed, released for more than 30 ms,
and pressed again within 2 seconds of the first press,
the QUE counter is incremented (Figure 2-7). The
transmission that the HCS410 is busy with is aborted
and a new transmission is begun with the new QUE bits
set. These bits can be used by the decoder to perform
secondary functions using only a single button without
the requirement that the decoder receive more than
one completed transmission. For example if none of
the QUE bits are set the decoder only unlocks the
driver’s door, if QUE0 is set (double press on the trans-
mitter) the decoder unlocks all the doors.
Note 1: The QUE will not overflow.
2: The button must be pressed for more than
50 ms.
FIGURE 2-13: QUE COUNTER TIMING DIAGRAM
1st Button Press
All Buttons Released
2nd Button Press
Input
Sx
DIO
Transmission
t=0
QUE = 00₂
TLOW>30 ms
t > 50 ms
QUE = 01₂
t <2S
t=0
© 2011 Microchip Technology Inc.
DS40158F-page 11