HCS361-IP Microchip Technology, HCS361-IP Datasheet - Page 12

• Manufacturer Part Number: HCS361-IP
• Description: KEELOQ CODE HOPPING ENCODER
• Manufacturer: Microchip Technology

HCS361
5.0
5.1
Code word completion is an automatic feature that
ensures that the entire code word is transmitted, even
if the button is released before the transmission is com-
plete and that a minimum of two words are completed.
The HCS361 encoder powers itself up when a button is
pushed and powers itself down after the current trans-
mission is finished, if the user has already released the
button. If the button is held down beyond the time for
two transmissions, then multiple transmissions will
result. The HCS361 transmits at least two transmis-
sions before powering down. If another button is acti-
vated during a transmission, the active transmission will
be aborted and the new code will be generated using
the new button information.
5.2
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
100ms 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. Using the
BACW allows the user to transmit a higher amplitude
transmission if the transmission length is shorter. The
FCC puts constraints on the average power that can be
transmitted by a device, and BACW effectively prevents
continuous transmission by only allowing the transmis-
sion of every second word. This reduces the average
power transmitted and hence, assists in FCC approval
of a transmitter device.
5.3
The CRC bits are calculated on the 65 previously trans-
mitted bits. The CRC bits can be used by the receiver
to check the data integrity before processing starts. The
CRC can detect all single bit and 66% of double bit
errors. The CRC is computed as follows:
FIGURE 5-1:
DS40146C-page 12
(1 out of 2 transmitted)
(All words transmitted)
BACW Disabled
BACW Enabled
SPECIAL FEATURES
Code Word Completion
CRC (Cycle Redundancy Check) Bits
Blank Alternate Code Word
BLANK ALTERNATE CODE WORD
Amplitude
2A
A
Min Tx Length
100ms
Preliminary
100ms
EQUATION 0-1:
and
with
and
Di
5.4
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 HCS361 which is stored in
EEPROM. Instead of the normal key generation
method being used to create the encryption key, this
seed value is used and there should not be any mathe-
matical relationship between serial numbers and seeds
for the best security.
5.5
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 time-out bit (Section 3.5.7). Setting
this bit will enable the function (turn Auto-shutoff func-
tion on) and clearing the bit will disable the function.
Time-out period is approximately 25 seconds.
5.6
The V
(Figure 4-2) and will be transmitted as a one if the
operating voltage has dropped below the low voltage
trip point, typically 3.8V at 25 C. This V
transmitted so the receiver can give an indication to the
user that the transmitter battery is low.
5.7
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
1Hz during the transmission.
n
Time
the nth transmission bit 0
CRC 0
LOW
One Code Word
Secure Learning
Auto-shutoff
V
LED Output Operation
CRC 1
LOW
bit is transmitted with every transmission
n
100ms
+
: Voltage LOW Indicator
1
n
=
CRC 1 0
+
1
CRC 0
CRC CALCULATION
=
CRC 0
1996 Microchip Technology Inc.
0
n
=
n
100ms
Di
n
0
64
n
Di
n
CRC 1
LOW
signal is
n