HCS300-IP Microchip Technology, HCS300-IP Datasheet - Page 9

• Manufacturer Part Number: HCS300-IP
• Description: KEE LOQ Code Hopping Encoder
• Manufacturer: Microchip Technology

5.0
5.1
Code word completion is an automatic feature that
makes sure that the entire code word is transmitted,
even if the button is released before the transmission is
complete. The HCS300 encoder powers itself up when
a button is pushed and powers itself down after the
command is finished, if the user has already released
the button. If the button is held down beyond the time
for one transmission, then multiple transmissions will
result. If another button is activated 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 that is
determined in conjunction with the baudrate selection
bits BSL0 and BSL1. 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 or every fourth code word. This
reduces the average power transmitted and hence,
assists in FCC approval of a transmitter device.
5.3
Envelope Encryption is a user selectable option which
is meant to offer a higher level of security for a code
hopping system. During a normal transmission with the
envelope encryption turned off, the 28-bit serial number
is transmitted in the clear (unencrypted). If envelope
encryption is selected, then the serial number is also
encrypted before transmission. The encryption for the
serial number is done using a different algorithm than
the transmission algorithm. The envelope encryption
scheme is not nearly as complex as the KeeLoq algo-
rithm and, hence, not as secure. When the envelope
encryption is used, the serial number must be
decrypted using the envelope key and envelope
decryption. After the serial number is obtained, the nor-
mal decryption method can be used to decrypt the hop-
ping code. All transmitters in a system must use the
same envelope key.
1996 Microchip Technology Inc.
SPECIAL FEATURES
Code Word Completion
Envelope Encryption Option
Blank Alternate Code Word
Preliminary
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 learn function. This can be done by utilizing the
seed value on the HCS300 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 and there need
not be any mathematical relationship between serial
numbers and seeds.
TABLE 5-1:
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 Auto-shutoff bit (see 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.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Note 1: Transmit generated 32-bit code hopping
2: Transmit 32-bit seed value.
Secure Learn
Auto-shutoff
word.
S3
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
PIN ACTIVATION TABLE
S2
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
S1
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
HCS300
S0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
DS21137D-page 9
Notes
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1
1
1
1
1
1
1
1
1
1
1
1
1
2